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Advanced Bash-Scripting GuideAdvanced Bash-Scripting GuideAn in-depth exploration of the art of shell scriptingMendel Cooper3.7
23 October 2005
Revision History |
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Revision 3.5 | 04 June 2005 | Revised by: mc | 'BOXBERRY' release: Important Update. | Revision 3.6 | 28 Aug 2005 | Revised by: mc | 'POKEBERRY' release: Bugfix Update. | Revision 3.7 | 23 Oct 2005 | Revised by: mc | 'WHORTLEBERRY' release: Bugfix Update. |
This tutorial assumes no previous knowledge of
scripting or programming, but progresses rapidly toward an
intermediate/advanced level of instruction . . . all
the while sneaking in little snippets of UNIX® wisdom and lore. It
serves as a textbook, a manual for self-study, and a reference and
source of knowledge on shell scripting techniques. The exercises
and heavily-commented examples invite active reader participation,
under the premise that the only way to really learn
scripting is to write scripts. This book is suitable for classroom use as a general
introduction to programming concepts. The latest update of this document, as an archived, bzip2-ed "tarball"
including both the SGML source and rendered HTML, may
be downloaded from the author's home site. A pdf
version is also available. See the change
log for a revision history.
DedicationFor Anita, the source of all the magic - List of Examples
- 2-1. cleanup: A script to clean up the log
files in /var/log
- 2-2. cleanup: An improved clean-up
script
- 2-3. cleanup: An enhanced
and generalized version of above scripts.
- 3-1. Code blocks and I/O redirection
- 3-2. Saving the results of a code block to a file
- 3-3. Running a loop in the background
- 3-4. Backup of all files changed in last day
- 4-1. Variable assignment and substitution
- 4-2. Plain Variable Assignment
- 4-3. Variable Assignment, plain and fancy
- 4-4. Integer or string?
- 4-5. Positional Parameters
- 4-6. wh, whois domain name lookup
- 4-7. Using shift
- 5-1. Echoing Weird Variables
- 5-2. Escaped Characters
- 6-1. exit / exit status
- 6-2. Negating a condition using !
- 7-1. What is truth?
- 7-2. Equivalence of test,
/usr/bin/test, [ ],
and /usr/bin/[
- 7-3. Arithmetic Tests using (( ))
- 7-4. Testing for broken links
- 7-5. Arithmetic and string comparisons
- 7-6. Testing whether a string is null
- 7-7. zmore
- 8-1. Greatest common divisor
- 8-2. Using Arithmetic Operations
- 8-3. Compound Condition Tests Using && and ||
- 8-4. Representation of numerical constants
- 9-1. $IFS and whitespace
- 9-2. Timed Input
- 9-3. Once more, timed input
- 9-4. Timed read
- 9-5. Am I root?
- 9-6. arglist: Listing arguments with $* and $@
- 9-7. Inconsistent $* and $@ behavior
- 9-8. $* and $@ when
$IFS is empty
- 9-9. Underscore variable
- 9-10. Inserting a blank line between paragraphs in a text file
- 9-11. Converting graphic file formats, with filename change
- 9-12. Emulating getopt
- 9-13. Alternate ways of extracting substrings
- 9-14. Using parameter substitution and error messages
- 9-15. Parameter substitution and "usage" messages
- 9-16. Length of a variable
- 9-17. Pattern matching in parameter substitution
- 9-18. Renaming file extensions:
- 9-19. Using pattern matching to parse arbitrary strings
- 9-20. Matching patterns at prefix or suffix of string
- 9-21. Using declare to type variables
- 9-22. Indirect References
- 9-23. Passing an indirect reference to awk
- 9-24. Generating random numbers
- 9-25. Picking a random card from a deck
- 9-26. Random between values
- 9-27. Rolling a single die with RANDOM
- 9-28. Reseeding RANDOM
- 9-29. Pseudorandom numbers, using awk
- 9-30. C-type manipulation of variables
- 10-1. Simple for loops
- 10-2. for loop with two parameters in each
[list] element
- 10-3. Fileinfo: operating on a file list
contained in a variable
- 10-4. Operating on files with a for loop
- 10-5. Missing in [list] in a
for loop
- 10-6. Generating the [list] in a for
loop with command substitution
- 10-7. A grep replacement
for binary files
- 10-8. Listing all users on the system
- 10-9. Checking all the binaries in a directory for
authorship
- 10-10. Listing the symbolic
links in a directory
- 10-11. Symbolic links in a directory, saved to a file
- 10-12. A C-like for loop
- 10-13. Using efax in batch mode
- 10-14. Simple while loop
- 10-15. Another while loop
- 10-16. while loop with multiple conditions
- 10-17. C-like syntax in a while loop
- 10-18. until loop
- 10-19. Nested Loop
- 10-20. Effects of break and
continue in a loop
- 10-21. Breaking out of multiple loop levels
- 10-22. Continuing at a higher loop level
- 10-23. Using "continue N" in an actual task
- 10-24. Using case
- 10-25. Creating menus using case
- 10-26. Using command substitution to generate the
case variable
- 10-27. Simple string matching
- 10-28. Checking for alphabetic input
- 10-29. Creating menus using select
- 10-30. Creating menus using select in a function
- 11-1. A script that forks off multiple instances of itself
- 11-2. printf in action
- 11-3. Variable assignment, using read
- 11-4. What happens when read has no
variable
- 11-5. Multi-line input to read
- 11-6. Detecting the arrow keys
- 11-7. Using read with
file redirection
- 11-8. Problems reading from a pipe
- 11-9. Changing the current working directory
- 11-10. Letting "let" do arithmetic.
- 11-11. Showing the effect of eval
- 11-12. Forcing a log-off
- 11-13. A version of "rot13"
- 11-14. Using eval to force variable
substitution in a Perl script
- 11-15. Using set with positional
parameters
- 11-16. Reassigning the positional parameters
- 11-17. "Unsetting" a variable
- 11-18. Using export to pass a variable to an
embedded awk script
- 11-19. Using getopts to read the
options/arguments passed to a script
- 11-20. "Including" a data file
- 11-21. A (useless) script that sources itself
- 11-22. Effects of exec
- 11-23. A script that exec's itself
- 11-24. Waiting for a process to finish before proceeding
- 11-25. A script that kills itself
- 12-1. Using ls to create a table of contents
for burning a CDR disk
- 12-2. Hello or Good-bye
- 12-3. Badname, eliminate file names
in current directory containing bad characters and whitespace.
- 12-4. Deleting a file by its inode
number
- 12-5. Logfile: Using xargs to monitor system log
- 12-6. Copying files in current directory to another
- 12-7. Killing processes by name
- 12-8. Word frequency analysis
using xargs
- 12-9. Using expr
- 12-10. Using date
- 12-11. Word Frequency Analysis
- 12-12. Which files are scripts?
- 12-13. Generating 10-digit random numbers
- 12-14. Using tail to monitor the system log
- 12-15. Emulating "grep" in a script
- 12-16. Looking up definitions in Webster's 1913 Dictionary
- 12-17. Checking words in a list for validity
- 12-18. toupper: Transforms a file to all uppercase.
- 12-19. lowercase: Changes all filenames in working directory to lowercase.
- 12-20. Du: DOS to UNIX text file conversion.
- 12-21. rot13: rot13, ultra-weak encryption.
- 12-22. Generating "Crypto-Quote" Puzzles
- 12-23. Formatted file listing.
- 12-24. Using column to format a directory
listing
- 12-25. nl: A self-numbering script.
- 12-26. manview: Viewing formatted manpages
- 12-27. Using cpio to move a directory tree
- 12-28. Unpacking an rpm archive
- 12-29. Stripping comments from C program files
- 12-30. Exploring /usr/X11R6/bin
- 12-31. An "improved" strings
command
- 12-32. Using cmp to compare two files
within a script.
- 12-33. basename and dirname
- 12-34. Checking file integrity
- 12-35. Uudecoding encoded files
- 12-36. Finding out where to report a spammer
- 12-37. Analyzing a spam domain
- 12-38. Getting a stock quote
- 12-39. Updating FC4
- 12-40. Using ssh
- 12-41. A script that mails itself
- 12-42. Monthly Payment on a Mortgage
- 12-43. Base Conversion
- 12-44. Invoking bc using a "here
document"
- 12-45. Calculating PI
- 12-46. Converting a decimal number to hexadecimal
- 12-47. Factoring
- 12-48. Calculating the hypotenuse of a triangle
- 12-49. Using seq to generate loop arguments
- 12-50. Letter Count"
- 12-51. Using getopt to parse command-line
options
- 12-52. A script that copies itself
- 12-53. Exercising dd
- 12-54. Capturing Keystrokes
- 12-55. Securely deleting a file
- 12-56. Filename generator
- 12-57. Converting meters to miles
- 12-58. Using m4
- 13-1. Setting a new password
- 13-2. Setting an erase character
- 13-3. secret password:
Turning off terminal echoing
- 13-4. Keypress detection
- 13-5. Checking a remote server for identd
- 13-6. pidof helps kill a process
- 13-7. Checking a CD image
- 13-8. Creating a filesystem in a file
- 13-9. Adding a new hard drive
- 13-10. Using umask to hide an output file
from prying eyes
- 13-11. killall, from /etc/rc.d/init.d
- 14-1. Stupid script tricks
- 14-2. Generating a variable from a loop
- 14-3. Finding anagrams
- 16-1. Redirecting stdin using
exec
- 16-2. Redirecting stdout using
exec
- 16-3. Redirecting both stdin and
stdout in the same script with
exec
- 16-4. Avoiding a subshell
- 16-5. Redirected while loop
- 16-6. Alternate form of redirected while loop
- 16-7. Redirected until loop
- 16-8. Redirected for loop
- 16-9. Redirected for loop (both
stdin and stdout
redirected)
- 16-10. Redirected if/then test
- 16-11. Data file "names.data" for above examples
- 16-12. Logging events
- 17-1. broadcast: Sends message to everyone logged in
- 17-2. dummyfile: Creates a 2-line dummy file
- 17-3. Multi-line message using cat
- 17-4. Multi-line message, with tabs suppressed
- 17-5. Here document with parameter substitution
- 17-6. Upload a file pair to "Sunsite" incoming
directory
- 17-7. Parameter substitution turned off
- 17-8. A script that generates another script
- 17-9. Here documents and functions
- 17-10. "Anonymous" Here Document
- 17-11. Commenting out a block of code
- 17-12. A self-documenting script
- 17-13. Prepending a line to a file
- 20-1. Variable scope in a subshell
- 20-2. List User Profiles
- 20-3. Running parallel processes in subshells
- 21-1. Running a script in restricted mode
- 23-1. Simple functions
- 23-2. Function Taking Parameters
- 23-3. Functions and command-line args passed to the script
- 23-4. Passing an indirect reference to a function
- 23-5. Dereferencing a parameter passed to a function
- 23-6. Again, dereferencing a parameter passed to a function
- 23-7. Maximum of two numbers
- 23-8. Converting numbers to Roman numerals
- 23-9. Testing large return values in a function
- 23-10. Comparing two large integers
- 23-11. Real name from username
- 23-12. Local variable visibility
- 23-13. Recursion, using a local variable
- 23-14. The Towers of Hanoi
- 24-1. Aliases within a script
- 24-2. unalias: Setting and unsetting an alias
- 25-1. Using an "and list" to test for command-line arguments
- 25-2. Another command-line arg test using an "and list"
- 25-3. Using "or lists" in combination with an "and list"
- 26-1. Simple array usage
- 26-2. Formatting a poem
- 26-3. Various array operations
- 26-4. String operations on arrays
- 26-5. Loading the contents of a script into an array
- 26-6. Some special properties of arrays
- 26-7. Of empty arrays and empty elements
- 26-8. Initializing arrays
- 26-9. Copying and concatenating arrays
- 26-10. More on concatenating arrays
- 26-11. An old friend:
The Bubble Sort
- 26-12. Embedded arrays and indirect references
- 26-13. Complex array application:
Sieve of Eratosthenes
- 26-14. Emulating a push-down stack
- 26-15. Complex array application:
Exploring a weird mathematical series
- 26-16. Simulating a two-dimensional array, then tilting it
- 27-1. Using /dev/tcp for troubleshooting
- 27-2. Finding the process associated with a PID
- 27-3. On-line connect status
- 28-1. Hiding the cookie jar
- 28-2. Setting up a swapfile using /dev/zero
- 28-3. Creating a ramdisk
- 29-1. A buggy script
- 29-2. Missing keyword
- 29-3. test24, another buggy script
- 29-4. Testing a condition with an "assert"
- 29-5. Trapping at exit
- 29-6. Cleaning up after Control-C
- 29-7. Tracing a variable
- 29-8. Running multiple processes (on an SMP box)
- 31-1. Numerical and string comparison are not equivalent
- 31-2. Subshell Pitfalls
- 31-3. Piping the output of echo to a read
- 33-1. shell wrapper
- 33-2. A slightly more complex shell wrapper
- 33-3. A generic shell wrapper that writes to a logfile
- 33-4. A shell wrapper around an awk script
- 33-5. A shell wrapper around another awk script
- 33-6. Perl embedded in a Bash script
- 33-7. Bash and Perl scripts combined
- 33-8. A (useless) script that recursively calls itself
- 33-9. A (useful) script that recursively calls itself
- 33-10. Another (useful) script that recursively calls itself
- 33-11. A "colorized" address database
- 33-12. Drawing a box
- 33-13. Echoing colored text
- 33-14. A "horserace" game
- 33-15. Return value trickery
- 33-16. Even more return value trickery
- 33-17. Passing and returning arrays
- 33-18. Fun with anagrams
- 33-19. Widgets invoked from a shell script
- 34-1. String expansion
- 34-2. Indirect variable references - the new way
- 34-3. Simple database application, using indirect variable
referencing
- 34-4. Using arrays and other miscellaneous trickery
to deal four random hands from a deck of cards
- A-1. mailformat: Formatting an e-mail message
- A-2. rn: A simple-minded file rename utility
- A-3. blank-rename: renames filenames containing
blanks
- A-4. encryptedpw: Uploading to an ftp site,
using a locally encrypted password
- A-5. copy-cd: Copying a data CD
- A-6. Collatz series
- A-7. days-between: Calculate number of days
between two dates
- A-8. Make a "dictionary"
- A-9. Soundex conversion
- A-10. "Game of Life"
- A-11. Data file for "Game of Life"
- A-12. behead: Removing mail and news message headers
- A-13. ftpget: Downloading files via ftp
- A-14. password: Generating random
8-character passwords
- A-15. fifo: Making daily backups, using named pipes
- A-16. Generating prime numbers using the modulo operator
- A-17. tree: Displaying a directory tree
- A-18. string functions: C-like string functions
- A-19. Directory information
- A-20. Object-oriented database
- A-21. Library of hash functions
- A-22. Colorizing text using hash functions
- A-23. Mounting USB keychain storage devices
- A-24. Preserving weblogs
- A-25. Protecting literal strings
- A-26. Unprotecting literal strings
- A-27. Spammer Identification
- A-28. Spammer Hunt
- A-29. Making wget easier to use
- A-30. A "podcasting" script
- A-31. Basics Reviewed
- A-32. An expanded cd command
- C-1. Counting Letter Occurrences
- K-1. Sample .bashrc file
- L-1. VIEWDATA.BAT: DOS Batch File
- L-2. viewdata.sh: Shell Script Conversion of VIEWDATA.BAT
- P-1. Print the server environment
Part 1. IntroductionThe shell is a command interpreter. More than just the
insulating layer between the operating system kernel and the user,
it's also a fairly powerful programming language. A shell program,
called a script, is an easy-to-use tool for
building applications by "gluing" together system
calls, tools, utilities, and compiled binaries. Virtually the
entire repertoire of UNIX commands, utilities, and tools is
available for invocation by a shell script. If that were
not enough, internal shell commands, such as testing and loop
constructs, give additional power and flexibility to scripts.
Shell scripts lend themselves exceptionally well to administrative
system tasks and other routine repetitive jobs not requiring the
bells and whistles of a full-blown tightly structured programming
language.
Chapter 1. Why Shell Programming? | No programming language is perfect. There is not even a single
best language; there are only languages well suited or perhaps poorly
suited for particular purposes. | | Herbert Mayer |
A working knowledge of shell scripting is essential to anyone
wishing to become reasonably proficient at system administration,
even if they do not anticipate ever having to actually write a
script. Consider that as a Linux machine boots up, it executes the
shell scripts in /etc/rc.d
to restore the system configuration and set up services. A detailed
understanding of these startup scripts is important for analyzing
the behavior of a system, and possibly modifying it. Writing shell scripts is not hard to learn, since the scripts
can be built in bite-sized sections and there is only a fairly
small set of shell-specific operators and options
to learn. The syntax is simple and straightforward, similar to
that of invoking and chaining together utilities at the command
line, and there are only a few "rules" to learn.
Most short scripts work right the first time, and debugging even
the longer ones is straightforward. A shell script is a "quick and dirty" method of
prototyping a complex application. Getting even a limited subset
of the functionality to work in a shell script is often a useful
first stage in project development. This way, the structure of
the application can be tested and played with, and the major
pitfalls found before proceeding to the final coding in C, C++,
Java, or Perl. Shell scripting hearkens back to the classic UNIX philosophy
of breaking complex projects into simpler subtasks, of chaining
together components and utilities. Many consider this a better,
or at least more esthetically pleasing approach to problem solving
than using one of the new generation of high powered all-in-one
languages, such as Perl, which attempt to be all things to all
people, but at the cost of forcing you to alter your thinking
processes to fit the tool. When not to use shell scripts
Resource-intensive tasks, especially where speed is
a factor (sorting, hashing, etc.) Procedures involving heavy-duty math operations,
especially floating point arithmetic, arbitrary precision
calculations, or complex numbers (use C++ or FORTRAN
instead) Cross-platform portability required (use C or Java instead) Complex applications, where structured programming is
a necessity (need type-checking of variables, function
prototypes, etc.) Mission-critical applications upon which you are betting the
ranch, or the future of the company Situations where security is important, where you need to
guarantee the integrity of your system and protect against
intrusion, cracking, and vandalism Project consists of subcomponents with interlocking
dependencies Extensive file operations required (Bash is limited to
serial file access, and that only in a particularly clumsy
and inefficient line-by-line fashion) Need native support for multi-dimensional arrays Need data structures, such as linked lists or trees Need to generate or manipulate graphics or GUIs Need direct access to system hardware Need port or socket I/O Need to use libraries or interface with legacy code Proprietary, closed-source applications (shell scripts put the
source code right out in the open for all the world to see)
If any of the above applies, consider a more powerful scripting
language -- perhaps Perl, Tcl, Python, Ruby -- or possibly a
high-level compiled language such as C, C++, or Java. Even then,
prototyping the application as a shell script might still be a
useful development step. We will be using Bash, an acronym for
"Bourne-Again shell" and a pun on Stephen Bourne's
now classic Bourne shell. Bash has become a de
facto standard for shell scripting on all flavors of
UNIX. Most of the principles this book covers apply equally
well to scripting with other shells, such as the Korn Shell,
from which Bash derives some of its features,
and the C Shell and its variants. (Note that C Shell programming
is not recommended due to certain inherent problems, as pointed out
in an October, 1993 Usenet
post by Tom Christiansen.)
What follows is a tutorial on shell scripting. It relies
heavily on examples to illustrate various features of the shell.
The example scripts work -- they've been tested, insofar as was
possible -- and some of them are even useful in real life. The
reader can play with the actual working code of the examples
in the source archive (scriptname.sh or
scriptname.bash),
give them execute permission (chmod
u+rx scriptname), then run them
to see what happens. Should the source archive
not be available, then cut-and-paste from the HTML,
pdf,
or text
rendered versions. Be aware that some of the scripts presented here
introduce features before they are explained, and this may require
the reader to temporarily skip ahead for enlightenment. Unless otherwise noted, the author of this
book wrote the example scripts that follow.
Chapter 2. Starting Off With a Sha-Bang | Shell programming is a 1950s juke box . . . | | Larry Wall |
In the simplest case, a script is nothing more than a list of system
commands stored in a file. At the very least, this saves the
effort of retyping that particular sequence of commands each time
it is invoked. Example 2-1. cleanup: A script to clean up the log
files in /var/log # Cleanup
# Run as root, of course.
cd /var/log
cat /dev/null > messages
cat /dev/null > wtmp
echo "Logs cleaned up." |
There is nothing unusual here, only a set of commands that
could just as easily be invoked one by one from the command line on
the console or in an xterm. The advantages of
placing the commands in a script go beyond not having to retype them
time and again. The script becomes a tool,
and can easily be modified or customized for a particular
application. Example 2-2. cleanup: An improved clean-up
script #!/bin/bash
# Proper header for a Bash script.
# Cleanup, version 2
# Run as root, of course.
# Insert code here to print error message and exit if not root.
LOG_DIR=/var/log
# Variables are better than hard-coded values.
cd $LOG_DIR
cat /dev/null > messages
cat /dev/null > wtmp
echo "Logs cleaned up."
exit # The right and proper method of "exiting" from a script. |
Now that's beginning to look like a real script. But we can
go even farther . . . Example 2-3. cleanup: An enhanced
and generalized version of above scripts. #!/bin/bash
# Cleanup, version 3
# Warning:
# -------
# This script uses quite a number of features that will be explained
#+ later on.
# By the time you've finished the first half of the book,
#+ there should be nothing mysterious about it.
LOG_DIR=/var/log
ROOT_UID=0 # Only users with $UID 0 have root privileges.
LINES=50 # Default number of lines saved.
E_XCD=66 # Can't change directory?
E_NOTROOT=67 # Non-root exit error.
# Run as root, of course.
if [ "$UID" -ne "$ROOT_UID" ]
then
echo "Must be root to run this script."
exit $E_NOTROOT
fi
if [ -n "$1" ]
# Test if command line argument present (non-empty).
then
lines=$1
else
lines=$LINES # Default, if not specified on command line.
fi
# Stephane Chazelas suggests the following,
#+ as a better way of checking command line arguments,
#+ but this is still a bit advanced for this stage of the tutorial.
#
# E_WRONGARGS=65 # Non-numerical argument (bad arg format)
#
# case "$1" in
# "" ) lines=50;;
# *[!0-9]*) echo "Usage: `basename $0` file-to-cleanup"; exit $E_WRONGARGS;;
# * ) lines=$1;;
# esac
#
#* Skip ahead to "Loops" chapter to decipher all this.
cd $LOG_DIR
if [ `pwd` != "$LOG_DIR" ] # or if [ "$PWD" != "$LOG_DIR" ]
# Not in /var/log?
then
echo "Can't change to $LOG_DIR."
exit $E_XCD
fi # Doublecheck if in right directory, before messing with log file.
# far more efficient is:
#
# cd /var/log || {
# echo "Cannot change to necessary directory." >&2
# exit $E_XCD;
# }
tail -$lines messages > mesg.temp # Saves last section of message log file.
mv mesg.temp messages # Becomes new log directory.
# cat /dev/null > messages
#* No longer needed, as the above method is safer.
cat /dev/null > wtmp # ': > wtmp' and '> wtmp' have the same effect.
echo "Logs cleaned up."
exit 0
# A zero return value from the script upon exit
#+ indicates success to the shell. |
Since you may not wish to wipe out the entire system log,
this version of the script keeps the last section of the message
log intact. You will constantly discover ways of refining previously
written scripts for increased effectiveness. The
sha-bang
( #!) at the head of a script
tells your system that this file is a set of commands to be fed
to the command interpreter indicated. The
#! is actually a two-byte
magic number, a special marker that
designates a file type, or in this case an executable shell
script (type man magic for more
details on this fascinating topic). Immediately following
the sha-bang is a path
name. This is the path to the program that interprets
the commands in the script, whether it be a shell, a programming
language, or a utility. This command interpreter then executes
the commands in the script, starting at the top (line following
the sha-bang line), ignoring comments.
#!/bin/sh
#!/bin/bash
#!/usr/bin/perl
#!/usr/bin/tcl
#!/bin/sed -f
#!/usr/awk -f |
Each of the above script header lines calls a different command
interpreter, be it /bin/sh, the default shell
(bash in a Linux system) or otherwise.
Using #!/bin/sh, the default Bourne shell
in most commercial variants of UNIX, makes the script portable to non-Linux machines,
though you sacrifice Bash-specific features.
The script will, however, conform to the
POSIX
sh standard. Note that the path given at the "sha-bang" must
be correct, otherwise an error message -- usually "Command
not found" -- will be the only result of running the
script. #! can be omitted if the script consists only
of a set of generic system commands, using no internal
shell directives. The second example, above, requires the
initial #!, since the variable assignment line,
lines=50, uses a shell-specific construct.
Note again that #!/bin/sh invokes the default
shell interpreter, which defaults to /bin/bash
on a Linux machine.  | This tutorial encourages a modular approach
to constructing a script. Make note of and collect
"boilerplate" code snippets that might be useful
in future scripts. Eventually you can build quite an extensive
library of nifty routines. As an example, the following script
prolog tests whether the script has been invoked with the correct
number of parameters. E_WRONG_ARGS=65
script_parameters="-a -h -m -z"
# -a = all, -h = help, etc.
if [ $# -ne $Number_of_expected_args ]
then
echo "Usage: `basename $0` $script_parameters"
# `basename $0` is the script's filename.
exit $E_WRONG_ARGS
fi |
Many times, you will write a script that carries out one
particular task. The first script in this chapter is an
example of this. Later, it might occur to you to generalize
the script to do other, similar tasks. Replacing the literal
("hard-wired") constants by variables is a step in
that direction, as is replacing repetitive code blocks by functions. |
2.1. Invoking the scriptHaving written the script, you can invoke it by sh
scriptname,
or alternatively bash scriptname. (Not
recommended is using sh <scriptname,
since this effectively disables reading from
stdin within the script.) Much more
convenient is to make the script itself directly executable with
a chmod.
- Either:
chmod 555 scriptname (gives
everyone read/execute permission)
- or
chmod +rx scriptname (gives
everyone read/execute permission) chmod
u+rx scriptname (gives only the
script owner read/execute permission)
Having made the script executable, you may now test it by
./scriptname.
If it begins with a "sha-bang" line, invoking the
script calls the correct command interpreter to run it. As a final step, after testing and debugging,
you would likely want to move it to /usr/local/bin (as root, of
course), to make the script available to yourself and all
other users as a system-wide executable. The script could
then be invoked by simply typing scriptname
[ENTER] from the command line.
Chapter 3. Special CharactersSpecial Characters Found In
Scripts and Elsewhere - #
# This line is a comment. |
Comments may also occur following the end of a command. echo "A comment will follow." # Comment here.
# ^ Note whitespace before # |
Comments may also follow whitespace at the beginning
of a line. # A tab precedes this comment. |
 | A command may not follow a comment on the
same line. There is no method of terminating the comment,
in order for "live code" to begin on the same
line. Use a new line for the next command. |
 | Of course, an escaped # in an
echo statement does
not begin a comment. Likewise, a
# appears in certain parameter
substitution constructs and in numerical constant expressions.
echo "The # here does not begin a comment."
echo 'The # here does not begin a comment.'
echo The \# here does not begin a comment.
echo The # here begins a comment.
echo ${PATH#*:} # Parameter substitution, not a comment.
echo $(( 2#101011 )) # Base conversion, not a comment.
# Thanks, S.C. |
The standard quoting and
escape characters (" ' \) escape the #.
|
Certain pattern matching
operations also use the #. - ;
echo hello; echo there
if [ -x "$filename" ]; then # Note that "if" and "then" need separation.
# Why?
echo "File $filename exists."; cp $filename $filename.bak
else
echo "File $filename not found."; touch $filename
fi; echo "File test complete." |
Note that the ";" sometimes
needs to be escaped. - ;;
case "$variable" in
abc) echo "\$variable = abc" ;;
xyz) echo "\$variable = xyz" ;;
esac |
- .
- .
When considering directory names, a single
dot represents the current working directory,
and two dots denote the parent
directory. bash$ pwd
/home/bozo/projects
bash$ cd .
bash$ pwd
/home/bozo/projects
bash$ cd ..
bash$ pwd
/home/bozo/
|
The dot often appears as the
destination (directory) of a file movement command. bash$ cp /home/bozo/current_work/junk/* .
|
- .
- "
- '
- ,
- \
\X
"escapes" the character
X. This has the effect of
"quoting" X, equivalent
to 'X'. The \ may
be used to quote " and ',
so they are expressed literally. See Chapter 5 for an in-depth explanation
of escaped characters. - /
This is also the division arithmetic operator. - `
- :
Endless loop: while :
do
operation-1
operation-2
...
operation-n
done
# Same as:
# while true
# do
# ...
# done |
Placeholder in if/then test: if condition
then : # Do nothing and branch ahead
else
take-some-action
fi |
Provide a placeholder where a binary operation is
expected, see Example 8-2 and default parameters. : ${username=`whoami`}
# ${username=`whoami`} Gives an error without the leading :
# unless "username" is a command or builtin... |
Provide a placeholder where a command is expected in a
here document. See Example 17-10. Evaluate string of variables using
parameter substitution
(as in Example 9-14).
: ${HOSTNAME?} ${USER?} ${MAIL?}
# Prints error message
#+ if one or more of essential environmental variables not set. |
Variable expansion / substring
replacement. In combination with the > redirection operator,
truncates a file to zero length, without changing its
permissions. If the file did not previously exist,
creates it.
: > data.xxx # File "data.xxx" now empty.
# Same effect as cat /dev/null >data.xxx
# However, this does not fork a new process, since ":" is a builtin. |
See also Example 12-14.In combination with the >>
redirection operator, has no effect on a pre-existing
target file (: >> target_file).
If the file did not previously exist, creates it.  | This applies to regular files, not pipes,
symlinks, and certain special files. |
May be used to begin a comment line, although this is not
recommended. Using # for a comment turns
off error checking for the remainder of that line, so
almost anything may appear in a comment. However,
this is not the case with
:.
: This is a comment that generates an error, ( if [ $x -eq 3] ). |
The ":" also serves as a field
separator, in /etc/passwd, and in the $PATH variable.
bash$ echo $PATH
/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/sbin:/usr/sbin:/usr/games |
- !
In a different context, the !
also appears in indirect variable
references. In yet another context, from the command
line, the ! invokes the
Bash history mechanism (see Appendix J). Note that within a script,
the history mechanism is disabled. - *
bash$ echo *
abs-book.sgml add-drive.sh agram.sh alias.sh
|
The * also represents any number
(or zero) characters in a regular expression. - *
A double asterisk, **, is the exponentiation
operator. - ?
In a double
parentheses construct, the ? serves
as a C-style trinary operator. See Example 9-30. In a parameter
substitution expression, the ?
tests whether a variable has been
set. - ?
- $
A $ prefixing a variable name
indicates the value the variable
holds. - $
- ${}
- $*, $@
- $?
- $$
- ()
 | A listing of commands within
parentheses starts a subshell. Variables inside parentheses, within the subshell, are not
visible to the rest of the script. The parent process,
the script, cannot read variables
created in the child process, the subshell.
a=123
( a=321; )
echo "a = $a" # a = 123
# "a" within parentheses acts like a local variable. |
|
- {xxx,yyy,zzz,...}
A command may act upon a comma-separated list of file specs within
braces.
Filename expansion (globbing)
applies to the file specs between the braces.  | No spaces allowed within the braces
unless the spaces are quoted or escaped. echo {file1,file2}\ :{\ A," B",' C'} file1 : A file1 : B file1 : C file2 : A file2 : B file2 : C |
- {}
bash$ { local a;
a=123; }
bash: local: can only be used in a
function
| a=123
{ a=321; }
echo "a = $a" # a = 321 (value inside code block)
# Thanks, S.C. |
The code block enclosed in braces may have I/O redirected to and from
it. Example 3-1. Code blocks and I/O redirection #!/bin/bash
# Reading lines in /etc/fstab.
File=/etc/fstab
{
read line1
read line2
} < $File
echo "First line in $File is:"
echo "$line1"
echo
echo "Second line in $File is:"
echo "$line2"
exit 0
# Now, how do you parse the separate fields of each line?
# Hint: use awk. |
Example 3-2. Saving the results of a code block to a file #!/bin/bash
# rpm-check.sh
# Queries an rpm file for description, listing, and whether it can be installed.
# Saves output to a file.
#
# This script illustrates using a code block.
SUCCESS=0
E_NOARGS=65
if [ -z "$1" ]
then
echo "Usage: `basename $0` rpm-file"
exit $E_NOARGS
fi
{
echo
echo "Archive Description:"
rpm -qpi $1 # Query description.
echo
echo "Archive Listing:"
rpm -qpl $1 # Query listing.
echo
rpm -i --test $1 # Query whether rpm file can be installed.
if [ "$?" -eq $SUCCESS ]
then
echo "$1 can be installed."
else
echo "$1 cannot be installed."
fi
echo
} > "$1.test" # Redirects output of everything in block to file.
echo "Results of rpm test in file $1.test"
# See rpm man page for explanation of options.
exit 0 |
 | Unlike a command group within (parentheses),
as above, a code block enclosed by {braces} will
not normally launch a subshell.
|
- {} \;
 | The ";" ends
the -exec option of a
find command sequence. It needs
to be escaped to protect it from interpretation by the
shell. |
- [ ]
Test expression between [
]. Note that [ is part of
the shell builtin test (and a synonym
for it), not a link to the external
command /usr/bin/test. - [[ ]]
Test expression between [[ ]] (shell
keyword). See the discussion on the [[ ... ]] construct. - [ ]
In the context of an array,
brackets set off the numbering of each element of that array.
Array[1]=slot_1
echo ${Array[1]} |
- [ ]
As part of a regular
expression, brackets delineate a range of characters to
match. - (( ))
Expand and evaluate integer expression between
(( )). See the discussion on the (( ... )) construct. - > &> >& >> <
scriptname >filename redirects the output of
scriptname to file
filename. Overwrite
filename if it already exists. command &>filename redirects
both the stdout and the
stderr of command
to filename. command >&2 redirects
stdout of command
to stderr. scriptname >>filename appends
the output of scriptname
to file filename. If
filename does not already exist,
it will be created. (command)> <(command) In a different context,
the "<" and
">" characters act
as string comparison
operators. In yet another context,
the "<" and
">" characters act
as integer comparison
operators. See also Example 12-9. - <<
- <<<
- <, >
- \<, \>
bash$ grep '\<the\>' textfile - |
echo ls -l | sh
# Passes the output of "echo ls -l" to the shell,
#+ with the same result as a simple "ls -l".
cat *.lst | sort | uniq
# Merges and sorts all ".lst" files, then deletes duplicate lines. |
The output of a command or commands
may be piped to a script.
#!/bin/bash
# uppercase.sh : Changes input to uppercase.
tr 'a-z' 'A-Z'
# Letter ranges must be quoted
#+ to prevent filename generation from single-letter filenames.
exit 0 |
Now, let us pipe the output of ls -l to this
script.
bash$ ls -l | ./uppercase.sh
-RW-RW-R-- 1 BOZO BOZO 109 APR 7 19:49 1.TXT
-RW-RW-R-- 1 BOZO BOZO 109 APR 14 16:48 2.TXT
-RW-R--R-- 1 BOZO BOZO 725 APR 20 20:56 DATA-FILE
|
 | The stdout of each process in
a pipe must be read as the stdin
of the next. If this is not the case, the data stream
will block, and the pipe will not
behave as expected.
cat file1 file2 | ls -l | sort
# The output from "cat file1 file2" disappears. |
A pipe runs as a child
process, and therefore cannot alter script
variables.
variable="initial_value"
echo "new_value" | read variable
echo "variable = $variable" # variable = initial_value |
If one of the commands in the pipe
aborts, this prematurely terminates execution of the
pipe. Called a broken pipe, this
condition sends a SIGPIPE signal. |
- >|
- ||
- &
bash$ sleep 10 &
[1] 850
[1]+ Done sleep 10
|
Within a script, commands and even loops may run in the
background. Example 3-3. Running a loop in the background #!/bin/bash
# background-loop.sh
for i in 1 2 3 4 5 6 7 8 9 10 # First loop.
do
echo -n "$i "
done & # Run this loop in background.
# Will sometimes execute after second loop.
echo # This 'echo' sometimes will not display.
for i in 11 12 13 14 15 16 17 18 19 20 # Second loop.
do
echo -n "$i "
done
echo # This 'echo' sometimes will not display.
# ======================================================
# The expected output from the script:
# 1 2 3 4 5 6 7 8 9 10
# 11 12 13 14 15 16 17 18 19 20
# Sometimes, though, you get:
# 11 12 13 14 15 16 17 18 19 20
# 1 2 3 4 5 6 7 8 9 10 bozo $
# (The second 'echo' doesn't execute. Why?)
# Occasionally also:
# 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
# (The first 'echo' doesn't execute. Why?)
# Very rarely something like:
# 11 12 13 1 2 3 4 5 6 7 8 9 10 14 15 16 17 18 19 20
# The foreground loop preempts the background one.
exit 0
# Nasimuddin Ansari suggests adding sleep 1
#+ after the echo -n "$i" in lines 6 and 14,
#+ for some real fun. |
 | A command run in the background within a
script may cause the script to hang, waiting
for a keystroke. Fortunately, there is a remedy for this. |
- &&
- -
COMMAND -[Option1][Option2][...] ls -al sort -dfu $filename set -- $variable if [ $file1 -ot $file2 ]
then
echo "File $file1 is older than $file2."
fi
if [ "$a" -eq "$b" ]
then
echo "$a is equal to $b."
fi
if [ "$c" -eq 24 -a "$d" -eq 47 ]
then
echo "$c equals 24 and $d equals 47."
fi |
- -
(cd /source/directory && tar cf - . ) | (cd /dest/directory && tar xpvf -)
# Move entire file tree from one directory to another
# [courtesy Alan Cox <a.cox@swansea.ac.uk>, with a minor change]
# 1) cd /source/directory Source directory, where the files to be moved are.
# 2) && "And-list": if the 'cd' operation successful, then execute the next command.
# 3) tar cf - . The 'c' option 'tar' archiving command creates a new archive,
# the 'f' (file) option, followed by '-' designates the target file as stdout,
# and do it in current directory tree ('.').
# 4) | Piped to...
# 5) ( ... ) a subshell
# 6) cd /dest/directory Change to the destination directory.
# 7) && "And-list", as above
# 8) tar xpvf - Unarchive ('x'), preserve ownership and file permissions ('p'),
# and send verbose messages to stdout ('v'),
# reading data from stdin ('f' followed by '-').
#
# Note that 'x' is a command, and 'p', 'v', 'f' are options.
# Whew!
# More elegant than, but equivalent to:
# cd source/directory
# tar cf - . | (cd ../dest/directory; tar xpvf -)
#
# Also having same effect:
# cp -a /source/directory/* /dest/directory
# Or:
# cp -a /source/directory/* /source/directory/.[^.]* /dest/directory
# If there are hidden files in /source/directory. |
bunzip2 linux-2.6.13.tar.bz2 | tar xvf -
# --uncompress tar file-- | --then pass it to "tar"--
# If "tar" has not been patched to handle "bunzip2",
# this needs to be done in two discrete steps, using a pipe.
# The purpose of the exercise is to unarchive "bzipped" kernel source. |
Note that in this context the "-" is not
itself a Bash operator, but rather an option recognized by
certain UNIX utilities that write to
stdout, such as tar,
cat, etc. bash$ echo "whatever" | cat -
whatever |
Where a filename is expected,
- redirects output to
stdout (sometimes seen with
tar cf), or accepts input from
stdin, rather than from a file. This
is a method of using a file-oriented utility as a filter
in a pipe. bash$ file
Usage: file [-bciknvzL] [-f namefile] [-m magicfiles] file...
|
By itself on the command line, file fails with an error message.
Add a "-" for a more useful result. This causes the
shell to await user input.
bash$ file -
abc
standard input: ASCII text
bash$ file -
#!/bin/bash
standard input: Bourne-Again shell script text executable
|
Now the command accepts input from stdin
and analyzes it.
The "-" can be used to pipe
stdout to other commands. This permits
such stunts as prepending lines
to a file. Using diff to
compare a file with a section
of another: grep Linux file1 | diff file2 - Finally, a real-world example using
- with tar. Example 3-4. Backup of all files changed in last day #!/bin/bash
# Backs up all files in current directory modified within last 24 hours
#+ in a "tarball" (tarred and gzipped file).
BACKUPFILE=backup-$(date +%m-%d-%Y)
# Embeds date in backup filename.
# Thanks, Joshua Tschida, for the idea.
archive=${1:-$BACKUPFILE}
# If no backup-archive filename specified on command line,
#+ it will default to "backup-MM-DD-YYYY.tar.gz."
tar cvf - `find . -mtime -1 -type f -print` > $archive.tar
gzip $archive.tar
echo "Directory $PWD backed up in archive file \"$archive.tar.gz\"."
# Stephane Chazelas points out that the above code will fail
#+ if there are too many files found
#+ or if any filenames contain blank characters.
# He suggests the following alternatives:
# -------------------------------------------------------------------
# find . -mtime -1 -type f -print0 | xargs -0 tar rvf "$archive.tar"
# using the GNU version of "find".
# find . -mtime -1 -type f -exec tar rvf "$archive.tar" '{}' \;
# portable to other UNIX flavors, but much slower.
# -------------------------------------------------------------------
exit 0 |
 | Filenames beginning with
"-" may cause problems when coupled with the
"-" redirection operator. A script should
check for this and add an appropriate prefix to such
filenames, for example ./-FILENAME,
$PWD/-FILENAME, or
$PATHNAME/-FILENAME. If the value of a variable begins with a
-, this may likewise create
problems.
var="-n"
echo $var
# Has the effect of "echo -n", and outputs nothing. |
|
- -
 | Do not confuse the "-" used in this
sense with the "-" redirection
operator just discussed. The interpretation of the
"-" depends on the context in which it
appears. |
- -
- =
In a different context,
the "=" is a string comparison
operator. - +
In a different context,
the + is a Regular
Expression operator. - +
Certain commands and builtins use the
+ to enable certain options and the
- to disable them. - %
In a different context,
the % is a pattern
matching operator. - ~
- ~+
- ~-
- =~
- ^
- Control Characters
Control characters are not normally useful inside a
script. Ctl-B Backspace (nondestructive). Ctl-C Break. Terminate a foreground job.
Ctl-D Log out from a shell (similar to
exit). "EOF" (end of file). This also
terminates input from stdin. When typing text on the console or in an
xterm window,
Ctl-D erases the character under the
cursor. When there are no characters present,
Ctl-D logs out of the session, as
expected. In an xterm window, this has the effect of closing
the window. Ctl-G "BEL" (beep). On some old-time teletype
terminals, this would actually ring a bell. Ctl-H "Rubout" (destructive backspace). Erases
characters the cursor backs over while backspacing. #!/bin/bash
# Embedding Ctl-H in a string.
a="^H^H" # Two Ctl-H's (backspaces).
echo "abcdef" # abcdef
echo -n "abcdef$a " # abcd f
# Space at end ^ ^ Backspaces twice.
echo -n "abcdef$a" # abcdef
# No space at end Doesn't backspace (why?).
# Results may not be quite as expected.
echo; echo |
Ctl-I Horizontal tab. Ctl-J Newline (line feed). Ctl-K Vertical tab. When typing text on the console or in an
xterm window,
Ctl-K erases from the character
under the cursor to end of line. Ctl-L Formfeed (clear the terminal screen). This has
the same effect as the clear command. Ctl-M Carriage return. #!/bin/bash
# Thank you, Lee Maschmeyer, for this example.
read -n 1 -s -p $'Control-M leaves cursor at beginning of this line. Press Enter. \x0d'
# Of course, '0d' is the hex equivalent of Control-M.
echo >&2 # The '-s' makes anything typed silent,
#+ so it is necessary to go to new line explicitly.
read -n 1 -s -p $'Control-J leaves cursor on next line. \x0a'
echo >&2 # Control-J is linefeed.
###
read -n 1 -s -p $'And Control-K\x0bgoes straight down.'
echo >&2 # Control-K is vertical tab.
# A better example of the effect of a vertical tab is:
var=$'\x0aThis is the bottom line\x0bThis is the top line\x0a'
echo "$var"
# This works the same way as the above example. However:
echo "$var" | col
# This causes the right end of the line to be higher than the left end.
# It also explains why we started and ended with a line feed --
#+ to avoid a garbled screen.
# As Lee Maschmeyer explains:
# --------------------------
# In the [first vertical tab example] . . . the vertical tab
#+ makes the printing go straight down without a carriage return.
# This is true only on devices, such as the Linux console,
#+ that can't go "backward."
# The real purpose of VT is to go straight UP, not down.
# It can be used to print superscripts on a printer.
# The col utility can be used to emulate the proper behavior of VT.
exit 0 |
Ctl-Q Resume (XON). This resumes stdin in a terminal. Ctl-S Suspend (XOFF). This freezes stdin in a terminal.
(Use Ctl-Q to restore input.) Ctl-U Erase a line of input, from the cursor backward to
beginning of line. In some settings,
Ctl-U erases the entire
line of input, regardless of cursor
position. Ctl-V When inputting text, Ctl-V
permits inserting control characters. For example, the
following two are equivalent:
echo -e '\x0a'
echo <Ctl-V><Ctl-J> |
Ctl-V is primarily useful from
within a text editor. Ctl-W When typing text on the console or in an xterm window,
Ctl-W erases from the character
under the cursor backwards to the first instance of
whitespace. In some settings, Ctl-W
erases backwards to first non-alphanumeric character. Ctl-Z Pause a foreground job.
- Whitespace
Blank lines have no effect on the action of a script,
and are therefore useful for visually separating functional
sections. $IFS, the special variable
separating fields of input to certain commands, defaults
to whitespace. To preserve whitespace within a string or in a variable,
use quoting.
Chapter 4. Introduction to Variables and ParametersVariables are how programming and
scripting languages represent data. They appear in arithmetic
operations and manipulation of quantities, in string parsing, and
they are indispensable for working in the abstract with symbols --
tokens that represent something else. A variable is nothing more
than a label assigned to a location or set
of locations in computer memory holding an item of data.
4.1. Variable SubstitutionThe name of a variable is a placeholder for
its value, the data it holds. Referencing its
value is called variable substitution. - $
Let us carefully distinguish between the
name of a variable
and its value. If
variable1 is the name of a
variable, then $variable1
is a reference to its value,
the data item it contains. The only time a
variable appears "naked" -- without
the $ prefix -- is when declared
or assigned, when unset,
when exported,
or in the special case of a variable representing
a signal (see
Example 29-5). Assignment may be with an
= (as in var1=27),
in a read statement,
and at the head of a loop (for var2 in 1 2
3). Enclosing a referenced value in
double quotes (" ")
does not interfere with variable substitution. This is
called partial quoting, sometimes
referred to as "weak quoting." Using single quotes (' ')
causes the variable name to be used literally, and no
substitution will take place. This is full
quoting, sometimes referred to as "strong
quoting." See Chapter 5 for a
detailed discussion. Note that $variable is actually a
simplified alternate form of
${variable}. In contexts
where the $variable syntax
causes an error, the longer form may work (see Section 9.3, below). Example 4-1. Variable assignment and substitution #!/bin/bash
# Variables: assignment and substitution
a=375
hello=$a
#-------------------------------------------------------------------------
# No space permitted on either side of = sign when initializing variables.
# What happens if there is a space?
# If "VARIABLE =value",
# ^
#+ script tries to run "VARIABLE" command with one argument, "=value".
# If "VARIABLE= value",
# ^
#+ script tries to run "value" command with
#+ the environmental variable "VARIABLE" set to "".
#-------------------------------------------------------------------------
echo hello # Not a variable reference, just the string "hello".
echo $hello
echo ${hello} # Identical to above.
echo "$hello"
echo "${hello}"
echo
hello="A B C D"
echo $hello # A B C D
echo "$hello" # A B C D
# As you see, echo $hello and echo "$hello" give different results.
# ^ ^
# Quoting a variable preserves whitespace.
echo
echo '$hello' # $hello
# ^ ^
# Variable referencing disabled by single quotes,
#+ which causes the "$" to be interpreted literally.
# Notice the effect of different types of quoting.
hello= # Setting it to a null value.
echo "\$hello (null value) = $hello"
# Note that setting a variable to a null value is not the same as
#+ unsetting it, although the end result is the same (see below).
# --------------------------------------------------------------
# It is permissible to set multiple variables on the same line,
#+ if separated by white space.
# Caution, this may reduce legibility, and may not be portable.
var1=21 var2=22 var3=$V3
echo
echo "var1=$var1 var2=$var2 var3=$var3"
# May cause problems with older versions of "sh".
# --------------------------------------------------------------
echo; echo
numbers="one two three"
# ^ ^
other_numbers="1 2 3"
# ^ ^
# If there is whitespace embedded within a variable,
#+ then quotes are necessary.
echo "numbers = $numbers"
echo "other_numbers = $other_numbers" # other_numbers = 1 2 3
echo
echo "uninitialized_variable = $uninitialized_variable"
# Uninitialized variable has null value (no value at all).
uninitialized_variable= # Declaring, but not initializing it --
#+ same as setting it to a null value, as above.
echo "uninitialized_variable = $uninitialized_variable"
# It still has a null value.
uninitialized_variable=23 # Set it.
unset uninitialized_variable # Unset it.
echo "uninitialized_variable = $uninitialized_variable"
# It still has a null value.
echo
exit 0 |
 | An uninitialized variable has a
"null" value - no assigned value at all
(not zero!). Using a variable before assigning a value
to it will usually cause problems. It is nevertheless possible to perform arithmetic operations
on an uninitialized variable.
echo "$uninitialized" # (blank line)
let "uninitialized += 5" # Add 5 to it.
echo "$uninitialized" # 5
# Conclusion:
# An uninitialized variable has no value, however
#+ it acts as if it were 0 in an arithmetic operation.
# This is undocumented (and probably non-portable) behavior. |
See also Example 11-21. |
4.2. Variable Assignment- =
the assignment operator (no space before
and after)  | Do not confuse this with = and -eq, which test, rather than
assign! Note that = can be either an assignment
or a test operator, depending on context. |
Example 4-2. Plain Variable Assignment #!/bin/bash
# Naked variables
echo
# When is a variable "naked", i.e., lacking the '$' in front?
# When it is being assigned, rather than referenced.
# Assignment
a=879
echo "The value of \"a\" is $a."
# Assignment using 'let'
let a=16+5
echo "The value of \"a\" is now $a."
echo
# In a 'for' loop (really, a type of disguised assignment):
echo -n "Values of \"a\" in the loop are: "
for a in 7 8 9 11
do
echo -n "$a "
done
echo
echo
# In a 'read' statement (also a type of assignment):
echo -n "Enter \"a\" "
read a
echo "The value of \"a\" is now $a."
echo
exit 0 |
Example 4-3. Variable Assignment, plain and fancy #!/bin/bash
a=23 # Simple case
echo $a
b=$a
echo $b
# Now, getting a little bit fancier (command substitution).
a=`echo Hello!` # Assigns result of 'echo' command to 'a'
echo $a
# Note that including an exclamation mark (!) within a
#+ command substitution construct #+ will not work from the command line,
#+ since this triggers the Bash "history mechanism."
# Inside a script, however, the history functions are disabled.
a=`ls -l` # Assigns result of 'ls -l' command to 'a'
echo $a # Unquoted, however, removes tabs and newlines.
echo
echo "$a" # The quoted variable preserves whitespace.
# (See the chapter on "Quoting.")
exit 0 |
Variable assignment using the $(...)
mechanism (a newer method than backquotes). This is
actually a form of command
substitution. # From /etc/rc.d/rc.local
R=$(cat /etc/redhat-release)
arch=$(uname -m) |
4.3. Bash Variables Are Untyped
Unlike many other programming languages, Bash does not segregate
its variables by "type". Essentially, Bash
variables are character strings, but, depending on context, Bash
permits integer operations and comparisons on variables. The
determining factor is whether the value of a variable contains
only digits. Example 4-4. Integer or string? #!/bin/bash
# int-or-string.sh: Integer or string?
a=2334 # Integer.
let "a += 1"
echo "a = $a " # a = 2335
echo # Integer, still.
b=${a/23/BB} # Substitute "BB" for "23".
# This transforms $b into a string.
echo "b = $b" # b = BB35
declare -i b # Declaring it an integer doesn't help.
echo "b = $b" # b = BB35
let "b += 1" # BB35 + 1 =
echo "b = $b" # b = 1
echo
c=BB34
echo "c = $c" # c = BB34
d=${c/BB/23} # Substitute "23" for "BB".
# This makes $d an integer.
echo "d = $d" # d = 2334
let "d += 1" # 2334 + 1 =
echo "d = $d" # d = 2335
echo
# What about null variables?
e=""
echo "e = $e" # e =
let "e += 1" # Arithmetic operations allowed on a null variable?
echo "e = $e" # e = 1
echo # Null variable transformed into an integer.
# What about undeclared variables?
echo "f = $f" # f =
let "f += 1" # Arithmetic operations allowed?
echo "f = $f" # f = 1
echo # Undeclared variable transformed into an integer.
# Variables in Bash are essentially untyped.
exit 0 |
Untyped variables are both a blessing and a curse. They permit
more flexibility in scripting (enough rope to hang yourself!) and
make it easier to grind out lines of code. However, they permit
errors to creep in and encourage sloppy programming habits. The burden is on the programmer to keep track of what type the
script variables are. Bash will not do it for you.
4.4. Special Variable Types- local variables
variables visible only within a code block or function
(see also local variables
in functions) - environmental variables
variables that affect the behavior of the shell and
user interface  | In a more general context, each process has an
"environment", that is, a group of
variables that hold information that the process
may reference. In this sense, the shell behaves like
any other process. Every time a shell starts, it creates shell variables that
correspond to its own environmental variables. Updating
or adding new environmental variables causes the shell
to update its environment, and all the shell's child
processes (the commands it executes) inherit this
environment. |
 | The space allotted to the environment is limited.
Creating too many environmental variables or ones that use up
excessive space may cause problems. bash$ eval "`seq 10000 | sed -e 's/.*/export var&=ZZZZZZZZZZZZZZ/'`"
bash$ du
bash: /usr/bin/du: Argument list too long
|
(Thank you, Stéphane Chazelas for the clarification,
and for providing the above example.) |
If a script sets environmental variables, they need to be
"exported", that is, reported to the
environment local to the script. This is the function of
the export command.  | A script can export variables only
to child processes, that is, only to commands or processes
which that particular script initiates. A script invoked
from the command line cannot
export variables back to the command line environment.
Child processes cannot export
variables back to the parent processes that spawned
them. |
--- - positional parameters
arguments passed to the script from the command
line: $0, $1,
$2, $3 . . . $0 is the name of the script itself,
$1 is the first argument,
$2 the second, $3
the third, and so forth.
After $9, the arguments must be enclosed
in brackets, for example, ${10},
${11}, ${12}. The special variables $* and $@
denote all the positional parameters. Example 4-5. Positional Parameters #!/bin/bash
# Call this script with at least 10 parameters, for example
# ./scriptname 1 2 3 4 5 6 7 8 9 10
MINPARAMS=10
echo
echo "The name of this script is \"$0\"."
# Adds ./ for current directory
echo "The name of this script is \"`basename $0`\"."
# Strips out path name info (see 'basename')
echo
if [ -n "$1" ] # Tested variable is quoted.
then
echo "Parameter #1 is $1" # Need quotes to escape #
fi
if [ -n "$2" ]
then
echo "Parameter #2 is $2"
fi
if [ -n "$3" ]
then
echo "Parameter #3 is $3"
fi
# ...
if [ -n "${10}" ] # Parameters > $9 must be enclosed in {brackets}.
then
echo "Parameter #10 is ${10}"
fi
echo "-----------------------------------"
echo "All the command-line parameters are: "$*""
if [ $# -lt "$MINPARAMS" ]
then
echo
echo "This script needs at least $MINPARAMS command-line arguments!"
fi
echo
exit 0 |
Bracket notation for positional
parameters leads to a fairly simple way of referencing
the last argument passed to a
script on the command line. This also requires indirect referencing. args=$# # Number of args passed.
lastarg=${!args}
# Or: lastarg=${!#}
# (Thanks, Chris Monson.)
# Note that lastarg=${!$#} doesn't work. |
Some scripts can perform different operations,
depending on which name they are invoked with. For this
to work, the script needs to check $0,
the name it was invoked by. There must also exist symbolic
links to all the alternate names of the script. See Example 12-2.  | If a script expects a command line parameter
but is invoked without one, this may cause a null variable
assignment, generally an undesirable result. One way to prevent
this is to append an extra character to both sides of the
assignment statement using the expected positional parameter.
|
variable1_=$1_ # Rather than variable1=$1
# This will prevent an error, even if positional parameter is absent.
critical_argument01=$variable1_
# The extra character can be stripped off later, like so.
variable1=${variable1_/_/}
# Side effects only if $variable1_ begins with an underscore.
# This uses one of the parameter substitution templates discussed later.
# (Leaving out the replacement pattern results in a deletion.)
# A more straightforward way of dealing with this is
#+ to simply test whether expected positional parameters have been passed.
if [ -z $1 ]
then
exit $E_MISSING_POS_PARAM
fi
# However, as Fabian Kreutz points out,
#+ the above method may have unexpected side-effects.
# A better method is parameter substitution:
# ${1:-$DefaultVal}
# See the "Parameter Substition" section
#+ in the "Variables Revisited" chapter. |
--- Example 4-6. wh, whois domain name lookup #!/bin/bash
# ex18.sh
# Does a 'whois domain-name' lookup on any of 3 alternate servers:
# ripe.net, cw.net, radb.net
# Place this script -- renamed 'wh' -- in /usr/local/bin
# Requires symbolic links:
# ln -s /usr/local/bin/wh /usr/local/bin/wh-ripe
# ln -s /usr/local/bin/wh /usr/local/bin/wh-cw
# ln -s /usr/local/bin/wh /usr/local/bin/wh-radb
E_NOARGS=65
if [ -z "$1" ]
then
echo "Usage: `basename $0` [domain-name]"
exit $E_NOARGS
fi
# Check script name and call proper server.
case `basename $0` in # Or: case ${0##*/} in
"wh" ) whois $1@whois.ripe.net;;
"wh-ripe") whois $1@whois.ripe.net;;
"wh-radb") whois $1@whois.radb.net;;
"wh-cw" ) whois $1@whois.cw.net;;
* ) echo "Usage: `basename $0` [domain-name]";;
esac
exit $? |
---
The shift command reassigns the positional
parameters, in effect shifting them to the left one notch. $1 <--- $2, $2 <--- $3, $3 <--- $4, etc. The old $1 disappears, but
$0 (the script name)
does not change. If you use a large number of
positional parameters to a script, shift
lets you access those past 10, although
{bracket} notation
also permits this. Example 4-7. Using shift #!/bin/bash
# Using 'shift' to step through all the positional parameters.
# Name this script something like shft,
#+ and invoke it with some parameters, for example
# ./shft a b c def 23 skidoo
until [ -z "$1" ] # Until all parameters used up...
do
echo -n "$1 "
shift
done
echo # Extra line feed.
exit 0 |
Chapter 5. Quoting
Quoting means just that, bracketing a string in quotes. This
has the effect of protecting special
characters in the string from reinterpretation
or expansion by the shell or shell script. (A character is
"special" if it has an interpretation other than its
literal meaning, such as the wild card character --
*.) bash$ ls -l [Vv]*
-rw-rw-r-- 1 bozo bozo 324 Apr 2 15:05 VIEWDATA.BAT
-rw-rw-r-- 1 bozo bozo 507 May 4 14:25 vartrace.sh
-rw-rw-r-- 1 bozo bozo 539 Apr 14 17:11 viewdata.sh
bash$ ls -l '[Vv]*'
ls: [Vv]*: No such file or directory |
Certain programs and utilities reinterpret or expand
special characters in a quoted string. An important use of
quoting is protecting a command-line parameter from the shell,
but still letting the calling program expand it. bash$ grep '[Ff]irst' *.txt
file1.txt:This is the first line of file1.txt.
file2.txt:This is the First line of file2.txt. |
Note that the unquoted grep [Ff]irst *.txt
works under the Bash shell.
Quoting can also suppress echo's
"appetite" for newlines. bash$ echo $(ls -l)
total 8 -rw-rw-r-- 1 bozo bozo 130 Aug 21 12:57 t222.sh -rw-rw-r-- 1 bozo bozo 78 Aug 21 12:57 t71.sh
bash$ echo "$(ls -l)"
total 8
-rw-rw-r-- 1 bozo bozo 130 Aug 21 12:57 t222.sh
-rw-rw-r-- 1 bozo bozo 78 Aug 21 12:57 t71.sh |
5.1. Quoting VariablesWhen referencing a variable, it is generally advisable to
enclose its name in double quotes.
This prevents reinterpretation of all special characters within
the quoted string -- the variable name
-- except $, ` (backquote), and
\ (escape).
Keeping $ as a special character within
double quotes permits referencing a quoted variable
("$variable"), that is, replacing the
variable with its value (see Example 4-1, above). Use double quotes to prevent word splitting.
An argument enclosed in double quotes presents
itself as a single word, even if it contains whitespace separators.
variable1="a variable containing five words"
COMMAND This is $variable1 # Executes COMMAND with 7 arguments:
# "This" "is" "a" "variable" "containing" "five" "words"
COMMAND "This is $variable1" # Executes COMMAND with 1 argument:
# "This is a variable containing five words"
variable2="" # Empty.
COMMAND $variable2 $variable2 $variable2 # Executes COMMAND with no arguments.
COMMAND "$variable2" "$variable2" "$variable2" # Executes COMMAND with 3 empty arguments.
COMMAND "$variable2 $variable2 $variable2" # Executes COMMAND with 1 argument (2 spaces).
# Thanks, Stéphane Chazelas. |
 | Enclosing the arguments to an echo
statement in double quotes is necessary only when word splitting
or preservation of whitespace
is an issue. |
Example 5-1. Echoing Weird Variables #!/bin/bash
# weirdvars.sh: Echoing weird variables.
var="'(]\\{}\$\""
echo $var # '(]\{}$"
echo "$var" # '(]\{}$" Doesn't make a difference.
echo
IFS='\'
echo $var # '(] {}$" \ converted to space. Why?
echo "$var" # '(]\{}$"
# Examples above supplied by Stephane Chazelas.
exit 0 |
Single quotes (' ') operate similarly to double
quotes, but do not permit referencing variables, since
the special meaning of $ is turned off.
Within single quotes, every special
character except ' gets interpreted literally.
Consider single quotes ("full quoting") to be a
stricter method of quoting than double quotes ("partial
quoting").  | Since even the escape character (\)
gets a literal interpretation within single quotes, trying to
enclose a single quote within single quotes will not yield the
expected result.
echo "Why can't I write 's between single quotes"
echo
# The roundabout method.
echo 'Why can'\''t I write '"'"'s between single quotes'
# |-------| |----------| |-----------------------|
# Three single-quoted strings, with escaped and quoted single quotes between.
# This example courtesy of Stéphane Chazelas. |
|
5.2. EscapingEscaping is a method
of quoting single characters. The escape
(\) preceding a character tells the shell to
interpret that character literally.  | With certain commands and utilities, such as echo and sed, escaping a character may have the
opposite effect - it can toggle on a special meaning for that
character. |
Special meanings of certain
escaped characters - used with echo and
sed
- \n
means newline - \r
means return - \t
means tab - \v
means vertical tab - \b
means backspace - \a
means "alert" (beep or flash) - \0xx
translates to the octal ASCII
equivalent of 0xx Example 5-2. Escaped Characters #!/bin/bash
# escaped.sh: escaped characters
echo; echo
echo "\v\v\v\v" # Prints \v\v\v\v literally.
# Use the -e option with 'echo' to print escaped characters.
echo "============="
echo "VERTICAL TABS"
echo -e "\v\v\v\v" # Prints 4 vertical tabs.
echo "=============="
echo "QUOTATION MARK"
echo -e "\042" # Prints " (quote, octal ASCII character 42).
echo "=============="
# The $'\X' construct makes the -e option unnecessary.
echo; echo "NEWLINE AND BEEP"
echo $'\n' # Newline.
echo $'\a' # Alert (beep).
echo "==============="
echo "QUOTATION MARKS"
# Version 2 and later of Bash permits using the $'\nnn' construct.
# Note that in this case, '\nnn' is an octal value.
echo $'\t \042 \t' # Quote (") framed by tabs.
# It also works with hexadecimal values, in an $'\xhhh' construct.
echo $'\t \x22 \t' # Quote (") framed by tabs.
# Thank you, Greg Keraunen, for pointing this out.
# Earlier Bash versions allowed '\x022'.
echo "==============="
echo
# Assigning ASCII characters to a variable.
# ----------------------------------------
quote=$'\042' # " assigned to a variable.
echo "$quote This is a quoted string, $quote and this lies outside the quotes."
echo
# Concatenating ASCII chars in a variable.
triple_underline=$'\137\137\137' # 137 is octal ASCII code for '_'.
echo "$triple_underline UNDERLINE $triple_underline"
echo
ABC=$'\101\102\103\010' # 101, 102, 103 are octal A, B, C.
echo $ABC
echo; echo
escape=$'\033' # 033 is octal for escape.
echo "\"escape\" echoes as $escape"
# no visible output.
echo; echo
exit 0 |
See Example 34-1 for another example of the
$' ' string expansion
construct. - \"
gives the quote its literal meaning echo "Hello" # Hello
echo "\"Hello\", he said." # "Hello", he said. |
- \$
gives the dollar sign its literal meaning
(variable name following \$ will not be
referenced) echo "\$variable01" # results in $variable01 |
- \\
gives the backslash its literal meaning echo "\\" # Results in \
# Whereas . . .
echo "\" # Invokes secondary prompt from the command line.
# In a script, gives an error message. |
 | The behavior of \ depends on whether
it is itself escaped, quoted, or appearing within command substitution or a here document.
# Simple escaping and quoting
echo \z # z
echo \\z # \z
echo '\z' # \z
echo '\\z' # \\z
echo "\z" # \z
echo "\\z" # \z
# Command substitution
echo `echo \z` # z
echo `echo \\z` # z
echo `echo \\\z` # \z
echo `echo \\\\z` # \z
echo `echo \\\\\\z` # \z
echo `echo \\\\\\\z` # \\z
echo `echo "\z"` # \z
echo `echo "\\z"` # \z
# Here document
cat <<EOF
\z
EOF # \z
cat <<EOF
\\z
EOF # \z
# These examples supplied by Stéphane Chazelas. |
Elements of a string assigned to a variable may be escaped, but
the escape character alone may not be assigned to a variable.
variable=\
echo "$variable"
# Will not work - gives an error message:
# test.sh: : command not found
# A "naked" escape cannot safely be assigned to a variable.
#
# What actually happens here is that the "\" escapes the newline and
#+ the effect is variable=echo "$variable"
#+ invalid variable assignment
variable=\
23skidoo
echo "$variable" # 23skidoo
# This works, since the second line
#+ is a valid variable assignment.
variable=\
# \^ escape followed by space
echo "$variable" # space
variable=\\
echo "$variable" # \
variable=\\\
echo "$variable"
# Will not work - gives an error message:
# test.sh: \: command not found
#
# First escape escapes second one, but the third one is left "naked",
#+ with same result as first instance, above.
variable=\\\\
echo "$variable" # \\
# Second and fourth escapes escaped.
# This is o.k. |
|
Escaping a space can prevent word splitting in a command's argument list.
file_list="/bin/cat /bin/gzip /bin/more /usr/bin/less /usr/bin/emacs-20.7"
# List of files as argument(s) to a command.
# Add two files to the list, and list all.
ls -l /usr/X11R6/bin/xsetroot /sbin/dump $file_list
echo "-------------------------------------------------------------------------"
# What happens if we escape a couple of spaces?
ls -l /usr/X11R6/bin/xsetroot\ /sbin/dump\ $file_list
# Error: the first three files concatenated into a single argument to 'ls -l'
# because the two escaped spaces prevent argument (word) splitting. |
The escape also provides a means of writing a
multi-line command. Normally, each separate line constitutes
a different command, but an escape at the end
of a line escapes the newline character,
and the command sequence continues on to the next line. (cd /source/directory && tar cf - . ) | \
(cd /dest/directory && tar xpvf -)
# Repeating Alan Cox's directory tree copy command,
# but split into two lines for increased legibility.
# As an alternative:
tar cf - -C /source/directory . |
tar xpvf - -C /dest/directory
# See note below.
# (Thanks, Stéphane Chazelas.) |
 | If a script line ends with a |, a pipe
character, then a \, an escape, is not strictly
necessary. It is, however, good programming practice to always
escape the end of a line of code that continues to the
following line. |
echo "foo
bar"
#foo
#bar
echo
echo 'foo
bar' # No difference yet.
#foo
#bar
echo
echo foo\
bar # Newline escaped.
#foobar
echo
echo "foo\
bar" # Same here, as \ still interpreted as escape within weak quotes.
#foobar
echo
echo 'foo\
bar' # Escape character \ taken literally because of strong quoting.
#foo\
#bar
# Examples suggested by Stéphane Chazelas. |
Chapter 6. Exit and Exit Status | ...there are dark corners in the Bourne shell, and people use all
of them. | | Chet Ramey |
The
exit
command may be used to terminate a script, just as in a
C program. It can also return a value,
which is available to the script's parent process. Every command returns an
exit status
(sometimes referred to as a
return status
). A successful command returns a
0, while an unsuccessful one returns
a non-zero value that usually may
be interpreted as an error code. Well-behaved UNIX commands,
programs, and utilities return a 0
exit code upon successful completion, though there are some
exceptions. Likewise, functions within a script and the script itself
return an exit status. The last command executed in the function
or script determines the exit status. Within a script, an
exit nnn
command may be used to deliver an
nnn exit status
to the shell (nnn
must be a decimal number in the 0 -
255 range).  | When a script ends with an exit that has
no parameter, the exit status of the script is the exit status
of the last command executed in the script (previous to the
exit). #!/bin/bash
COMMAND_1
. . .
# Will exit with status of last command.
COMMAND_LAST
exit |
The equivalent of a bare exit is
exit $? or even just omitting the
exit. #!/bin/bash
COMMAND_1
. . .
# Will exit with status of last command.
COMMAND_LAST
exit $? |
#!/bin/bash
COMMAND1
. . .
# Will exit with status of last command.
COMMAND_LAST |
|
$? reads the exit status of the last
command executed. After a function returns,
$? gives the exit status of the last
command executed in the function. This is Bash's way of
giving functions a "return value." After a
script terminates, a $? from the command
line gives the exit status of the script, that is, the last
command executed in the script, which is, by convention,
0 on success or an integer in the
range 1 - 255 on error. Example 6-1. exit / exit status #!/bin/bash
echo hello
echo $? # Exit status 0 returned because command executed successfully.
lskdf # Unrecognized command.
echo $? # Non-zero exit status returned because command failed to execute.
echo
exit 113 # Will return 113 to shell.
# To verify this, type "echo $?" after script terminates.
# By convention, an 'exit 0' indicates success,
#+ while a non-zero exit value means an error or anomalous condition. |
$? is especially useful
for testing the result of a command in a script (see Example 12-32 and Example 12-17).  | The !, the logical
"not" qualifier, reverses the outcome of a test or
command, and this affects its exit
status.
Example 6-2. Negating a condition using ! true # the "true" builtin.
echo "exit status of \"true\" = $?" # 0
! true
echo "exit status of \"! true\" = $?" # 1
# Note that the "!" needs a space.
# !true leads to a "command not found" error
#
# The '!' operator prefixing a command invokes the Bash history mechanism.
true
!true
# No error this time, but no negation either.
# It just repeats the previous command (true).
# Thanks, Stéphane Chazelas and Kristopher Newsome. |
|
 | Certain exit status codes have reserved meanings and should not
be user-specified in a script. |
Chapter 7. Tests
Every reasonably complete programming language can test
for a condition, then act according to the result of
the test. Bash has the test command,
various bracket and parenthesis operators, and the
if/then construct.
7.1. Test Constructs
An if/then construct tests whether the
exit status of a list
of commands is 0 (since 0 means
"success" by UNIX convention), and if so, executes
one or more commands. There exists a dedicated command called [ (left bracket
special character). It is a synonym for test,
and a builtin for efficiency
reasons. This command considers its arguments as comparison
expressions or file tests and returns an exit status corresponding
to the result of the comparison (0 for true, 1 for false). With version 2.02, Bash introduced the [[ ... ]] extended
test command, which performs comparisons
in a manner more familiar to programmers from other
languages. Note that [[ is a keyword, not a command. Bash sees [[ $a -lt $b ]] as a
single element, which returns an exit status. The (( ... )) and let ... constructs also return an
exit status of 0 if the arithmetic
expressions they evaluate expand to a non-zero value. These
arithmetic expansion
constructs may therefore be used to perform arithmetic
comparisons.
let "1<2" returns 0 (as "1<2" expands to "1")
(( 0 && 1 )) returns 1 (as "0 && 1" expands to "0") |
An if can test any command, not just
conditions enclosed within brackets.
if cmp a b &> /dev/null # Suppress output.
then echo "Files a and b are identical."
else echo "Files a and b differ."
fi
# The very useful "if-grep" construct:
# -----------------------------------
if grep -q Bash file
then echo "File contains at least one occurrence of Bash."
fi
word=Linux
letter_sequence=inu
if echo "$word" | grep -q "$letter_sequence"
# The "-q" option to grep suppresses output.
then
echo "$letter_sequence found in $word"
else
echo "$letter_sequence not found in $word"
fi
if COMMAND_WHOSE_EXIT_STATUS_IS_0_UNLESS_ERROR_OCCURRED
then echo "Command succeeded."
else echo "Command failed."
fi |
An if/then construct can contain nested
comparisons and tests.
if echo "Next *if* is part of the comparison for the first *if*."
if [[ $comparison = "integer" ]]
then (( a < b ))
else
[[ $a < $b ]]
fi
then
echo '$a is less than $b'
fi |
This detailed "if-test" explanation
courtesy of Stéphane Chazelas.
Example 7-1. What is truth? #!/bin/bash
# Tip:
# If you're unsure of how a certain condition would evaluate,
#+ test it in an if-test.
echo
echo "Testing \"0\""
if [ 0 ] # zero
then
echo "0 is true."
else
echo "0 is false."
fi # 0 is true.
echo
echo "Testing \"1\""
if [ 1 ] # one
then
echo "1 is true."
else
echo "1 is false."
fi # 1 is true.
echo
echo "Testing \"-1\""
if [ -1 ] # minus one
then
echo "-1 is true."
else
echo "-1 is false."
fi # -1 is true.
echo
echo "Testing \"NULL\""
if [ ] # NULL (empty condition)
then
echo "NULL is true."
else
echo "NULL is false."
fi # NULL is false.
echo
echo "Testing \"xyz\""
if [ xyz ] # string
then
echo "Random string is true."
else
echo "Random string is false."
fi # Random string is true.
echo
echo "Testing \"\$xyz\""
if [ $xyz ] # Tests if $xyz is null, but...
# it's only an uninitialized variable.
then
echo "Uninitialized variable is true."
else
echo "Uninitialized variable is false."
fi # Uninitialized variable is false.
echo
echo "Testing \"-n \$xyz\""
if [ -n "$xyz" ] # More pedantically correct.
then
echo "Uninitialized variable is true."
else
echo "Uninitialized variable is false."
fi # Uninitialized variable is false.
echo
xyz= # Initialized, but set to null value.
echo "Testing \"-n \$xyz\""
if [ -n "$xyz" ]
then
echo "Null variable is true."
else
echo "Null variable is false."
fi # Null variable is false.
echo
# When is "false" true?
echo "Testing \"false\""
if [ "false" ] # It seems that "false" is just a string.
then
echo "\"false\" is true." #+ and it tests true.
else
echo "\"false\" is false."
fi # "false" is true.
echo
echo "Testing \"\$false\"" # Again, uninitialized variable.
if [ "$false" ]
then
echo "\"\$false\" is true."
else
echo "\"\$false\" is false."
fi # "$false" is false.
# Now, we get the expected result.
# What would happen if we tested the uninitialized variable "$true"?
echo
exit 0 |
if [ condition-true ]
then
command 1
command 2
...
else
# Optional (may be left out if not needed).
# Adds default code block executing if original condition tests false.
command 3
command 4
...
fi |
 | When if and then
are on same line in a condition test, a semicolon must
terminate the if statement. Both
if and then are keywords. Keywords (or commands)
begin statements, and before a new statement on the same line
begins, the old one must terminate. if [ -x "$filename" ]; then |
|
Else if and elif - elif
elif is a contraction
for else if. The effect is to nest an
inner if/then construct within an outer
one. if [ condition1 ]
then
command1
command2
command3
elif [ condition2 ]
# Same as else if
then
command4
command5
else
default-command
fi |
The if test condition-true construct is the
exact equivalent of if [ condition-true ].
As it happens, the left bracket, [ , is a token
which invokes the test command. The closing
right bracket, ] , in an if/test should not
therefore be strictly necessary, however newer versions of Bash
require it.  | The test command is a Bash builtin which tests file
types and compares strings. Therefore, in a Bash script,
test does not call
the external /usr/bin/test binary,
which is part of the sh-utils
package. Likewise, [ does not call
/usr/bin/[, which is linked to
/usr/bin/test. bash$ type test
test is a shell builtin
bash$ type '['
[ is a shell builtin
bash$ type '[['
[[ is a shell keyword
bash$ type ']]'
]] is a shell keyword
bash$ type ']'
bash: type: ]: not found
|
|
Example 7-2. Equivalence of test,
/usr/bin/test, [ ],
and /usr/bin/[ #!/bin/bash
echo
if test -z "$1"
then
echo "No command-line arguments."
else
echo "First command-line argument is $1."
fi
echo
if /usr/bin/test -z "$1" # Same result as "test" builtin".
then
echo "No command-line arguments."
else
echo "First command-line argument is $1."
fi
echo
if [ -z "$1" ] # Functionally identical to above code blocks.
# if [ -z "$1" should work, but...
#+ Bash responds to a missing close-bracket with an error message.
then
echo "No command-line arguments."
else
echo "First command-line argument is $1."
fi
echo
if /usr/bin/[ -z "$1" ] # Again, functionally identical to above.
# if /usr/bin/[ -z "$1" # Works, but gives an error message.
# # Note:
# This has been fixed in Bash, version 3.x.
then
echo "No command-line arguments."
else
echo "First command-line argument is $1."
fi
echo
exit 0 |
The [[ ]] construct
is the more versatile Bash version of [ ]. This
is the extended test command, adopted from
ksh88.  | No filename expansion or word splitting takes place
between [[ and ]], but there is
parameter expansion and command substitution. |
file=/etc/passwd
if [[ -e $file ]]
then
echo "Password file exists."
fi |
 | Using the [[ ... ]] test construct,
rather than [ ... ] can prevent many
logic errors in scripts. For example, the &&,
||, <, and >
operators work within a [[ ]] test, despite
giving an error within a [ ] construct. |
 | Following an if, neither the
test command nor the test brackets ( [ ] or [[ ]] )
are strictly necessary.
dir=/home/bozo
if cd "$dir" 2>/dev/null; then # "2>/dev/null" hides error message.
echo "Now in $dir."
else
echo "Can't change to $dir."
fi |
The "if COMMAND" construct returns the exit status of COMMAND.Similarly, a condition within test brackets may stand alone
without an if, when used in combination
with a list construct.
var1=20
var2=22
[ "$var1" -ne "$var2" ] && echo "$var1 is not equal to $var2"
home=/home/bozo
[ -d "$home" ] || echo "$home directory does not exist." |
|
The (( )) construct expands
and evaluates an arithmetic expression. If
the expression evaluates as zero, it returns an
exit status of
1, or "false". A non-zero
expression returns an exit status of 0,
or "true". This is in marked contrast to using
the test and [ ] constructs
previously discussed. Example 7-3. Arithmetic Tests using (( )) #!/bin/bash
# Arithmetic tests.
# The (( ... )) construct evaluates and tests numerical expressions.
# Exit status opposite from [ ... ] construct!
(( 0 ))
echo "Exit status of \"(( 0 ))\" is $?." # 1
(( 1 ))
echo "Exit status of \"(( 1 ))\" is $?." # 0
(( 5 > 4 )) # true
echo "Exit status of \"(( 5 > 4 ))\" is $?." # 0
(( 5 > 9 )) # false
echo "Exit status of \"(( 5 > 9 ))\" is $?." # 1
(( 5 - 5 )) # 0
echo "Exit status of \"(( 5 - 5 ))\" is $?." # 1
(( 5 / 4 )) # Division o.k.
echo "Exit status of \"(( 5 / 4 ))\" is $?." # 0
(( 1 / 2 )) # Division result < 1.
echo "Exit status of \"(( 1 / 2 ))\" is $?." # Rounded off to 0.
# 1
(( 1 / 0 )) 2>/dev/null # Illegal division by 0.
# ^^^^^^^^^^^
echo "Exit status of \"(( 1 / 0 ))\" is $?." # 1
# What effect does the "2>/dev/null" have?
# What would happen if it were removed?
# Try removing it, then rerunning the script.
exit 0 |
7.2. File test operatorsReturns true if... - -e
file exists - -a
file exists This is identical in effect to -e.
It has been "deprecated," and its use is
discouraged. - -f
file is a regular
file (not a directory or device file) - -s
file is not zero size - -d
file is a directory - -b
file is a block device (floppy, cdrom, etc.)
- -c
file is a character device (keyboard, modem, sound
card, etc.) - -p
file is a pipe - -h
file is a symbolic
link - -L
file is a symbolic link - -S
file is a socket - -t
file (descriptor) is
associated with a terminal device This test option may be used to check whether the
stdin ([ -t 0 ])
or stdout ([ -t 1 ])
in a given script is a terminal. - -r
file has read permission (for the
user running the test) - -w
file has write permission (for the user running
the test) - -x
file has execute permission (for the user running
the test) - -g
set-group-id (sgid) flag set on file or directory If a directory has the sgid
flag set, then a file created within that directory belongs
to the group that owns the directory, not necessarily to
the group of the user who created the file. This may be
useful for a directory shared by a workgroup. - -u
set-user-id (suid) flag set on file A binary owned by root
with set-user-id flag set
runs with root privileges, even
when an ordinary user invokes it.
This is useful for executables (such as
pppd and cdrecord)
that need to access system hardware. Lacking the
suid flag, these binaries could not
be invoked by a non-root user.
-rwsr-xr-t 1 root 178236 Oct 2 2000 /usr/sbin/pppd
|
A file with the suid flag set shows
an s in its permissions.- -k
sticky bit set Commonly known as the "sticky bit," the
save-text-mode flag is a special
type of file permission. If a file has this flag set,
that file will be kept in cache memory, for quicker access.
If set on a directory, it restricts write permission.
Setting the sticky bit adds a t
to the permissions on the file or directory listing.
drwxrwxrwt 7 root 1024 May 19 21:26 tmp/
|
If a user does not own a directory that has the sticky
bit set, but has write permission in that directory,
he can only delete files in it that he owns. This keeps
users from inadvertently overwriting or deleting each
other's files in a publicly accessible directory, such
as /tmp.
(The owner of the directory or root
can, of course, delete or rename files there.)- -O
you are owner of file - -G
group-id of file same as yours - -N
file modified since it was last read - f1 -nt f2
file f1 is newer than
f2 - f1 -ot f2
file f1 is older than
f2 - f1 -ef f2
files f1 and
f2 are hard links to the same
file - !
"not" -- reverses the sense of the
tests above (returns true if condition absent).
Example 7-4. Testing for broken links #!/bin/bash
# broken-link.sh
# Written by Lee bigelow <ligelowbee@yahoo.com>
# Used with permission.
#A pure shell script to find dead symlinks and output them quoted
#so they can be fed to xargs and dealt with :)
#eg. broken-link.sh /somedir /someotherdir|xargs rm
#
#This, however, is a better method:
#
#find "somedir" -type l -print0|\
#xargs -r0 file|\
#grep "broken symbolic"|
#sed -e 's/^\|: *broken symbolic.*$/"/g'
#
#but that wouldn't be pure bash, now would it.
#Caution: beware the /proc file system and any circular links!
##############################################################
#If no args are passed to the script set directorys to search
#to current directory. Otherwise set the directorys to search
#to the agrs passed.
####################
[ $# -eq 0 ] && directorys=`pwd` || directorys=$@
#Setup the function linkchk to check the directory it is passed
#for files that are links and don't exist, then print them quoted.
#If one of the elements in the directory is a subdirectory then
#send that send that subdirectory to the linkcheck function.
##########
linkchk () {
for element in $1/*; do
[ -h "$element" -a ! -e "$element" ] && echo \"$element\"
[ -d "$element" ] && linkchk $element
# Of course, '-h' tests for symbolic link, '-d' for directory.
done
}
#Send each arg that was passed to the script to the linkchk function
#if it is a valid directoy. If not, then print the error message
#and usage info.
################
for directory in $directorys; do
if [ -d $directory ]
then linkchk $directory
else
echo "$directory is not a directory"
echo "Usage: $0 dir1 dir2 ..."
fi
done
exit 0 |
Example 28-1, Example 10-7,
Example 10-3, Example 28-3, and Example A-1 also illustrate uses of the file test
operators.
7.3. Other Comparison OperatorsA binary comparison operator compares two
variables or quantities. Note the separation between integer and
string comparison. integer comparison - -eq
is equal to if [ "$a" -eq "$b" ] - -ne
is not equal to if [ "$a" -ne "$b" ] - -gt
is greater than if [ "$a" -gt "$b" ] - -ge
is greater than or equal to if [ "$a" -ge "$b" ] - -lt
is less than if [ "$a" -lt "$b" ] - -le
is less than or equal to if [ "$a" -le "$b" ] - <
is less than (within double
parentheses) (("$a" < "$b")) - <=
is less than or equal to (within double parentheses) (("$a" <= "$b")) - >
is greater than (within double parentheses) (("$a" > "$b")) - >=
is greater than or equal to (within double parentheses) (("$a" >= "$b"))
string comparison - =
is equal to if [ "$a" = "$b" ] - ==
is equal to if [ "$a" == "$b" ] This is a synonym for =.  | The == comparison operator behaves differently
within a double-brackets
test than within single brackets.
[[ $a == z* ]] # True if $a starts with an "z" (pattern matching).
[[ $a == "z*" ]] # True if $a is equal to z* (literal matching).
[ $a == z* ] # File globbing and word splitting take place.
[ "$a" == "z*" ] # True if $a is equal to z* (literal matching).
# Thanks, Stéphane Chazelas |
|
- !=
is not equal to if [ "$a" != "$b" ] This operator uses pattern matching within a [[ ... ]] construct. - <
is less than, in ASCII alphabetical order if [[ "$a" < "$b" ]] if [ "$a" \< "$b" ] Note that the "<" needs to be
escaped within a [ ]
construct. - >
is greater than, in ASCII alphabetical order if [[ "$a" > "$b" ]] if [ "$a" \> "$b" ] Note that the ">" needs to be
escaped within a [ ] construct. See Example 26-11 for an application of this
comparison operator. - -z
string is "null", that is, has zero length - -n
string is not "null".  | The -n test absolutely
requires that the string be quoted within the
test brackets. Using an unquoted string with
! -z, or even just the
unquoted string alone within test brackets (see Example 7-6) normally works, however, this is
an unsafe practice. Always quote
a tested string.
|
Example 7-5. Arithmetic and string comparisons #!/bin/bash
a=4
b=5
# Here "a" and "b" can be treated either as integers or strings.
# There is some blurring between the arithmetic and string comparisons,
#+ since Bash variables are not strongly typed.
# Bash permits integer operations and comparisons on variables
#+ whose value consists of all-integer characters.
# Caution advised, however.
echo
if [ "$a" -ne "$b" ]
then
echo "$a is not equal to $b"
echo "(arithmetic comparison)"
fi
echo
if [ "$a" != "$b" ]
then
echo "$a is not equal to $b."
echo "(string comparison)"
# "4" != "5"
# ASCII 52 != ASCII 53
fi
# In this particular instance, both "-ne" and "!=" work.
echo
exit 0 |
Example 7-6. Testing whether a string is null #!/bin/bash
# str-test.sh: Testing null strings and unquoted strings,
#+ but not strings and sealing wax, not to mention cabbages and kings . . .
# Using if [ ... ]
# If a string has not been initialized, it has no defined value.
# This state is called "null" (not the same as zero).
if [ -n $string1 ] # $string1 has not been declared or initialized.
then
echo "String \"string1\" is not null."
else
echo "String \"string1\" is null."
fi
# Wrong result.
# Shows $string1 as not null, although it was not initialized.
echo
# Lets try it again.
if [ -n "$string1" ] # This time, $string1 is quoted.
then
echo "String \"string1\" is not null."
else
echo "String \"string1\" is null."
fi # Quote strings within test brackets!
echo
if [ $string1 ] # This time, $string1 stands naked.
then
echo "String \"string1\" is not null."
else
echo "String \"string1\" is null."
fi
# This works fine.
# The [ ] test operator alone detects whether the string is null.
# However it is good practice to quote it ("$string1").
#
# As Stephane Chazelas points out,
# if [ $string1 ] has one argument, "]"
# if [ "$string1" ] has two arguments, the empty "$string1" and "]"
echo
string1=initialized
if [ $string1 ] # Again, $string1 stands naked.
then
echo "String \"string1\" is not null."
else
echo "String \"string1\" is null."
fi
# Again, gives correct result.
# Still, it is better to quote it ("$string1"), because . . .
string1="a = b"
if [ $string1 ] # Again, $string1 stands naked.
then
echo "String \"string1\" is not null."
else
echo "String \"string1\" is null."
fi
# Not quoting "$string1" now gives wrong result!
exit 0
# Thank you, also, Florian Wisser, for the "heads-up". |
Example 7-7. zmore #!/bin/bash
# zmore
#View gzipped files with 'more'
NOARGS=65
NOTFOUND=66
NOTGZIP=67
if [ $# -eq 0 ] # same effect as: if [ -z "$1" ]
# $1 can exist, but be empty: zmore "" arg2 arg3
then
echo "Usage: `basename $0` filename" >&2
# Error message to stderr.
exit $NOARGS
# Returns 65 as exit status of script (error code).
fi
filename=$1
if [ ! -f "$filename" ] # Quoting $filename allows for possible spaces.
then
echo "File $filename not found!" >&2
# Error message to stderr.
exit $NOTFOUND
fi
if [ ${filename##*.} != "gz" ]
# Using bracket in variable substitution.
then
echo "File $1 is not a gzipped file!"
exit $NOTGZIP
fi
zcat $1 | more
# Uses the filter 'more.'
# May substitute 'less', if desired.
exit $? # Script returns exit status of pipe.
# Actually "exit $?" is unnecessary, as the script will, in any case,
# return the exit status of the last command executed. |
compound comparison - -a
logical and exp1 -a exp2 returns true if
both exp1 and exp2 are true. - -o
logical or exp1 -o exp2 returns
true if either exp1 or exp2 are
true.
These are similar to the Bash comparison operators
&& and ||, used
within double brackets.
[[ condition1 && condition2 ]] |
The -o and -a operators
work with the test command or occur within
single test brackets.
if [ "$exp1" -a "$exp2" ] |
Refer to Example 8-3, Example 26-16,
and Example A-28 to see compound comparison operators
in action.
7.4. Nested if/then Condition TestsCondition tests using the if/then
construct may be nested. The net result is identical to using the
&& compound comparison operator above. if [ condition1 ]
then
if [ condition2 ]
then
do-something # But only if both "condition1" and "condition2" valid.
fi
fi |
See Example 34-4 for an example of nested
if/then condition tests.
7.5. Testing Your Knowledge of TestsThe systemwide xinitrc file can be used
to launch the X server. This file contains quite a number of
if/then tests, as the following excerpt
shows. if [ -f $HOME/.Xclients ]; then
exec $HOME/.Xclients
elif [ -f /etc/X11/xinit/Xclients ]; then
exec /etc/X11/xinit/Xclients
else
# failsafe settings. Although we should never get here
# (we provide fallbacks in Xclients as well) it can't hurt.
xclock -geometry 100x100-5+5 &
xterm -geometry 80x50-50+150 &
if [ -f /usr/bin/netscape -a -f /usr/share/doc/HTML/index.html ]; then
netscape /usr/share/doc/HTML/index.html &
fi
fi |
Explain the "test" constructs in the above excerpt,
then examine the entire file, /etc/X11/xinit/xinitrc,
and analyze the if/then test constructs
there. You may need to refer ahead to the discussions of grep, sed,
and regular expressions.
Chapter 8. Operations and Related Topics8.1. Operatorsassignment - variable assignment
Initializing or changing the value of a variable - =
All-purpose assignment operator, which works for both
arithmetic and string assignments. var=27
category=minerals # No spaces allowed after the "=". |
 | Do not confuse the "=" assignment
operator with the = test
operator. # = as a test operator
if [ "$string1" = "$string2" ]
# if [ "X$string1" = "X$string2" ] is safer,
# to prevent an error message should one of the variables be empty.
# (The prepended "X" characters cancel out.)
then
command
fi |
|
arithmetic operators - +
plus - -
minus - *
multiplication - /
division - **
exponentiation
# Bash, version 2.02, introduced the "**" exponentiation operator.
let "z=5**3"
echo "z = $z" # z = 125 |
- %
modulo, or mod (returns the
remainder of an integer division
operation)
5/3 = 1 with remainder 2
This operator finds use in, among other things,
generating numbers within a specific range (see Example 9-24 and Example 9-27) and
formatting program output (see Example 26-15 and
Example A-6). It can even be used to generate
prime numbers, (see Example A-16). Modulo turns
up surprisingly often in various numerical recipes. Example 8-1. Greatest common divisor #!/bin/bash
# gcd.sh: greatest common divisor
# Uses Euclid's algorithm
# The "greatest common divisor" (gcd) of two integers
#+ is the largest integer that will divide both, leaving no remainder.
# Euclid's algorithm uses successive division.
# In each pass,
#+ dividend <--- divisor
#+ divisor <--- remainder
#+ until remainder = 0.
#+ The gcd = dividend, on the final pass.
#
# For an excellent discussion of Euclid's algorithm, see
# Jim Loy's site, http://www.jimloy.com/number/euclids.htm.
# ------------------------------------------------------
# Argument check
ARGS=2
E_BADARGS=65
if [ $# -ne "$ARGS" ]
then
echo "Usage: `basename $0` first-number second-number"
exit $E_BADARGS
fi
# ------------------------------------------------------
gcd ()
{
dividend=$1 # Arbitrary assignment.
divisor=$2 #+ It doesn't matter which of the two is larger.
# Why not?
remainder=1 # If uninitialized variable used in loop,
#+ it results in an error message
#+ on the first pass through loop.
until [ "$remainder" -eq 0 ]
do
let "remainder = $dividend % $divisor"
dividend=$divisor # Now repeat with 2 smallest numbers.
divisor=$remainder
done # Euclid's algorithm
} # Last $dividend is the gcd.
gcd $1 $2
echo; echo "GCD of $1 and $2 = $dividend"; echo
# Exercise :
# --------
# Check command-line arguments to make sure they are integers,
#+ and exit the script with an appropriate error message if not.
exit 0 |
- +=
"plus-equal" (increment variable by a constant) let "var += 5" results in
var being incremented by
5. - -=
"minus-equal" (decrement variable by a constant) - *=
"times-equal" (multiply variable by a constant) let "var *= 4" results in var
being multiplied by 4. - /=
"slash-equal" (divide variable by a constant) - %=
"mod-equal" (remainder of dividing variable by a constant) Arithmetic operators often occur in an
expr or let expression. Example 8-2. Using Arithmetic Operations #!/bin/bash
# Counting to 11 in 10 different ways.
n=1; echo -n "$n "
let "n = $n + 1" # let "n = n + 1" also works.
echo -n "$n "
: $((n = $n + 1))
# ":" necessary because otherwise Bash attempts
#+ to interpret "$((n = $n + 1))" as a command.
echo -n "$n "
(( n = n + 1 ))
# A simpler alternative to the method above.
# Thanks, David Lombard, for pointing this out.
echo -n "$n "
n=$(($n + 1))
echo -n "$n "
: $[ n = $n + 1 ]
# ":" necessary because otherwise Bash attempts
#+ to interpret "$[ n = $n + 1 ]" as a command.
# Works even if "n" was initialized as a string.
echo -n "$n "
n=$[ $n + 1 ]
# Works even if "n" was initialized as a string.
#* Avoid this type of construct, since it is obsolete and nonportable.
# Thanks, Stephane Chazelas.
echo -n "$n "
# Now for C-style increment operators.
# Thanks, Frank Wang, for pointing this out.
let "n++" # let "++n" also works.
echo -n "$n "
(( n++ )) # (( ++n ) also works.
echo -n "$n "
: $(( n++ )) # : $(( ++n )) also works.
echo -n "$n "
: $[ n++ ] # : $[ ++n ]] also works
echo -n "$n "
echo
exit 0 |
 | Integer variables in Bash are actually signed
long (32-bit) integers, in the range of
-2147483648 to 2147483647. An operation that takes a variable
outside these limits will give an erroneous result.
a=2147483646
echo "a = $a" # a = 2147483646
let "a+=1" # Increment "a".
echo "a = $a" # a = 2147483647
let "a+=1" # increment "a" again, past the limit.
echo "a = $a" # a = -2147483648
# ERROR (out of range) |
As of version 2.05b, Bash supports 64-bit integers. |
 | Bash does not understand floating point arithmetic. It
treats numbers containing a decimal point as strings.
a=1.5
let "b = $a + 1.3" # Error.
# t2.sh: let: b = 1.5 + 1.3: syntax error in expression (error token is ".5 + 1.3")
echo "b = $b" # b=1 |
Use bc in scripts that that need floating
point calculations or math library functions. |
bitwise operators - <<
bitwise left shift (multiplies by 2
for each shift position) - <<=
"left-shift-equal" let "var <<= 2" results in var
left-shifted 2 bits (multiplied by 4) - >>
bitwise right shift (divides by 2
for each shift position) - >>=
"right-shift-equal" (inverse of <<=) - &
bitwise and - &=
"bitwise and-equal" - |
bitwise OR - |=
"bitwise OR-equal" - ~
bitwise negate - !
bitwise NOT - ^
bitwise XOR - ^=
"bitwise XOR-equal"
logical operators - &&
and (logical) if [ $condition1 ] && [ $condition2 ]
# Same as: if [ $condition1 -a $condition2 ]
# Returns true if both condition1 and condition2 hold true...
if [[ $condition1 && $condition2 ]] # Also works.
# Note that && operator not permitted within [ ... ] construct. |
 | && may also, depending on context, be
used in an and list
to concatenate commands. |
- ||
or (logical) if [ $condition1 ] || [ $condition2 ]
# Same as: if [ $condition1 -o $condition2 ]
# Returns true if either condition1 or condition2 holds true...
if [[ $condition1 || $condition2 ]] # Also works.
# Note that || operator not permitted within [ ... ] construct. |
 | Bash tests the exit
status of each statement linked with a logical
operator. |
Example 8-3. Compound Condition Tests Using && and || #!/bin/bash
a=24
b=47
if [ "$a" -eq 24 ] && [ "$b" -eq 47 ]
then
echo "Test #1 succeeds."
else
echo "Test #1 fails."
fi
# ERROR: if [ "$a" -eq 24 && "$b" -eq 47 ]
#+ attempts to execute ' [ "$a" -eq 24 '
#+ and fails to finding matching ']'.
#
# Note: if [[ $a -eq 24 && $b -eq 24 ]] works.
# The double-bracket if-test is more flexible
#+ than the single-bracket version.
# (The "&&" has a different meaning in line 17 than in line 6.)
# Thanks, Stephane Chazelas, for pointing this out.
if [ "$a" -eq 98 ] || [ "$b" -eq 47 ]
then
echo "Test #2 succeeds."
else
echo "Test #2 fails."
fi
# The -a and -o options provide
#+ an alternative compound condition test.
# Thanks to Patrick Callahan for pointing this out.
if [ "$a" -eq 24 -a "$b" -eq 47 ]
then
echo "Test #3 succeeds."
else
echo "Test #3 fails."
fi
if [ "$a" -eq 98 -o "$b" -eq 47 ]
then
echo "Test #4 succeeds."
else
echo "Test #4 fails."
fi
a=rhino
b=crocodile
if [ "$a" = rhino ] && [ "$b" = crocodile ]
then
echo "Test #5 succeeds."
else
echo "Test #5 fails."
fi
exit 0 |
The && and || operators also
find use in an arithmetic context. bash$ echo $(( 1 && 2 )) $((3 && 0)) $((4 || 0)) $((0 || 0))
1 0 1 0
|
miscellaneous operators - ,
comma operator The comma operator chains together
two or more arithmetic operations. All the operations are
evaluated (with possible side effects),
but only the last operation is returned. let "t1 = ((5 + 3, 7 - 1, 15 - 4))"
echo "t1 = $t1" # t1 = 11
let "t2 = ((a = 9, 15 / 3))" # Set "a" and calculate "t2".
echo "t2 = $t2 a = $a" # t2 = 5 a = 9 |
The comma operator finds use mainly in for loops. See Example 10-12.
8.2. Numerical ConstantsA shell script interprets a number
as decimal (base 10), unless that number has a
special prefix or notation. A number preceded by a
0 is octal
(base 8). A number preceded by 0x
is hexadecimal (base 16). A number
with an embedded # evaluates as
BASE#NUMBER (with range and notational
restrictions). Example 8-4. Representation of numerical constants #!/bin/bash
# numbers.sh: Representation of numbers in different bases.
# Decimal: the default
let "dec = 32"
echo "decimal number = $dec" # 32
# Nothing out of the ordinary here.
# Octal: numbers preceded by '0' (zero)
let "oct = 032"
echo "octal number = $oct" # 26
# Expresses result in decimal.
# --------- ------ -- -------
# Hexadecimal: numbers preceded by '0x' or '0X'
let "hex = 0x32"
echo "hexadecimal number = $hex" # 50
# Expresses result in decimal.
# Other bases: BASE#NUMBER
# BASE between 2 and 64.
# NUMBER must use symbols within the BASE range, see below.
let "bin = 2#111100111001101"
echo "binary number = $bin" # 31181
let "b32 = 32#77"
echo "base-32 number = $b32" # 231
let "b64 = 64#@_"
echo "base-64 number = $b64" # 4031
# This notation only works for a limited range (2 - 64) of ASCII characters.
# 10 digits + 26 lowercase characters + 26 uppercase characters + @ + _
echo
echo $((36#zz)) $((2#10101010)) $((16#AF16)) $((53#1aA))
# 1295 170 44822 3375
# Important note:
# --------------
# Using a digit out of range of the specified base notation
#+ gives an error message.
let "bad_oct = 081"
# (Partial) error message output:
# bad_oct = 081: value too great for base (error token is "081")
# Octal numbers use only digits in the range 0 - 7.
exit 0 # Thanks, Rich Bartell and Stephane Chazelas, for clarification. |
Part 3. Beyond the Basics
Chapter 9. Variables RevisitedUsed properly, variables can add power and flexibility
to scripts. This requires learning their subtleties and
nuances.
9.1. Internal Variables- Builtin variables
variables affecting bash script behavior - $BASH
the path to the Bash
binary itself
bash$ echo $BASH
/bin/bash |
- $BASH_ENV
an environmental
variable pointing to a Bash startup file to be read
when a script is invoked - $BASH_SUBSHELL
a variable indicating the subshell level. This is a
new addition to Bash, version 3. See Example 20-1 for usage. - $BASH_VERSINFO[n]
a 6-element array
containing version information about the installed release
of Bash. This is similar to $BASH_VERSION,
below, but a bit more detailed. # Bash version info:
for n in 0 1 2 3 4 5
do
echo "BASH_VERSINFO[$n] = ${BASH_VERSINFO[$n]}"
done
# BASH_VERSINFO[0] = 3 # Major version no.
# BASH_VERSINFO[1] = 00 # Minor version no.
# BASH_VERSINFO[2] = 14 # Patch level.
# BASH_VERSINFO[3] = 1 # Build version.
# BASH_VERSINFO[4] = release # Release status.
# BASH_VERSINFO[5] = i386-redhat-linux-gnu # Architecture
# (same as $MACHTYPE). |
- $BASH_VERSION
the version of Bash installed on the system bash$ echo $BASH_VERSION
3.00.14(1)-release
|
tcsh% echo $BASH_VERSION
BASH_VERSION: Undefined variable.
|
Checking $BASH_VERSION is a good method of determining which
shell is running. $SHELL
does not necessarily give the correct answer. - $DIRSTACK
the top value in the directory stack
(affected by pushd and popd) This builtin
variable corresponds to the dirs
command, however dirs shows the entire
contents of the directory stack. - $EDITOR
the default editor invoked by a script, usually
vi or emacs. - $EUID
"effective" user ID number Identification number of whatever identity the
current user has assumed, perhaps by means of su.  | The $EUID is not necessarily
the same as the $UID. |
- $FUNCNAME
name of the current function xyz23 ()
{
echo "$FUNCNAME now executing." # xyz23 now executing.
}
xyz23
echo "FUNCNAME = $FUNCNAME" # FUNCNAME =
# Null value outside a function. |
- $GLOBIGNORE
A list of filename patterns to be excluded from
matching in globbing. - $GROUPS
groups current user belongs to This is a listing (array) of the group id numbers for
current user, as recorded in
/etc/passwd.
root# echo $GROUPS
0
root# echo ${GROUPS[1]}
1
root# echo ${GROUPS[5]}
6
|
- $HOME
home directory of the user, usually /home/username (see Example 9-14) - $HOSTNAME
The hostname command
assigns the system name at bootup in an init script.
However, the gethostname() function
sets the Bash internal variable $HOSTNAME.
See also Example 9-14. - $HOSTTYPE
host type Like $MACHTYPE,
identifies the system hardware. bash$ echo $HOSTTYPE
i686 |
- $IFS
internal field separator This variable determines how Bash recognizes fields, or word
boundaries when it interprets character strings. $IFS defaults to whitespace (space,
tab, and newline), but may be changed, for example,
to parse a comma-separated data file. Note that
$* uses the first
character held in $IFS. See Example 5-1. bash$ echo $IFS | cat -vte
$
bash$ bash -c 'set w x y z; IFS=":-;"; echo "$*"'
w:x:y:z
|
 | $IFS does not handle whitespace
the same as it does other characters.
Example 9-1. $IFS and whitespace #!/bin/bash
# $IFS treats whitespace differently than other characters.
output_args_one_per_line()
{
for arg
do echo "[$arg]"
done
}
echo; echo "IFS=\" \""
echo "-------"
IFS=" "
var=" a b c "
output_args_one_per_line $var # output_args_one_per_line `echo " a b c "`
#
# [a]
# [b]
# [c]
echo; echo "IFS=:"
echo "-----"
IFS=:
var=":a::b:c:::" # Same as above, but substitute ":" for " ".
output_args_one_per_line $var
#
# []
# [a]
# []
# [b]
# [c]
# []
# []
# []
# The same thing happens with the "FS" field separator in awk.
# Thank you, Stephane Chazelas.
echo
exit 0 |
|
(Thanks, S. C., for clarification and examples.) See also Example 12-37 for an instructive
example of using $IFS. - $IGNOREEOF
ignore EOF: how many end-of-files (control-D)
the shell will ignore before logging out. - $LC_COLLATE
Often set in the .bashrc or
/etc/profile files, this
variable controls collation order in filename
expansion and pattern matching. If mishandled,
LC_COLLATE can cause unexpected results
in filename
globbing.  | As of version 2.05 of Bash,
filename globbing no longer distinguishes between lowercase
and uppercase letters in a character range between
brackets. For example, ls [A-M]*
would match both File1.txt
and file1.txt. To revert to
the customary behavior of bracket matching, set
LC_COLLATE to C
by an export LC_COLLATE=C
in /etc/profile and/or
~/.bashrc. |
- $LC_CTYPE
This internal variable controls character interpretation
in globbing and pattern
matching. - $LINENO
This variable is the line number of the shell
script in which this variable appears. It has significance only
within the script in which it appears, and is chiefly useful for
debugging purposes. # *** BEGIN DEBUG BLOCK ***
last_cmd_arg=$_ # Save it.
echo "At line number $LINENO, variable \"v1\" = $v1"
echo "Last command argument processed = $last_cmd_arg"
# *** END DEBUG BLOCK *** |
- $MACHTYPE
machine type Identifies the system hardware. bash$ echo $MACHTYPE
i686 |
- $OLDPWD
old working directory
("OLD-print-working-directory",
previous directory you were in) - $OSTYPE
operating system type - $PATH
path to binaries, usually
/usr/bin/,
/usr/X11R6/bin/,
/usr/local/bin, etc. When given a command, the shell automatically does
a hash table search on the directories listed in the
path for the executable. The path
is stored in the environmental
variable, $PATH, a list
of directories, separated by colons. Normally,
the system stores the $PATH
definition in /etc/profile
and/or ~/.bashrc (see Appendix G). bash$ echo $PATH
/bin:/usr/bin:/usr/local/bin:/usr/X11R6/bin:/sbin:/usr/sbin |
PATH=${PATH}:/opt/bin appends
the /opt/bin
directory to the current path. In a script, it may be
expedient to temporarily add a directory to the path
in this way. When the script exits, this restores the
original $PATH (a child process, such
as a script, may not change the environment of the parent
process, the shell).  | The current "working directory",
./, is usually
omitted from the $PATH as a security
measure. |
- $PIPESTATUS
Array variable holding
exit status(es) of last executed
foreground pipe. Interestingly enough, this
does not necessarily give the same result as the exit status of the last
executed command. bash$ echo $PIPESTATUS
0
bash$ ls -al | bogus_command
bash: bogus_command: command not found
bash$ echo $PIPESTATUS
141
bash$ ls -al | bogus_command
bash: bogus_command: command not found
bash$ echo $?
127
|
The members of the $PIPESTATUS array
hold the exit status of each respective command
executed in a pipe. $PIPESTATUS[0]
holds the exit status of the first command in the pipe,
$PIPESTATUS[1] the exit status of
the second command, and so on.  | The $PIPESTATUS variable
may contain an erroneous 0 value
in a login shell (in releases prior to 3.0 of Bash).
tcsh% bash
bash$ who | grep nobody | sort
bash$ echo ${PIPESTATUS[*]}
0
|
The above lines contained in a script would produce the expected
0 1 0 output.
Thank you, Wayne Pollock for pointing this out and supplying the
above example.
|
 | The $PIPESTATUS variable gives
unexpected results in some contexts. bash$ echo $BASH_VERSION
3.00.14(1)-release
bash$ $ ls | bogus_command | wc
bash: bogus_command: command not found
0 0 0
bash$ echo ${PIPESTATUS[@]}
141 127 0
|
Chet Ramey attributes the above output to the behavior of
ls. If ls
writes to a pipe whose output is not read, then SIGPIPE
kills it, and its exit
status is 141. Otherwise
its exit status is 0,
as expected. This likewise is the case for tr. |
 | $PIPESTATUS is a
"volatile" variable. It needs to be
captured immediately after the pipe in question, before
any other command intervenes. bash$ $ ls | bogus_command | wc
bash: bogus_command: command not found
0 0 0
bash$ echo ${PIPESTATUS[@]}
0 127 0
bash$ echo ${PIPESTATUS[@]}
0
|
|
- $PPID
The $PPID of a process is
the process ID (pid) of its parent process.
Compare this with the pidof command. - $PROMPT_COMMAND
A variable holding a command to be executed
just before the primary prompt, $PS1
is to be displayed. - $PS1
This is the main prompt, seen at the command line. - $PS2
The secondary prompt, seen when additional input is
expected. It displays as ">". - $PS3
The tertiary prompt, displayed in a
select loop (see Example 10-29). - $PS4
The quartenary prompt, shown at the beginning of
each line of output when invoking a script with the
-x option.
It displays as "+". - $PWD
working directory (directory you are in at the time) This is the analog to the pwd
builtin command. #!/bin/bash
E_WRONG_DIRECTORY=73
clear # Clear screen.
TargetDirectory=/home/bozo/projects/GreatAmericanNovel
cd $TargetDirectory
echo "Deleting stale files in $TargetDirectory."
if [ "$PWD" != "$TargetDirectory" ]
then # Keep from wiping out wrong directory by accident.
echo "Wrong directory!"
echo "In $PWD, rather than $TargetDirectory!"
echo "Bailing out!"
exit $E_WRONG_DIRECTORY
fi
rm -rf *
rm .[A-Za-z0-9]* # Delete dotfiles.
# rm -f .[^.]* ..?* to remove filenames beginning with multiple dots.
# (shopt -s dotglob; rm -f *) will also work.
# Thanks, S.C. for pointing this out.
# Filenames may contain all characters in the 0 - 255 range, except "/".
# Deleting files beginning with weird characters is left as an exercise.
# Various other operations here, as necessary.
echo
echo "Done."
echo "Old files deleted in $TargetDirectory."
echo
exit 0 |
- $REPLY
The default value when a variable is not
supplied to read. Also
applicable to select menus,
but only supplies the item number of the variable chosen,
not the value of the variable itself. #!/bin/bash
# reply.sh
# REPLY is the default value for a 'read' command.
echo
echo -n "What is your favorite vegetable? "
read
echo "Your favorite vegetable is $REPLY."
# REPLY holds the value of last "read" if and only if
#+ no variable supplied.
echo
echo -n "What is your favorite fruit? "
read fruit
echo "Your favorite fruit is $fruit."
echo "but..."
echo "Value of \$REPLY is still $REPLY."
# $REPLY is still set to its previous value because
#+ the variable $fruit absorbed the new "read" value.
echo
exit 0 |
- $SECONDS
The number of seconds the script has been running. #!/bin/bash
TIME_LIMIT=10
INTERVAL=1
echo
echo "Hit Control-C to exit before $TIME_LIMIT seconds."
echo
while [ "$SECONDS" -le "$TIME_LIMIT" ]
do
if [ "$SECONDS" -eq 1 ]
then
units=second
else
units=seconds
fi
echo "This script has been running $SECONDS $units."
# On a slow or overburdened machine, the script may skip a count
#+ every once in a while.
sleep $INTERVAL
done
echo -e "\a" # Beep!
exit 0 |
- $SHELLOPTS
the list of enabled shell options, a readonly variable
bash$ echo $SHELLOPTS
braceexpand:hashall:histexpand:monitor:history:interactive-comments:emacs
|
- $SHLVL
Shell level, how deeply Bash is nested. If,
at the command line, $SHLVL is 1, then in a script it will
increment to 2. - $TMOUT
If the $TMOUT
environmental variable is set to a non-zero value
time, then the shell prompt will time out
after time seconds. This will cause a
logout. As of version 2.05b of Bash, it is now possible to use
$TMOUT in a script in combination
with read. # Works in scripts for Bash, versions 2.05b and later.
TMOUT=3 # Prompt times out at three seconds.
echo "What is your favorite song?"
echo "Quickly now, you only have $TMOUT seconds to answer!"
read song
if [ -z "$song" ]
then
song="(no answer)"
# Default response.
fi
echo "Your favorite song is $song." |
There are other, more complex, ways of implementing
timed input in a script. One alternative is to set up
a timing loop to signal the script when it times out.
This also requires a signal handling routine to trap (see
Example 29-5) the interrupt generated by the timing
loop (whew!). Example 9-2. Timed Input #!/bin/bash
# timed-input.sh
# TMOUT=3 Also works, as of newer versions of Bash.
TIMELIMIT=3 # Three seconds in this instance. May be set to different value.
PrintAnswer()
{
if [ "$answer" = TIMEOUT ]
then
echo $answer
else # Don't want to mix up the two instances.
echo "Your favorite veggie is $answer"
kill $! # Kills no longer needed TimerOn function running in background.
# $! is PID of last job running in background.
fi
}
TimerOn()
{
sleep $TIMELIMIT && kill -s 14 $$ &
# Waits 3 seconds, then sends sigalarm to script.
}
Int14Vector()
{
answer="TIMEOUT"
PrintAnswer
exit 14
}
trap Int14Vector 14 # Timer interrupt (14) subverted for our purposes.
echo "What is your favorite vegetable "
TimerOn
read answer
PrintAnswer
# Admittedly, this is a kludgy implementation of timed input,
#+ however the "-t" option to "read" simplifies this task.
# See "t-out.sh", below.
# If you need something really elegant...
#+ consider writing the application in C or C++,
#+ using appropriate library functions, such as 'alarm' and 'setitimer'.
exit 0 |
An alternative is using stty. Example 9-3. Once more, timed input #!/bin/bash
# timeout.sh
# Written by Stephane Chazelas,
#+ and modified by the document author.
INTERVAL=5 # timeout interval
timedout_read() {
timeout=$1
varname=$2
old_tty_settings=`stty -g`
stty -icanon min 0 time ${timeout}0
eval read $varname # or just read $varname
stty "$old_tty_settings"
# See man page for "stty".
}
echo; echo -n "What's your name? Quick! "
timedout_read $INTERVAL your_name
# This may not work on every terminal type.
# The maximum timeout depends on the terminal.
#+ (it is often 25.5 seconds).
echo
if [ ! -z "$your_name" ] # If name input before timeout...
then
echo "Your name is $your_name."
else
echo "Timed out."
fi
echo
# The behavior of this script differs somewhat from "timed-input.sh".
# At each keystroke, the counter resets.
exit 0 |
Perhaps the simplest method is using the
-t option to read. Example 9-4. Timed read #!/bin/bash
# t-out.sh
# Inspired by a suggestion from "syngin seven" (thanks).
TIMELIMIT=4 # 4 seconds
read -t $TIMELIMIT variable <&1
# ^^^
# In this instance, "<&1" is needed for Bash 1.x and 2.x,
# but unnecessary for Bash 3.x.
echo
if [ -z "$variable" ] # Is null?
then
echo "Timed out, variable still unset."
else
echo "variable = $variable"
fi
exit 0 |
- $UID
user ID number current user's user identification number, as
recorded in /etc/passwd
This is the current user's real id, even if she has
temporarily assumed another identity through su. $UID is a
readonly variable, not subject to change from the command
line or within a script, and is the counterpart to the
id builtin. Example 9-5. Am I root? #!/bin/bash
# am-i-root.sh: Am I root or not?
ROOT_UID=0 # Root has $UID 0.
if [ "$UID" -eq "$ROOT_UID" ] # Will the real "root" please stand up?
then
echo "You are root."
else
echo "You are just an ordinary user (but mom loves you just the same)."
fi
exit 0
# ============================================================= #
# Code below will not execute, because the script already exited.
# An alternate method of getting to the root of matters:
ROOTUSER_NAME=root
username=`id -nu` # Or... username=`whoami`
if [ "$username" = "$ROOTUSER_NAME" ]
then
echo "Rooty, toot, toot. You are root."
else
echo "You are just a regular fella."
fi |
See also Example 2-3.  | The variables $ENV,
$LOGNAME, $MAIL,
$TERM, $USER, and
$USERNAME are not
Bash builtins. These are,
however, often set as environmental
variables in one of the Bash startup files. $SHELL,
the name of the user's login shell, may be set from
/etc/passwd or in an "init"
script, and it is likewise not a Bash builtin. tcsh% echo $LOGNAME
bozo
tcsh% echo $SHELL
/bin/tcsh
tcsh% echo $TERM
rxvt
bash$ echo $LOGNAME
bozo
bash$ echo $SHELL
/bin/tcsh
bash$ echo $TERM
rxvt
|
|
Positional Parameters - $0, $1,
$2, etc.
positional parameters, passed from command
line to script, passed to a function, or set to a variable (see Example 4-5 and Example 11-15) - $#
number of command line arguments
or positional parameters (see Example 33-2) - $*
All of the positional parameters, seen as a single word  | "$*" must be
quoted. |
- $@
Same as $*, but each parameter is a
quoted string, that is, the parameters are passed on
intact, without interpretation or expansion. This means,
among other things, that each parameter in the argument
list is seen as a separate word.  | Of course, "$@"
should be quoted. |
Example 9-6. arglist: Listing arguments with $* and $@ #!/bin/bash
# arglist.sh
# Invoke this script with several arguments, such as "one two three".
E_BADARGS=65
if [ ! -n "$1" ]
then
echo "Usage: `basename $0` argument1 argument2 etc."
exit $E_BADARGS
fi
echo
index=1 # Initialize count.
echo "Listing args with \"\$*\":"
for arg in "$*" # Doesn't work properly if "$*" isn't quoted.
do
echo "Arg #$index = $arg"
let "index+=1"
done # $* sees all arguments as single word.
echo "Entire arg list seen as single word."
echo
index=1 # Reset count.
# What happens if you forget to do this?
echo "Listing args with \"\$@\":"
for arg in "$@"
do
echo "Arg #$index = $arg"
let "index+=1"
done # $@ sees arguments as separate words.
echo "Arg list seen as separate words."
echo
index=1 # Reset count.
echo "Listing args with \$* (unquoted):"
for arg in $*
do
echo "Arg #$index = $arg"
let "index+=1"
done # Unquoted $* sees arguments as separate words.
echo "Arg list seen as separate words."
exit 0 |
Following a shift, the
$@ holds the remaining command-line
parameters, lacking the previous $1,
which was lost.
#!/bin/bash
# Invoke with ./scriptname 1 2 3 4 5
echo "$@" # 1 2 3 4 5
shift
echo "$@" # 2 3 4 5
shift
echo "$@" # 3 4 5
# Each "shift" loses parameter $1.
# "$@" then contains the remaining parameters. |
The $@ special parameter finds
use as a tool for filtering input into shell scripts. The
cat "$@" construction accepts input
to a script either from stdin or
from files given as parameters to the script. See Example 12-21 and Example 12-22.  | The $* and $@
parameters sometimes display inconsistent and
puzzling behavior, depending on the setting of $IFS. |
Example 9-7. Inconsistent $* and $@ behavior #!/bin/bash
# Erratic behavior of the "$*" and "$@" internal Bash variables,
#+ depending on whether they are quoted or not.
# Inconsistent handling of word splitting and linefeeds.
set -- "First one" "second" "third:one" "" "Fifth: :one"
# Setting the script arguments, $1, $2, etc.
echo
echo 'IFS unchanged, using "$*"'
c=0
for i in "$*" # quoted
do echo "$((c+=1)): [$i]" # This line remains the same in every instance.
# Echo args.
done
echo ---
echo 'IFS unchanged, using $*'
c=0
for i in $* # unquoted
do echo "$((c+=1)): [$i]"
done
echo ---
echo 'IFS unchanged, using "$@"'
c=0
for i in "$@"
do echo "$((c+=1)): [$i]"
done
echo ---
echo 'IFS unchanged, using $@'
c=0
for i in $@
do echo "$((c+=1)): [$i]"
done
echo ---
IFS=:
echo 'IFS=":", using "$*"'
c=0
for i in "$*"
do echo "$((c+=1)): [$i]"
done
echo ---
echo 'IFS=":", using $*'
c=0
for i in $*
do echo "$((c+=1)): [$i]"
done
echo ---
var=$*
echo 'IFS=":", using "$var" (var=$*)'
c=0
for i in "$var"
do echo "$((c+=1)): [$i]"
done
echo ---
echo 'IFS=":", using $var (var=$*)'
c=0
for i in $var
do echo "$((c+=1)): [$i]"
done
echo ---
var="$*"
echo 'IFS=":", using $var (var="$*")'
c=0
for i in $var
do echo "$((c+=1)): [$i]"
done
echo ---
echo 'IFS=":", using "$var" (var="$*")'
c=0
for i in "$var"
do echo "$((c+=1)): [$i]"
done
echo ---
echo 'IFS=":", using "$@"'
c=0
for i in "$@"
do echo "$((c+=1)): [$i]"
done
echo ---
echo 'IFS=":", using $@'
c=0
for i in $@
do echo "$((c+=1)): [$i]"
done
echo ---
var=$@
echo 'IFS=":", using $var (var=$@)'
c=0
for i in $var
do echo "$((c+=1)): [$i]"
done
echo ---
echo 'IFS=":", using "$var" (var=$@)'
c=0
for i in "$var"
do echo "$((c+=1)): [$i]"
done
echo ---
var="$@"
echo 'IFS=":", using "$var" (var="$@")'
c=0
for i in "$var"
do echo "$((c+=1)): [$i]"
done
echo ---
echo 'IFS=":", using $var (var="$@")'
c=0
for i in $var
do echo "$((c+=1)): [$i]"
done
echo
# Try this script with ksh or zsh -y.
exit 0
# This example script by Stephane Chazelas,
# and slightly modified by the document author. |
 | The $@ and $*
parameters differ only when between double quotes. |
Example 9-8. $* and $@ when
$IFS is empty #!/bin/bash
# If $IFS set, but empty,
#+ then "$*" and "$@" do not echo positional params as expected.
mecho () # Echo positional parameters.
{
echo "$1,$2,$3";
}
IFS="" # Set, but empty.
set a b c # Positional parameters.
mecho "$*" # abc,,
mecho $* # a,b,c
mecho $@ # a,b,c
mecho "$@" # a,b,c
# The behavior of $* and $@ when $IFS is empty depends
#+ on whatever Bash or sh version being run.
# It is therefore inadvisable to depend on this "feature" in a script.
# Thanks, Stephane Chazelas.
exit 0 |
Other Special Parameters - $-
Flags passed to script (using set). See Example 11-15.  | This was originally a ksh
construct adopted into Bash, and unfortunately it does not
seem to work reliably in Bash scripts. One possible use
for it is to have a script self-test
whether it is interactive. |
- $!
PID (process ID) of last job run in background LOG=$0.log
COMMAND1="sleep 100"
echo "Logging PIDs background commands for script: $0" >> "$LOG"
# So they can be monitored, and killed as necessary.
echo >> "$LOG"
# Logging commands.
echo -n "PID of \"$COMMAND1\": " >> "$LOG"
${COMMAND1} &
echo $! >> "$LOG"
# PID of "sleep 100": 1506
# Thank you, Jacques Lederer, for suggesting this. |
possibly_hanging_job & { sleep ${TIMEOUT}; eval 'kill -9 $!' &> /dev/null; }
# Forces completion of an ill-behaved program.
# Useful, for example, in init scripts.
# Thank you, Sylvain Fourmanoit, for this creative use of the "!" variable. |
- $_
Special variable set to last argument of previous command
executed. Example 9-9. Underscore variable #!/bin/bash
echo $_ # /bin/bash
# Just called /bin/bash to run the script.
du >/dev/null # So no output from command.
echo $_ # du
ls -al >/dev/null # So no output from command.
echo $_ # -al (last argument)
:
echo $_ # : |
- $?
Exit status
of a command, function,
or the script itself (see Example 23-7) - $$
Process ID of the script itself. The
$$ variable often finds use
in scripts to construct "unique"
temp file names (see Example A-13, Example 29-6, Example 12-28, and Example 11-25). This is usually simpler than
invoking mktemp.
9.2. Manipulating Strings
Bash supports a surprising number of string manipulation
operations. Unfortunately, these tools lack
a unified focus. Some are a subset of parameter substitution, and
others fall under the functionality of the UNIX expr command. This results in
inconsistent command syntax and overlap of functionality,
not to mention confusion. String Length - ${#string}
- expr length $string
- expr "$string" : '.*'
stringZ=abcABC123ABCabc
echo ${#stringZ} # 15
echo `expr length $stringZ` # 15
echo `expr "$stringZ" : '.*'` # 15 |
Example 9-10. Inserting a blank line between paragraphs in a text file #!/bin/bash
# paragraph-space.sh
# Inserts a blank line between paragraphs of a single-spaced text file.
# Usage: $0 <FILENAME
MINLEN=45 # May need to change this value.
# Assume lines shorter than $MINLEN characters
#+ terminate a paragraph.
while read line # For as many lines as the input file has...
do
echo "$line" # Output the line itself.
len=${#line}
if [ "$len" -lt "$MINLEN" ]
then echo # Add a blank line after short line.
fi
done
exit 0 |
Length of Matching Substring at Beginning of String - expr match "$string" '$substring'
$substring is a regular expression. - expr "$string" : '$substring'
$substring is a regular
expression.
stringZ=abcABC123ABCabc
# |------|
echo `expr match "$stringZ" 'abc[A-Z]*.2'` # 8
echo `expr "$stringZ" : 'abc[A-Z]*.2'` # 8 |
Index - expr index $string $substring
Numerical position in $string of first character in
$substring that matches. stringZ=abcABC123ABCabc
echo `expr index "$stringZ" C12` # 6
# C position.
echo `expr index "$stringZ" 1c` # 3
# 'c' (in #3 position) matches before '1'. |
This is the near equivalent of
strchr() in C.
Substring Extraction - ${string:position}
Extracts substring from $string at
$position. If the $string parameter is
"*"
or "@", then this extracts the
positional parameters,
starting at $position. - ${string:position:length}
Extracts $length characters
of substring from $string at
$position. stringZ=abcABC123ABCabc
# 0123456789.....
# 0-based indexing.
echo ${stringZ:0} # abcABC123ABCabc
echo ${stringZ:1} # bcABC123ABCabc
echo ${stringZ:7} # 23ABCabc
echo ${stringZ:7:3} # 23A
# Three characters of substring.
# Is it possible to index from the right end of the string?
echo ${stringZ:-4} # abcABC123ABCabc
# Defaults to full string, as in ${parameter:-default}.
# However . . .
echo ${stringZ:(-4)} # Cabc
echo ${stringZ: -4} # Cabc
# Now, it works.
# Parentheses or added space "escape" the position parameter.
# Thank you, Dan Jacobson, for pointing this out. |
If the $string parameter is
"*" or
"@", then this extracts a maximum
of $length positional parameters, starting
at $position. echo ${*:2} # Echoes second and following positional parameters.
echo ${@:2} # Same as above.
echo ${*:2:3} # Echoes three positional parameters, starting at second. |
- expr substr $string $position $length
Extracts $length characters
from $string starting at
$position. stringZ=abcABC123ABCabc
# 123456789......
# 1-based indexing.
echo `expr substr $stringZ 1 2` # ab
echo `expr substr $stringZ 4 3` # ABC |
- expr match "$string" '\($substring\)'
Extracts $substring
at beginning of $string,
where $substring is a regular expression. - expr "$string" : '\($substring\)'
Extracts $substring
at beginning of $string,
where $substring is a regular
expression. stringZ=abcABC123ABCabc
# =======
echo `expr match "$stringZ" '\(.[b-c]*[A-Z]..[0-9]\)'` # abcABC1
echo `expr "$stringZ" : '\(.[b-c]*[A-Z]..[0-9]\)'` # abcABC1
echo `expr "$stringZ" : '\(.......\)'` # abcABC1
# All of the above forms give an identical result. |
- expr match "$string" '.*\($substring\)'
Extracts $substring
at end of
$string, where
$substring is a regular
expression. - expr "$string" : '.*\($substring\)'
Extracts $substring
at end of $string,
where $substring is a regular
expression. stringZ=abcABC123ABCabc
# ======
echo `expr match "$stringZ" '.*\([A-C][A-C][A-C][a-c]*\)'` # ABCabc
echo `expr "$stringZ" : '.*\(......\)'` # ABCabc |
Substring Removal - ${string#substring}
Strips shortest match of
$substring from
front of
$string. - ${string##substring}
Strips longest match of
$substring from
front of
$string.
stringZ=abcABC123ABCabc
# |----|
# |----------|
echo ${stringZ#a*C} # 123ABCabc
# Strip out shortest match between 'a' and 'C'.
echo ${stringZ##a*C} # abc
# Strip out longest match between 'a' and 'C'. |
- ${string%substring}
Strips shortest match of
$substring from
back of
$string. - ${string%%substring}
Strips longest match of
$substring from
back of
$string.
stringZ=abcABC123ABCabc
# ||
# |------------|
echo ${stringZ%b*c} # abcABC123ABCa
# Strip out shortest match between 'b' and 'c', from back of $stringZ.
echo ${stringZ%%b*c} # a
# Strip out longest match between 'b' and 'c', from back of $stringZ. |
Example 9-11. Converting graphic file formats, with filename change #!/bin/bash
# cvt.sh:
# Converts all the MacPaint image files in a directory to "pbm" format.
# Uses the "macptopbm" binary from the "netpbm" package,
#+ which is maintained by Brian Henderson (bryanh@giraffe-data.com).
# Netpbm is a standard part of most Linux distros.
OPERATION=macptopbm
SUFFIX=pbm # New filename suffix.
if [ -n "$1" ]
then
directory=$1 # If directory name given as a script argument...
else
directory=$PWD # Otherwise use current working directory.
fi
# Assumes all files in the target directory are MacPaint image files,
#+ with a ".mac" filename suffix.
for file in $directory/* # Filename globbing.
do
filename=${file%.*c} # Strip ".mac" suffix off filename
#+ ('.*c' matches everything
#+ between '.' and 'c', inclusive).
$OPERATION $file > "$filename.$SUFFIX"
# Redirect conversion to new filename.
rm -f $file # Delete original files after converting.
echo "$filename.$SUFFIX" # Log what is happening to stdout.
done
exit 0
# Exercise:
# --------
# As it stands, this script converts *all* the files in the current
#+ working directory.
# Modify it to work *only* on files with a ".mac" suffix. |
A simple emulation of getopt
using substring extraction constructs. Example 9-12. Emulating getopt #!/bin/bash
# getopt-simple.sh
# Author: Chris Morgan
# Used in the ABS Guide with permission.
getopt_simple()
{
echo "getopt_simple()"
echo "Parameters are '$*'"
until [ -z "$1" ]
do
echo "Processing parameter of: '$1'"
if [ ${1:0:1} = '/' ]
then
tmp=${1:1} # Strip off leading '/' . . .
parameter=${tmp%%=*} # Extract name.
value=${tmp##*=} # Extract value.
echo "Parameter: '$parameter', value: '$value'"
eval $parameter=$value
fi
shift
done
}
# Pass all options to getopt_simple().
getopt_simple $*
echo "test is '$test'"
echo "test2 is '$test2'"
exit 0
---
sh getopt_example.sh /test=value1 /test2=value2
Parameters are '/test=value1 /test2=value2'
Processing parameter of: '/test=value1'
Parameter: 'test', value: 'value1'
Processing parameter of: '/test2=value2'
Parameter: 'test2', value: 'value2'
test is 'value1'
test2 is 'value2' |
Substring Replacement - ${string/substring/replacement}
Replace first match of
$substring with
$replacement. - ${string//substring/replacement}
Replace all matches of
$substring with
$replacement.
stringZ=abcABC123ABCabc
echo ${stringZ/abc/xyz} # xyzABC123ABCabc
# Replaces first match of 'abc' with 'xyz'.
echo ${stringZ//abc/xyz} # xyzABC123ABCxyz
# Replaces all matches of 'abc' with # 'xyz'. |
- ${string/#substring/replacement}
If $substring matches
front end of
$string, substitute
$replacement for
$substring. - ${string/%substring/replacement}
If $substring matches
back end of
$string, substitute
$replacement for
$substring.
stringZ=abcABC123ABCabc
echo ${stringZ/#abc/XYZ} # XYZABC123ABCabc
# Replaces front-end match of 'abc' with 'XYZ'.
echo ${stringZ/%abc/XYZ} # abcABC123ABCXYZ
# Replaces back-end match of 'abc' with 'XYZ'. |
9.2.1. Manipulating strings using awkA Bash script may invoke the string manipulation facilities of
awk as an alternative to using its
built-in operations. Example 9-13. Alternate ways of extracting substrings #!/bin/bash
# substring-extraction.sh
String=23skidoo1
# 012345678 Bash
# 123456789 awk
# Note different string indexing system:
# Bash numbers first character of string as '0'.
# Awk numbers first character of string as '1'.
echo ${String:2:4} # position 3 (0-1-2), 4 characters long
# skid
# The awk equivalent of ${string:pos:length} is substr(string,pos,length).
echo | awk '
{ print substr("'"${String}"'",3,4) # skid
}
'
# Piping an empty "echo" to awk gives it dummy input,
#+ and thus makes it unnecessary to supply a filename.
exit 0 |
9.3. Parameter Substitution
Manipulating and/or expanding variables - ${parameter}
Same as $parameter, i.e.,
value of the variable
parameter.
In certain contexts, only the less ambiguous
${parameter} form
works. May be used for concatenating variables with strings. your_id=${USER}-on-${HOSTNAME}
echo "$your_id"
#
echo "Old \$PATH = $PATH"
PATH=${PATH}:/opt/bin #Add /opt/bin to $PATH for duration of script.
echo "New \$PATH = $PATH" |
- ${parameter-default}, ${parameter:-default}
If parameter not set, use default. echo ${username-`whoami`}
# Echoes the result of `whoami`, if variable $username is still unset. |
 | ${parameter-default}
and ${parameter:-default}
are almost equivalent. The extra : makes
a difference only when parameter
has been declared, but is null. |
#!/bin/bash
# param-sub.sh
# Whether a variable has been declared
#+ affects triggering of the default option
#+ even if the variable is null.
username0=
echo "username0 has been declared, but is set to null."
echo "username0 = ${username0-`whoami`}"
# Will not echo.
echo
echo username1 has not been declared.
echo "username1 = ${username1-`whoami`}"
# Will echo.
username2=
echo "username2 has been declared, but is set to null."
echo "username2 = ${username2:-`whoami`}"
# ^
# Will echo because of :- rather than just - in condition test.
# Compare to first instance, above.
#
# Once again:
variable=
# variable has been declared, but is set to null.
echo "${variable-0}" # (no output)
echo "${variable:-1}" # 1
# ^
unset variable
echo "${variable-2}" # 2
echo "${variable:-3}" # 3
exit 0 |
The default parameter construct
finds use in providing "missing" command-line
arguments in scripts. DEFAULT_FILENAME=generic.data
filename=${1:-$DEFAULT_FILENAME}
# If not otherwise specified, the following command block operates
#+ on the file "generic.data".
#
# Commands follow. |
See also Example 3-4, Example 28-2, and Example A-6. Compare this method with using an and
list to supply a default command-line
argument. - ${parameter=default}, ${parameter:=default}
If parameter not set, set it to default. Both forms nearly equivalent. The :
makes a difference only when $parameter
has been declared and is null,
as above.
echo ${username=`whoami`}
# Variable "username" is now set to `whoami`. |
- ${parameter+alt_value}, ${parameter:+alt_value}
If parameter set, use
alt_value, else use null
string. Both forms nearly equivalent. The :
makes a difference only when parameter
has been declared and is null, see below. echo "###### \${parameter+alt_value} ########"
echo
a=${param1+xyz}
echo "a = $a" # a =
param2=
a=${param2+xyz}
echo "a = $a" # a = xyz
param3=123
a=${param3+xyz}
echo "a = $a" # a = xyz
echo
echo "###### \${parameter:+alt_value} ########"
echo
a=${param4:+xyz}
echo "a = $a" # a =
param5=
a=${param5:+xyz}
echo "a = $a" # a =
# Different result from a=${param5+xyz}
param6=123
a=${param6+xyz}
echo "a = $a" # a = xyz |
- ${parameter?err_msg}, ${parameter:?err_msg}
If parameter set, use it, else print err_msg. Both forms nearly equivalent. The :
makes a difference only when parameter
has been declared and is null, as above.
Example 9-14. Using parameter substitution and error messages #!/bin/bash
# Check some of the system's environmental variables.
# This is good preventative maintenance.
# If, for example, $USER, the name of the person at the console, is not set,
#+ the machine will not recognize you.
: ${HOSTNAME?} ${USER?} ${HOME?} ${MAIL?}
echo
echo "Name of the machine is $HOSTNAME."
echo "You are $USER."
echo "Your home directory is $HOME."
echo "Your mail INBOX is located in $MAIL."
echo
echo "If you are reading this message,"
echo "critical environmental variables have been set."
echo
echo
# ------------------------------------------------------
# The ${variablename?} construction can also check
#+ for variables set within the script.
ThisVariable=Value-of-ThisVariable
# Note, by the way, that string variables may be set
#+ to characters disallowed in their names.
: ${ThisVariable?}
echo "Value of ThisVariable is $ThisVariable".
echo
echo
: ${ZZXy23AB?"ZZXy23AB has not been set."}
# If ZZXy23AB has not been set,
#+ then the script terminates with an error message.
# You can specify the error message.
# : ${variablename?"ERROR MESSAGE"}
# Same result with: dummy_variable=${ZZXy23AB?}
# dummy_variable=${ZZXy23AB?"ZXy23AB has not been set."}
#
# echo ${ZZXy23AB?} >/dev/null
# Compare these methods of checking whether a variable has been set
#+ with "set -u" . . .
echo "You will not see this message, because script already terminated."
HERE=0
exit $HERE # Will NOT exit here.
# In fact, this script will return an exit status (echo $?) of 1. |
Example 9-15. Parameter substitution and "usage" messages #!/bin/bash
# usage-message.sh
: ${1?"Usage: $0 ARGUMENT"}
# Script exits here if command-line parameter absent,
#+ with following error message.
# usage-message.sh: 1: Usage: usage-message.sh ARGUMENT
echo "These two lines echo only if command-line parameter given."
echo "command line parameter = \"$1\""
exit 0 # Will exit here only if command-line parameter present.
# Check the exit status, both with and without command-line parameter.
# If command-line parameter present, then "$?" is 0.
# If not, then "$?" is 1. |
Variable length / Substring removal - ${#var}
String length (number
of characters in $var). For
an array,
${#array} is the length of the
first element in the array.  | Exceptions:
${#*} and
${#@} give the number
of positional parameters.
For an array, ${#array[*]} and
${#array[@]} give the number
of elements in the array.
|
Example 9-16. Length of a variable #!/bin/bash
# length.sh
E_NO_ARGS=65
if [ $# -eq 0 ] # Must have command-line args to demo script.
then
echo "Please invoke this script with one or more command-line arguments."
exit $E_NO_ARGS
fi
var01=abcdEFGH28ij
echo "var01 = ${var01}"
echo "Length of var01 = ${#var01}"
# Now, let's try embedding a space.
var02="abcd EFGH28ij"
echo "var02 = ${var02}"
echo "Length of var02 = ${#var02}"
echo "Number of command-line arguments passed to script = ${#@}"
echo "Number of command-line arguments passed to script = ${#*}"
exit 0 |
- ${var#Pattern}, ${var##Pattern}
Remove from $var
the shortest/longest part of $Pattern
that matches the front end
of $var.
A usage illustration from Example A-7:
# Function from "days-between.sh" example.
# Strips leading zero(s) from argument passed.
strip_leading_zero () # Strip possible leading zero(s)
{ #+ from argument passed.
return=${1#0} # The "1" refers to "$1" -- passed arg.
} # The "0" is what to remove from "$1" -- strips zeros. |
Manfred Schwarb's more elaborate variation of the above:
strip_leading_zero2 () # Strip possible leading zero(s), since otherwise
{ # Bash will interpret such numbers as octal values.
shopt -s extglob # Turn on extended globbing.
local val=${1##+(0)} # Use local variable, longest matching series of 0's.
shopt -u extglob # Turn off extended globbing.
_strip_leading_zero2=${val:-0}
# If input was 0, return 0 instead of "".
} |
Another usage illustration:
echo `basename $PWD` # Basename of current working directory.
echo "${PWD##*/}" # Basename of current working directory.
echo
echo `basename $0` # Name of script.
echo $0 # Name of script.
echo "${0##*/}" # Name of script.
echo
filename=test.data
echo "${filename##*.}" # data
# Extension of filename. |
- ${var%Pattern}, ${var%%Pattern}
Remove from $var
the shortest/longest part of $Pattern
that matches the back end
of $var.
Version 2 of Bash added
additional options. Example 9-17. Pattern matching in parameter substitution #!/bin/bash
# patt-matching.sh
# Pattern matching using the # ## % %% parameter substitution operators.
var1=abcd12345abc6789
pattern1=a*c # * (wild card) matches everything between a - c.
echo
echo "var1 = $var1" # abcd12345abc6789
echo "var1 = ${var1}" # abcd12345abc6789
# (alternate form)
echo "Number of characters in ${var1} = ${#var1}"
echo
echo "pattern1 = $pattern1" # a*c (everything between 'a' and 'c')
echo "--------------"
echo '${var1#$pattern1} =' "${var1#$pattern1}" # d12345abc6789
# Shortest possible match, strips out first 3 characters abcd12345abc6789
# ^^^^^ |-|
echo '${var1##$pattern1} =' "${var1##$pattern1}" # 6789
# Longest possible match, strips out first 12 characters abcd12345abc6789
# ^^^^^ |----------|
echo; echo; echo
pattern2=b*9 # everything between 'b' and '9'
echo "var1 = $var1" # Still abcd12345abc6789
echo
echo "pattern2 = $pattern2"
echo "--------------"
echo '${var1%pattern2} =' "${var1%$pattern2}" # abcd12345a
# Shortest possible match, strips out last 6 characters abcd12345abc6789
# ^^^^ |----|
echo '${var1%%pattern2} =' "${var1%%$pattern2}" # a
# Longest possible match, strips out last 12 characters abcd12345abc6789
# ^^^^ |-------------|
# Remember, # and ## work from the left end (beginning) of string,
# % and %% work from the right end.
echo
exit 0 |
Example 9-18. Renaming file extensions: #!/bin/bash
# rfe.sh: Renaming file extensions.
#
# rfe old_extension new_extension
#
# Example:
# To rename all *.gif files in working directory to *.jpg,
# rfe gif jpg
E_BADARGS=65
case $# in
0|1) # The vertical bar means "or" in this context.
echo "Usage: `basename $0` old_file_suffix new_file_suffix"
exit $E_BADARGS # If 0 or 1 arg, then bail out.
;;
esac
for filename in *.$1
# Traverse list of files ending with 1st argument.
do
mv $filename ${filename%$1}$2
# Strip off part of filename matching 1st argument,
#+ then append 2nd argument.
done
exit 0 |
Variable expansion / Substring
replacement These constructs have been adopted from
ksh. - ${var:pos}
Variable var expanded,
starting from offset pos.
- ${var:pos:len}
Expansion to a max of len
characters of variable var, from offset
pos. See Example A-14
for an example of the creative use of this operator.
- ${var/Pattern/Replacement}
First match of Pattern,
within var replaced with
Replacement. If Replacement is
omitted, then the first match of
Pattern is replaced by
nothing, that is, deleted. - ${var//Pattern/Replacement}
As above, if Replacement
is omitted, then all occurrences of
Pattern are replaced by
nothing, that is, deleted. Example 9-19. Using pattern matching to parse arbitrary strings #!/bin/bash
var1=abcd-1234-defg
echo "var1 = $var1"
t=${var1#*-*}
echo "var1 (with everything, up to and including first - stripped out) = $t"
# t=${var1#*-} works just the same,
#+ since # matches the shortest string,
#+ and * matches everything preceding, including an empty string.
# (Thanks, Stephane Chazelas, for pointing this out.)
t=${var1##*-*}
echo "If var1 contains a \"-\", returns empty string... var1 = $t"
t=${var1%*-*}
echo "var1 (with everything from the last - on stripped out) = $t"
echo
# -------------------------------------------
path_name=/home/bozo/ideas/thoughts.for.today
# -------------------------------------------
echo "path_name = $path_name"
t=${path_name##/*/}
echo "path_name, stripped of prefixes = $t"
# Same effect as t=`basename $path_name` in this particular case.
# t=${path_name%/}; t=${t##*/} is a more general solution,
#+ but still fails sometimes.
# If $path_name ends with a newline, then `basename $path_name` will not work,
#+ but the above expression will.
# (Thanks, S.C.)
t=${path_name%/*.*}
# Same effect as t=`dirname $path_name`
echo "path_name, stripped of suffixes = $t"
# These will fail in some cases, such as "../", "/foo////", # "foo/", "/".
# Removing suffixes, especially when the basename has no suffix,
#+ but the dirname does, also complicates matters.
# (Thanks, S.C.)
echo
t=${path_name:11}
echo "$path_name, with first 11 chars stripped off = $t"
t=${path_name:11:5}
echo "$path_name, with first 11 chars stripped off, length 5 = $t"
echo
t=${path_name/bozo/clown}
echo "$path_name with \"bozo\" replaced by \"clown\" = $t"
t=${path_name/today/}
echo "$path_name with \"today\" deleted = $t"
t=${path_name//o/O}
echo "$path_name with all o's capitalized = $t"
t=${path_name//o/}
echo "$path_name with all o's deleted = $t"
exit 0 |
- ${var/#Pattern/Replacement}
If prefix of
var matches
Pattern, then substitute
Replacement for
Pattern. - ${var/%Pattern/Replacement}
If suffix of
var matches
Pattern, then substitute
Replacement for
Pattern. Example 9-20. Matching patterns at prefix or suffix of string #!/bin/bash
# var-match.sh:
# Demo of pattern replacement at prefix / suffix of string.
v0=abc1234zip1234abc # Original variable.
echo "v0 = $v0" # abc1234zip1234abc
echo
# Match at prefix (beginning) of string.
v1=${v0/#abc/ABCDEF} # abc1234zip1234abc
# |-|
echo "v1 = $v1" # ABCDEF1234zip1234abc
# |----|
# Match at suffix (end) of string.
v2=${v0/%abc/ABCDEF} # abc1234zip123abc
# |-|
echo "v2 = $v2" # abc1234zip1234ABCDEF
# |----|
echo
# ----------------------------------------------------
# Must match at beginning / end of string,
#+ otherwise no replacement results.
# ----------------------------------------------------
v3=${v0/#123/000} # Matches, but not at beginning.
echo "v3 = $v3" # abc1234zip1234abc
# NO REPLACEMENT.
v4=${v0/%123/000} # Matches, but not at end.
echo "v4 = $v4" # abc1234zip1234abc
# NO REPLACEMENT.
exit 0 |
- ${!varprefix*}, ${!varprefix@}
Matches all previously declared variables beginning
with varprefix.
xyz23=whatever
xyz24=
a=${!xyz*} # Expands to names of declared variables beginning with "xyz".
echo "a = $a" # a = xyz23 xyz24
a=${!xyz@} # Same as above.
echo "a = $a" # a = xyz23 xyz24
# Bash, version 2.04, adds this feature. |
9.4. Typing variables: declare or
typesetThe declare or
typeset builtins (they are exact synonyms)
permit restricting the properties of variables. This is a
very weak form of the typing available in certain programming
languages. The declare command is specific
to version 2 or later of Bash. The typeset
command also works in ksh scripts. declare/typeset options - -r readonly
(declare -r var1 works the same as
readonly var1) This is the rough equivalent of the C
const type qualifier. An
attempt to change the value of a readonly variable fails with an
error message. - -i integer
declare -i number
# The script will treat subsequent occurrences of "number" as an integer.
number=3
echo "Number = $number" # Number = 3
number=three
echo "Number = $number" # Number = 0
# Tries to evaluate the string "three" as an integer. |
Certain arithmetic operations are permitted
for declared integer variables without the need
for expr or let. n=6/3
echo "n = $n" # n = 6/3
declare -i n
n=6/3
echo "n = $n" # n = 2 |
- -a array
The variable indices will be treated as
an array. - -f functions
A declare -f line with no
arguments in a script causes a listing of all the
functions previously defined in that script. A declare -f function_name
in a script lists just the function named. - -x export
This declares a variable as available for exporting outside the
environment of the script itself. - -x var=$value
The declare command permits
assigning a value to a variable in the same statement
as setting its properties.
Example 9-21. Using declare to type variables #!/bin/bash
func1 ()
{
echo This is a function.
}
declare -f # Lists the function above.
echo
declare -i var1 # var1 is an integer.
var1=2367
echo "var1 declared as $var1"
var1=var1+1 # Integer declaration eliminates the need for 'let'.
echo "var1 incremented by 1 is $var1."
# Attempt to change variable declared as integer.
echo "Attempting to change var1 to floating point value, 2367.1."
var1=2367.1 # Results in error message, with no change to variable.
echo "var1 is still $var1"
echo
declare -r var2=13.36 # 'declare' permits setting a variable property
#+ and simultaneously assigning it a value.
echo "var2 declared as $var2" # Attempt to change readonly variable.
var2=13.37 # Generates error message, and exit from script.
echo "var2 is still $var2" # This line will not execute.
exit 0 # Script will not exit here. |
 | Using the declare builtin
restricts the scope of a variable.
foo ()
{
FOO="bar"
}
bar ()
{
foo
echo $FOO
}
bar # Prints bar. |
However . . .
foo (){
declare FOO="bar"
}
bar ()
{
foo
echo $FOO
}
bar # Prints nothing.
# Thank you, Michael Iatrou, for pointing this out. |
|
9.5. Indirect References to Variables
Assume that the value of a variable is the name of a second
variable. Is it somehow possible to retrieve the value
of this second variable from the first one? For example,
if a=letter_of_alphabet
and letter_of_alphabet=z,
can a reference to a return
z? This can indeed be done, and
it is called an indirect reference. It
uses the unusual eval var1=\$$var2
notation. Example 9-22. Indirect References #!/bin/bash
# ind-ref.sh: Indirect variable referencing.
# Accessing the contents of the contents of a variable.
a=letter_of_alphabet # Variable "a" holds the name of another variable.
letter_of_alphabet=z
echo
# Direct reference.
echo "a = $a" # a = letter_of_alphabet
# Indirect reference.
eval a=\$$a
echo "Now a = $a" # Now a = z
echo
# Now, let's try changing the second-order reference.
t=table_cell_3
table_cell_3=24
echo "\"table_cell_3\" = $table_cell_3" # "table_cell_3" = 24
echo -n "dereferenced \"t\" = "; eval echo \$$t # dereferenced "t" = 24
# In this simple case, the following also works (why?).
# eval t=\$$t; echo "\"t\" = $t"
echo
t=table_cell_3
NEW_VAL=387
table_cell_3=$NEW_VAL
echo "Changing value of \"table_cell_3\" to $NEW_VAL."
echo "\"table_cell_3\" now $table_cell_3"
echo -n "dereferenced \"t\" now "; eval echo \$$t
# "eval" takes the two arguments "echo" and "\$$t" (set equal to $table_cell_3)
echo
# (Thanks, Stephane Chazelas, for clearing up the above behavior.)
# Another method is the ${!t} notation, discussed in "Bash, version 2" section.
# See also ex78.sh.
exit 0 |
Of what practical use is indirect referencing of variables? It
gives Bash a little of the functionality of
pointers in C,
for instance, in table lookup.
And, it also has some other very interesting applications. . . . Nils Radtke shows how to build "dynamic"
variable names and evaluate their contents. This can be useful
when sourcing configuration files.
#!/bin/bash
# ---------------------------------------------
# This could be "sourced" from a separate file.
isdnMyProviderRemoteNet=172.16.0.100
isdnYourProviderRemoteNet=10.0.0.10
isdnOnlineService="MyProvider"
# ---------------------------------------------
remoteNet=$(eval "echo \$$(echo isdn${isdnOnlineService}RemoteNet)")
remoteNet=$(eval "echo \$$(echo isdnMyProviderRemoteNet)")
remoteNet=$(eval "echo \$isdnMyProviderRemoteNet")
remoteNet=$(eval "echo $isdnMyProviderRemoteNet")
echo "$remoteNet" # 172.16.0.100
# ================================================================
# And, it gets even better.
# Consider the following snippet given a variable named getSparc,
#+ but no such variable getIa64:
chkMirrorArchs () {
arch="$1";
if [ "$(eval "echo \${$(echo get$(echo -ne $arch |
sed 's/^\(.\).*/\1/g' | tr 'a-z' 'A-Z'; echo $arch |
sed 's/^.\(.*\)/\1/g')):-false}")" = true ]
then
return 0;
else
return 1;
fi;
}
getSparc="true"
unset getIa64
chkMirrorArchs sparc
echo $? # 0
# True
chkMirrorArchs Ia64
echo $? # 1
# False
# Notes:
# -----
# Even the to-be-substituted variable name part is built explicitly.
# The parameters to the chkMirrorArchs calls are all lower case.
# The variable name is composed of two parts: "get" and "Sparc" . . . |
Example 9-23. Passing an indirect reference to awk #!/bin/bash
# Another version of the "column totaler" script
#+ that adds up a specified column (of numbers) in the target file.
# This one uses indirect references.
ARGS=2
E_WRONGARGS=65
if [ $# -ne "$ARGS" ] # Check for proper no. of command line args.
then
echo "Usage: `basename $0` filename column-number"
exit $E_WRONGARGS
fi
filename=$1
column_number=$2
#===== Same as original script, up to this point =====#
# A multi-line awk script is invoked by awk ' ..... '
# Begin awk script.
# ------------------------------------------------
awk "
{ total += \$${column_number} # indirect reference
}
END {
print total
}
" "$filename"
# ------------------------------------------------
# End awk script.
# Indirect variable reference avoids the hassles
#+ of referencing a shell variable within the embedded awk script.
# Thanks, Stephane Chazelas.
exit 0 |
 | This method of indirect referencing is a bit tricky.
If the second order variable changes its value, then the first
order variable must be properly dereferenced (as in the above
example). Fortunately, the
${!variable} notation introduced
with version 2 of Bash
(see Example 34-2) makes indirect referencing more
intuitive. |
9.6. $RANDOM: generate random integer$RANDOM is an internal Bash function (not a constant) that
returns a pseudorandom
integer in the range 0 - 32767. It should
not be used to generate an encryption
key. Example 9-24. Generating random numbers #!/bin/bash
# $RANDOM returns a different random integer at each invocation.
# Nominal range: 0 - 32767 (signed 16-bit integer).
MAXCOUNT=10
count=1
echo
echo "$MAXCOUNT random numbers:"
echo "-----------------"
while [ "$count" -le $MAXCOUNT ] # Generate 10 ($MAXCOUNT) random integers.
do
number=$RANDOM
echo $number
let "count += 1" # Increment count.
done
echo "-----------------"
# If you need a random int within a certain range, use the 'modulo' operator.
# This returns the remainder of a division operation.
RANGE=500
echo
number=$RANDOM
let "number %= $RANGE"
# ^^
echo "Random number less than $RANGE --- $number"
echo
# If you need a random integer greater than a lower bound,
#+ then set up a test to discard all numbers below that.
FLOOR=200
number=0 #initialize
while [ "$number" -le $FLOOR ]
do
number=$RANDOM
done
echo "Random number greater than $FLOOR --- $number"
echo
# Let's examine a simple alternative to the above loop, namely
# let "number = $RANDOM + $FLOOR"
# That would eliminate the while-loop and run faster.
# But, there might be a problem with that. What is it?
# Combine above two techniques to retrieve random number between two limits.
number=0 #initialize
while [ "$number" -le $FLOOR ]
do
number=$RANDOM
let "number %= $RANGE" # Scales $number down within $RANGE.
done
echo "Random number between $FLOOR and $RANGE --- $number"
echo
# Generate binary choice, that is, "true" or "false" value.
BINARY=2
T=1
number=$RANDOM
let "number %= $BINARY"
# Note that let "number >>= 14" gives a better random distribution
#+ (right shifts out everything except last binary digit).
if [ "$number" -eq $T ]
then
echo "TRUE"
else
echo "FALSE"
fi
echo
# Generate a toss of the dice.
SPOTS=6 # Modulo 6 gives range 0 - 5.
# Incrementing by 1 gives desired range of 1 - 6.
# Thanks, Paulo Marcel Coelho Aragao, for the simplification.
die1=0
die2=0
# Would it be better to just set SPOTS=7 and not add 1? Why or why not?
# Tosses each die separately, and so gives correct odds.
let "die1 = $RANDOM % $SPOTS +1" # Roll first one.
let "die2 = $RANDOM % $SPOTS +1" # Roll second one.
# Which arithmetic operation, above, has greater precedence --
#+ modulo (%) or addition (+)?
let "throw = $die1 + $die2"
echo "Throw of the dice = $throw"
echo
exit 0 |
Example 9-25. Picking a random card from a deck #!/bin/bash
# pick-card.sh
# This is an example of choosing random elements of an array.
# Pick a card, any card.
Suites="Clubs
Diamonds
Hearts
Spades"
Denominations="2
3
4
5
6
7
8
9
10
Jack
Queen
King
Ace"
# Note variables spread over multiple lines.
suite=($Suites) # Read into array variable.
denomination=($Denominations)
num_suites=${#suite[*]} # Count how many elements.
num_denominations=${#denomination[*]}
echo -n "${denomination[$((RANDOM%num_denominations))]} of "
echo ${suite[$((RANDOM%num_suites))]}
# $bozo sh pick-cards.sh
# Jack of Clubs
# Thank you, "jipe," for pointing out this use of $RANDOM.
exit 0 |
Jipe points out a set of techniques for
generating random numbers within a range.
# Generate random number between 6 and 30.
rnumber=$((RANDOM%25+6))
# Generate random number in the same 6 - 30 range,
#+ but the number must be evenly divisible by 3.
rnumber=$(((RANDOM%30/3+1)*3))
# Note that this will not work all the time.
# It fails if $RANDOM returns 0.
# Frank Wang suggests the following alternative:
rnumber=$(( RANDOM%27/3*3+6 )) |
Bill Gradwohl came up with an improved
formula that works for positive numbers.
rnumber=$(((RANDOM%(max-min+divisibleBy))/divisibleBy*divisibleBy+min)) |
Here Bill presents a versatile function that returns
a random number between two specified values. Example 9-26. Random between values #!/bin/bash
# random-between.sh
# Random number between two specified values.
# Script by Bill Gradwohl, with minor modifications by the document author.
# Used with permission.
randomBetween() {
# Generates a positive or negative random number
#+ between $min and $max
#+ and divisible by $divisibleBy.
# Gives a "reasonably random" distribution of return values.
#
# Bill Gradwohl - Oct 1, 2003
syntax() {
# Function embedded within function.
echo
echo "Syntax: randomBetween [min] [max] [multiple]"
echo
echo "Expects up to 3 passed parameters, but all are completely optional."
echo "min is the minimum value"
echo "max is the maximum value"
echo "multiple specifies that the answer must be a multiple of this value."
echo " i.e. answer must be evenly divisible by this number."
echo
echo "If any value is missing, defaults area supplied as: 0 32767 1"
echo "Successful completion returns 0, unsuccessful completion returns"
echo "function syntax and 1."
echo "The answer is returned in the global variable randomBetweenAnswer"
echo "Negative values for any passed parameter are handled correctly."
}
local min=${1:-0}
local max=${2:-32767}
local divisibleBy=${3:-1}
# Default values assigned, in case parameters not passed to function.
local x
local spread
# Let's make sure the divisibleBy value is positive.
[ ${divisibleBy} -lt 0 ] && divisibleBy=$((0-divisibleBy))
# Sanity check.
if [ $# -gt 3 -o ${divisibleBy} -eq 0 -o ${min} -eq ${max} ]; then
syntax
return 1
fi
# See if the min and max are reversed.
if [ ${min} -gt ${max} ]; then
# Swap them.
x=${min}
min=${max}
max=${x}
fi
# If min is itself not evenly divisible by $divisibleBy,
#+ then fix the min to be within range.
if [ $((min/divisibleBy*divisibleBy)) -ne ${min} ]; then
if [ ${min} -lt 0 ]; then
min=$((min/divisibleBy*divisibleBy))
else
min=$((((min/divisibleBy)+1)*divisibleBy))
fi
fi
# If max is itself not evenly divisible by $divisibleBy,
#+ then fix the max to be within range.
if [ $((max/divisibleBy*divisibleBy)) -ne ${max} ]; then
if [ ${max} -lt 0 ]; then
max=$((((max/divisibleBy)-1)*divisibleBy))
else
max=$((max/divisibleBy*divisibleBy))
fi
fi
# ---------------------------------------------------------------------
# Now, to do the real work.
# Note that to get a proper distribution for the end points,
#+ the range of random values has to be allowed to go between
#+ 0 and abs(max-min)+divisibleBy, not just abs(max-min)+1.
# The slight increase will produce the proper distribution for the
#+ end points.
# Changing the formula to use abs(max-min)+1 will still produce
#+ correct answers, but the randomness of those answers is faulty in
#+ that the number of times the end points ($min and $max) are returned
#+ is considerably lower than when the correct formula is used.
# ---------------------------------------------------------------------
spread=$((max-min))
[ ${spread} -lt 0 ] && spread=$((0-spread))
let spread+=divisibleBy
randomBetweenAnswer=$(((RANDOM%spread)/divisibleBy*divisibleBy+min))
return 0
# However, Paulo Marcel Coelho Aragao points out that
#+ when $max and $min are not divisible by $divisibleBy,
#+ the formula fails.
#
# He suggests instead the following formula:
# rnumber = $(((RANDOM%(max-min+1)+min)/divisibleBy*divisibleBy))
}
# Let's test the function.
min=-14
max=20
divisibleBy=3
# Generate an array of expected answers and check to make sure we get
#+ at least one of each answer if we loop long enough.
declare -a answer
minimum=${min}
maximum=${max}
if [ $((minimum/divisibleBy*divisibleBy)) -ne ${minimum} ]; then
if [ ${minimum} -lt 0 ]; then
minimum=$((minimum/divisibleBy*divisibleBy))
else
minimum=$((((minimum/divisibleBy)+1)*divisibleBy))
fi
fi
# If max is itself not evenly divisible by $divisibleBy,
#+ then fix the max to be within range.
if [ $((maximum/divisibleBy*divisibleBy)) -ne ${maximum} ]; then
if [ ${maximum} -lt 0 ]; then
maximum=$((((maximum/divisibleBy)-1)*divisibleBy))
else
maximum=$((maximum/divisibleBy*divisibleBy))
fi
fi
# We need to generate only positive array subscripts,
#+ so we need a displacement that that will guarantee
#+ positive results.
displacement=$((0-minimum))
for ((i=${minimum}; i<=${maximum}; i+=divisibleBy)); do
answer[i+displacement]=0
done
# Now loop a large number of times to see what we get.
loopIt=1000 # The script author suggests 100000,
#+ but that takes a good long while.
for ((i=0; i<${loopIt}; ++i)); do
# Note that we are specifying min and max in reversed order here to
#+ make the function correct for this case.
randomBetween ${max} ${min} ${divisibleBy}
# Report an error if an answer is unexpected.
[ ${randomBetweenAnswer} -lt ${min} -o ${randomBetweenAnswer} -gt ${max} ] && echo MIN or MAX error - ${randomBetweenAnswer}!
[ $((randomBetweenAnswer%${divisibleBy})) -ne 0 ] && echo DIVISIBLE BY error - ${randomBetweenAnswer}!
# Store the answer away statistically.
answer[randomBetweenAnswer+displacement]=$((answer[randomBetweenAnswer+displacement]+1))
done
# Let's check the results
for ((i=${minimum}; i<=${maximum}; i+=divisibleBy)); do
[ ${answer[i+displacement]} -eq 0 ] && echo "We never got an answer of $i." || echo "${i} occurred ${answer[i+displacement]} times."
done
exit 0 |
Just how random is $RANDOM? The best
way to test this is to write a script that tracks
the distribution of "random" numbers
generated by $RANDOM. Let's roll a
$RANDOM die a few times . . . Example 9-27. Rolling a single die with RANDOM #!/bin/bash
# How random is RANDOM?
RANDOM=$$ # Reseed the random number generator using script process ID.
PIPS=6 # A die has 6 pips.
MAXTHROWS=600 # Increase this if you have nothing better to do with your time.
throw=0 # Throw count.
ones=0 # Must initialize counts to zero,
twos=0 #+ since an uninitialized variable is null, not zero.
threes=0
fours=0
fives=0
sixes=0
print_result ()
{
echo
echo "ones = $ones"
echo "twos = $twos"
echo "threes = $threes"
echo "fours = $fours"
echo "fives = $fives"
echo "sixes = $sixes"
echo
}
update_count()
{
case "$1" in
0) let "ones += 1";; # Since die has no "zero", this corresponds to 1.
1) let "twos += 1";; # And this to 2, etc.
2) let "threes += 1";;
3) let "fours += 1";;
4) let "fives += 1";;
5) let "sixes += 1";;
esac
}
echo
while [ "$throw" -lt "$MAXTHROWS" ]
do
let "die1 = RANDOM % $PIPS"
update_count $die1
let "throw += 1"
done
print_result
exit 0
# The scores should distribute fairly evenly, assuming RANDOM is fairly random.
# With $MAXTHROWS at 600, all should cluster around 100, plus-or-minus 20 or so.
#
# Keep in mind that RANDOM is a pseudorandom generator,
#+ and not a spectacularly good one at that.
# Randomness is a deep and complex subject.
# Sufficiently long "random" sequences may exhibit
#+ chaotic and other "non-random" behavior.
# Exercise (easy):
# ---------------
# Rewrite this script to flip a coin 1000 times.
# Choices are "HEADS" and "TAILS". |
As we have seen in the last example, it is best to
"reseed" the RANDOM
generator each time it is invoked. Using the same seed
for RANDOM repeats the same series
of numbers.
(This mirrors the behavior of the
random() function in
C.) Example 9-28. Reseeding RANDOM #!/bin/bash
# seeding-random.sh: Seeding the RANDOM variable.
MAXCOUNT=25 # How many numbers to generate.
random_numbers ()
{
count=0
while [ "$count" -lt "$MAXCOUNT" ]
do
number=$RANDOM
echo -n "$number "
let "count += 1"
done
}
echo; echo
RANDOM=1 # Setting RANDOM seeds the random number generator.
random_numbers
echo; echo
RANDOM=1 # Same seed for RANDOM...
random_numbers # ...reproduces the exact same number series.
#
# When is it useful to duplicate a "random" number series?
echo; echo
RANDOM=2 # Trying again, but with a different seed...
random_numbers # gives a different number series.
echo; echo
# RANDOM=$$ seeds RANDOM from process id of script.
# It is also possible to seed RANDOM from 'time' or 'date' commands.
# Getting fancy...
SEED=$(head -1 /dev/urandom | od -N 1 | awk '{ print $2 }')
# Pseudo-random output fetched
#+ from /dev/urandom (system pseudo-random device-file),
#+ then converted to line of printable (octal) numbers by "od",
#+ finally "awk" retrieves just one number for SEED.
RANDOM=$SEED
random_numbers
echo; echo
exit 0 |
 | The /dev/urandom device-file provides
a method of generating much more "random"
pseudorandom numbers than the $RANDOM
variable. dd if=/dev/urandom of=targetfile
bs=1 count=XX creates a file of well-scattered
pseudorandom numbers. However, assigning these numbers
to a variable in a script requires a workaround, such as
filtering through od (as in
above example and Example 12-13), or using dd (see Example 12-55),
or even piping to md5sum
(see Example 33-14).
There are also other ways to generate pseudorandom
numbers in a script. Awk provides a
convenient means of doing this. Example 9-29. Pseudorandom numbers, using awk #!/bin/bash
# random2.sh: Returns a pseudorandom number in the range 0 - 1.
# Uses the awk rand() function.
AWKSCRIPT=' { srand(); print rand() } '
# Command(s) / parameters passed to awk
# Note that srand() reseeds awk's random number generator.
echo -n "Random number between 0 and 1 = "
echo | awk "$AWKSCRIPT"
# What happens if you leave out the 'echo'?
exit 0
# Exercises:
# ---------
# 1) Using a loop construct, print out 10 different random numbers.
# (Hint: you must reseed the "srand()" function with a different seed
#+ in each pass through the loop. What happens if you fail to do this?)
# 2) Using an integer multiplier as a scaling factor, generate random numbers
#+ in the range between 10 and 100.
# 3) Same as exercise #2, above, but generate random integers this time. |
The date command also lends
itself to generating pseudorandom
integer sequences. |
9.7. The Double Parentheses ConstructSimilar to the let command,
the ((...)) construct permits arithmetic
expansion and evaluation. In its simplest form, a=$((
5 + 3 )) would set "a" to "5 +
3", or 8. However, this double parentheses construct is
also a mechanism for allowing C-type manipulation of variables
in Bash. Example 9-30. C-type manipulation of variables #!/bin/bash
# Manipulating a variable, C-style, using the ((...)) construct.
echo
(( a = 23 )) # Setting a value, C-style, with spaces on both sides of the "=".
echo "a (initial value) = $a"
(( a++ )) # Post-increment 'a', C-style.
echo "a (after a++) = $a"
(( a-- )) # Post-decrement 'a', C-style.
echo "a (after a--) = $a"
(( ++a )) # Pre-increment 'a', C-style.
echo "a (after ++a) = $a"
(( --a )) # Pre-decrement 'a', C-style.
echo "a (after --a) = $a"
echo
########################################################
# Note that, as in C, pre- and post-decrement operators
#+ have slightly different side-effects.
n=1; let --n && echo "True" || echo "False" # False
n=1; let n-- && echo "True" || echo "False" # True
# Thanks, Jeroen Domburg.
########################################################
echo
(( t = a<45?7:11 )) # C-style trinary operator.
echo "If a < 45, then t = 7, else t = 11."
echo "t = $t " # Yes!
echo
# -----------------
# Easter Egg alert!
# -----------------
# Chet Ramey apparently snuck a bunch of undocumented C-style constructs
#+ into Bash (actually adapted from ksh, pretty much).
# In the Bash docs, Ramey calls ((...)) shell arithmetic,
#+ but it goes far beyond that.
# Sorry, Chet, the secret is now out.
# See also "for" and "while" loops using the ((...)) construct.
# These work only with Bash, version 2.04 or later.
exit 0 |
See also Example 10-12.
Chapter 10. Loops and BranchesOperations on code blocks are the key to structured and organized
shell scripts. Looping and branching constructs provide the tools for
accomplishing this.
10.1. LoopsA loop is a block of code that
iterates (repeats) a list of commands
as long as the loop control condition
is true. for loops - for arg in [list]
This is the basic looping construct. It differs significantly
from its C counterpart. for arg in [list] do command(s)... done  | During each pass through the loop,
arg takes on the
value of each successive variable in the
list. |
for arg in "$var1" "$var2" "$var3" ... "$varN"
# In pass 1 of the loop, arg = $var1
# In pass 2 of the loop, arg = $var2
# In pass 3 of the loop, arg = $var3
# ...
# In pass N of the loop, arg = $varN
# Arguments in [list] quoted to prevent possible word splitting. |
The argument list may contain wild cards. If do is on same line as
for, there needs to be a semicolon
after list. for arg in [list] ; do
Example 10-1. Simple for loops #!/bin/bash
# Listing the planets.
for planet in Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto
do
echo $planet # Each planet on a separate line.
done
echo
for planet in "Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto"
# All planets on same line.
# Entire 'list' enclosed in quotes creates a single variable.
do
echo $planet
done
exit 0 |
 | Each [list] element
may contain multiple parameters. This is useful when
processing parameters in groups. In such cases,
use the set command
(see Example 11-15) to force parsing of each
[list] element and assignment of
each component to the positional parameters. |
Example 10-2. for loop with two parameters in each
[list] element #!/bin/bash
# Planets revisited.
# Associate the name of each planet with its distance from the sun.
for planet in "Mercury 36" "Venus 67" "Earth 93" "Mars 142" "Jupiter 483"
do
set -- $planet # Parses variable "planet" and sets positional parameters.
# the "--" prevents nasty surprises if $planet is null or begins with a dash.
# May need to save original positional parameters, since they get overwritten.
# One way of doing this is to use an array,
# original_params=("$@")
echo "$1 $2,000,000 miles from the sun"
#-------two tabs---concatenate zeroes onto parameter $2
done
# (Thanks, S.C., for additional clarification.)
exit 0 |
A variable may supply the [list] in a
for loop. Example 10-3. Fileinfo: operating on a file list
contained in a variable #!/bin/bash
# fileinfo.sh
FILES="/usr/sbin/accept
/usr/sbin/pwck
/usr/sbin/chroot
/usr/bin/fakefile
/sbin/badblocks
/sbin/ypbind" # List of files you are curious about.
# Threw in a dummy file, /usr/bin/fakefile.
echo
for file in $FILES
do
if [ ! -e "$file" ] # Check if file exists.
then
echo "$file does not exist."; echo
continue # On to next.
fi
ls -l $file | awk '{ print $9 " file size: " $5 }' # Print 2 fields.
whatis `basename $file` # File info.
# Note that the whatis database needs to have been set up for this to work.
# To do this, as root run /usr/bin/makewhatis.
echo
done
exit 0 |
If the [list] in a
for loop contains wildcards
(* and ?) used in filename
expansion, then globbing
takes place. Example 10-4. Operating on files with a for loop #!/bin/bash
# list-glob.sh: Generating [list] in a for-loop, using "globbing"
echo
for file in *
# ^ Bash performs filename expansion
#+ on expressions that globbing recognizes.
do
ls -l "$file" # Lists all files in $PWD (current directory).
# Recall that the wild card character "*" matches every filename,
#+ however, in "globbing," it doesn't match dot-files.
# If the pattern matches no file, it is expanded to itself.
# To prevent this, set the nullglob option
#+ (shopt -s nullglob).
# Thanks, S.C.
done
echo; echo
for file in [jx]*
do
rm -f $file # Removes only files beginning with "j" or "x" in $PWD.
echo "Removed file \"$file\"".
done
echo
exit 0 |
Omitting the in [list] part of a
for loop causes the loop to operate
on $@ -- the list of arguments given
on the command line to the script. A particularly clever
illustration of this is Example A-16. Example 10-5. Missing in [list] in a
for loop #!/bin/bash
# Invoke this script both with and without arguments,
#+ and see what happens.
for a
do
echo -n "$a "
done
# The 'in list' missing, therefore the loop operates on '$@'
#+ (command-line argument list, including whitespace).
echo
exit 0 |
It is possible to use command substitution
to generate the [list] in a
for loop. See also Example 12-49,
Example 10-10 and Example 12-43. Example 10-6. Generating the [list] in a for
loop with command substitution #!/bin/bash
# for-loopcmd.sh: for-loop with [list]
#+ generated by command substitution.
NUMBERS="9 7 3 8 37.53"
for number in `echo $NUMBERS` # for number in 9 7 3 8 37.53
do
echo -n "$number "
done
echo
exit 0 |
Here is a somewhat more complex example of using command
substitution to create the [list]. Example 10-7. A grep replacement
for binary files #!/bin/bash
# bin-grep.sh: Locates matching strings in a binary file.
# A "grep" replacement for binary files.
# Similar effect to "grep -a"
E_BADARGS=65
E_NOFILE=66
if [ $# -ne 2 ]
then
echo "Usage: `basename $0` search_string filename"
exit $E_BADARGS
fi
if [ ! -f "$2" ]
then
echo "File \"$2\" does not exist."
exit $E_NOFILE
fi
IFS="\n" # Per suggestion of Paulo Marcel Coelho Aragao.
for word in $( strings "$2" | grep "$1" )
# The "strings" command lists strings in binary files.
# Output then piped to "grep", which tests for desired string.
do
echo $word
done
# As S.C. points out, lines 23 - 29 could be replaced with the simpler
# strings "$2" | grep "$1" | tr -s "$IFS" '[\n*]'
# Try something like "./bin-grep.sh mem /bin/ls" to exercise this script.
exit 0 |
More of the same. Example 10-8. Listing all users on the system #!/bin/bash
# userlist.sh
PASSWORD_FILE=/etc/passwd
n=1 # User number
for name in $(awk 'BEGIN{FS=":"}{print $1}' < "$PASSWORD_FILE" )
# Field separator = : ^^^^^^
# Print first field ^^^^^^^^
# Get input from password file ^^^^^^^^^^^^^^^^^
do
echo "USER #$n = $name"
let "n += 1"
done
# USER #1 = root
# USER #2 = bin
# USER #3 = daemon
# ...
# USER #30 = bozo
exit 0
# Exercise:
# --------
# How is it that an ordinary user (or a script run by same)
#+ can read /etc/passwd?
# Isn't this a security hole? Why or why not? |
A final example of the [list] resulting from command
substitution. Example 10-9. Checking all the binaries in a directory for
authorship #!/bin/bash
# findstring.sh:
# Find a particular string in binaries in a specified directory.
directory=/usr/bin/
fstring="Free Software Foundation" # See which files come from the FSF.
for file in $( find $directory -type f -name '*' | sort )
do
strings -f $file | grep "$fstring" | sed -e "s%$directory%%"
# In the "sed" expression,
#+ it is necessary to substitute for the normal "/" delimiter
#+ because "/" happens to be one of the characters filtered out.
# Failure to do so gives an error message (try it).
done
exit 0
# Exercise (easy):
# ---------------
# Convert this script to taking command-line parameters
#+ for $directory and $fstring. |
The output of a for loop may be piped to
a command or commands. Example 10-10. Listing the symbolic
links in a directory #!/bin/bash
# symlinks.sh: Lists symbolic links in a directory.
directory=${1-`pwd`}
# Defaults to current working directory,
#+ if not otherwise specified.
# Equivalent to code block below.
# ----------------------------------------------------------
# ARGS=1 # Expect one command-line argument.
#
# if [ $# -ne "$ARGS" ] # If not 1 arg...
# then
# directory=`pwd` # current working directory
# else
# directory=$1
# fi
# ----------------------------------------------------------
echo "symbolic links in directory \"$directory\""
for file in "$( find $directory -type l )" # -type l = symbolic links
do
echo "$file"
done | sort # Otherwise file list is unsorted.
# Strictly speaking, a loop isn't really necessary here,
#+ since the output of the "find" command is expanded into a single word.
# However, it's easy to understand and illustrative this way.
# As Dominik 'Aeneas' Schnitzer points out,
#+ failing to quote $( find $directory -type l )
#+ will choke on filenames with embedded whitespace.
# Even this will only pick up the first field of each argument.
exit 0
# Jean Helou proposes the following alternative:
echo "symbolic links in directory \"$directory\""
# Backup of the current IFS. One can never be too cautious.
OLDIFS=$IFS
IFS=:
for file in $(find $directory -type l -printf "%p$IFS")
do # ^^^^^^^^^^^^^^^^
echo "$file"
done|sort |
The stdout of a loop may be redirected to a file, as this slight
modification to the previous example shows. Example 10-11. Symbolic links in a directory, saved to a file #!/bin/bash
# symlinks.sh: Lists symbolic links in a directory.
OUTFILE=symlinks.list # save file
directory=${1-`pwd`}
# Defaults to current working directory,
#+ if not otherwise specified.
echo "symbolic links in directory \"$directory\"" > "$OUTFILE"
echo "---------------------------" >> "$OUTFILE"
for file in "$( find $directory -type l )" # -type l = symbolic links
do
echo "$file"
done | sort >> "$OUTFILE" # stdout of loop
# ^^^^^^^^^^^^^ redirected to save file.
exit 0 |
There is an alternative syntax to a for
loop that will look very familiar to C
programmers. This requires double parentheses. Example 10-12. A C-like for loop #!/bin/bash
# Two ways to count up to 10.
echo
# Standard syntax.
for a in 1 2 3 4 5 6 7 8 9 10
do
echo -n "$a "
done
echo; echo
# +==========================================+
# Now, let's do the same, using C-like syntax.
LIMIT=10
for ((a=1; a <= LIMIT ; a++)) # Double parentheses, and "LIMIT" with no "$".
do
echo -n "$a "
done # A construct borrowed from 'ksh93'.
echo; echo
# +=========================================================================+
# Let's use the C "comma operator" to increment two variables simultaneously.
for ((a=1, b=1; a <= LIMIT ; a++, b++)) # The comma chains together operations.
do
echo -n "$a-$b "
done
echo; echo
exit 0 |
See also Example 26-15, Example 26-16, and Example A-6. --- Now, a for loop used in a
"real-life" context. Example 10-13. Using efax in batch mode #!/bin/bash
# Faxing (must have 'fax' installed).
EXPECTED_ARGS=2
E_BADARGS=65
if [ $# -ne $EXPECTED_ARGS ]
# Check for proper no. of command line args.
then
echo "Usage: `basename $0` phone# text-file"
exit $E_BADARGS
fi
if [ ! -f "$2" ]
then
echo "File $2 is not a text file"
exit $E_BADARGS
fi
fax make $2 # Create fax formatted files from text files.
for file in $(ls $2.0*) # Concatenate the converted files.
# Uses wild card in variable list.
do
fil="$fil $file"
done
efax -d /dev/ttyS3 -o1 -t "T$1" $fil # Do the work.
# As S.C. points out, the for-loop can be eliminated with
# efax -d /dev/ttyS3 -o1 -t "T$1" $2.0*
# but it's not quite as instructive [grin].
exit 0 |
- while
This construct tests for a condition at the top of a
loop, and keeps looping as long as that condition
is true (returns a 0 exit status). In contrast
to a for loop, a
while loop finds use in situations
where the number of loop repetitions is not known
beforehand. while [condition] do command... done As is the case with for loops,
placing the do on the same line as
the condition test requires a semicolon. while [condition] ; do Note that certain specialized while
loops, as, for example, a getopts construct, deviate
somewhat from the standard template given here. Example 10-14. Simple while loop #!/bin/bash
var0=0
LIMIT=10
while [ "$var0" -lt "$LIMIT" ]
do
echo -n "$var0 " # -n suppresses newline.
# ^ Space, to separate printed out numbers.
var0=`expr $var0 + 1` # var0=$(($var0+1)) also works.
# var0=$((var0 + 1)) also works.
# let "var0 += 1" also works.
done # Various other methods also work.
echo
exit 0 |
Example 10-15. Another while loop #!/bin/bash
echo
# Equivalent to:
while [ "$var1" != "end" ] # while test "$var1" != "end"
do
echo "Input variable #1 (end to exit) "
read var1 # Not 'read $var1' (why?).
echo "variable #1 = $var1" # Need quotes because of "#" . . .
# If input is 'end', echoes it here.
# Does not test for termination condition until top of loop.
echo
done
exit 0 |
A while loop may have multiple
conditions. Only the final condition determines when the loop
terminates. This necessitates a slightly different loop syntax,
however. Example 10-16. while loop with multiple conditions #!/bin/bash
var1=unset
previous=$var1
while echo "previous-variable = $previous"
echo
previous=$var1
[ "$var1" != end ] # Keeps track of what $var1 was previously.
# Four conditions on "while", but only last one controls loop.
# The *last* exit status is the one that counts.
do
echo "Input variable #1 (end to exit) "
read var1
echo "variable #1 = $var1"
done
# Try to figure out how this all works.
# It's a wee bit tricky.
exit 0 |
As with a for loop, a
while loop may employ C-like syntax
by using the double parentheses construct (see also Example 9-30). Example 10-17. C-like syntax in a while loop #!/bin/bash
# wh-loopc.sh: Count to 10 in a "while" loop.
LIMIT=10
a=1
while [ "$a" -le $LIMIT ]
do
echo -n "$a "
let "a+=1"
done # No surprises, so far.
echo; echo
# +=================================================================+
# Now, repeat with C-like syntax.
((a = 1)) # a=1
# Double parentheses permit space when setting a variable, as in C.
while (( a <= LIMIT )) # Double parentheses, and no "$" preceding variables.
do
echo -n "$a "
((a += 1)) # let "a+=1"
# Yes, indeed.
# Double parentheses permit incrementing a variable with C-like syntax.
done
echo
# Now, C programmers can feel right at home in Bash.
exit 0 |
- until
This construct tests for a condition at the top of a loop, and keeps
looping as long as that condition is false (opposite of
while loop). until [condition-is-true] do command... done Note that an until loop tests for the
terminating condition at the top of the loop, differing from a
similar construct in some programming languages. As is the case with for loops,
placing the do on the same line as
the condition test requires a semicolon. until [condition-is-true] ; do Example 10-18. until loop #!/bin/bash
END_CONDITION=end
until [ "$var1" = "$END_CONDITION" ]
# Tests condition here, at top of loop.
do
echo "Input variable #1 "
echo "($END_CONDITION to exit)"
read var1
echo "variable #1 = $var1"
echo
done
exit 0 |
10.2. Nested LoopsA nested loop is a loop within a
loop, an inner loop within the body of an outer one. How
this works is that the first pass of the outer loop triggers
the inner loop, which executes to completion. Then the
second pass of the outer loop triggers the inner loop
again. This repeats until the outer loop finishes. Of course,
a break within either the inner or outer
loop would interrupt this process. Example 10-19. Nested Loop #!/bin/bash
# nested-loop.sh: Nested "for" loops.
outer=1 # Set outer loop counter.
# Beginning of outer loop.
for a in 1 2 3 4 5
do
echo "Pass $outer in outer loop."
echo "---------------------"
inner=1 # Reset inner loop counter.
# ===============================================
# Beginning of inner loop.
for b in 1 2 3 4 5
do
echo "Pass $inner in inner loop."
let "inner+=1" # Increment inner loop counter.
done
# End of inner loop.
# ===============================================
let "outer+=1" # Increment outer loop counter.
echo # Space between output blocks in pass of outer loop.
done
# End of outer loop.
exit 0 |
See Example 26-11 for an illustration of nested
while loops, and Example 26-13 to see a while loop nested inside an until loop.
10.3. Loop ControlCommands Affecting Loop Behavior - break, continue
The break and continue
loop control commands
correspond exactly to their counterparts in other
programming languages. The break
command terminates the loop (breaks out of it), while
continue causes a jump to the next
iteration (repetition) of the loop,
skipping all the remaining commands in that particular
loop cycle. Example 10-20. Effects of break and
continue in a loop #!/bin/bash
LIMIT=19 # Upper limit
echo
echo "Printing Numbers 1 through 20 (but not 3 and 11)."
a=0
while [ $a -le "$LIMIT" ]
do
a=$(($a+1))
if [ "$a" -eq 3 ] || [ "$a" -eq 11 ] # Excludes 3 and 11.
then
continue # Skip rest of this particular loop iteration.
fi
echo -n "$a " # This will not execute for 3 and 11.
done
# Exercise:
# Why does loop print up to 20?
echo; echo
echo Printing Numbers 1 through 20, but something happens after 2.
##################################################################
# Same loop, but substituting 'break' for 'continue'.
a=0
while [ "$a" -le "$LIMIT" ]
do
a=$(($a+1))
if [ "$a" -gt 2 ]
then
break # Skip entire rest of loop.
fi
echo -n "$a "
done
echo; echo; echo
exit 0 |
The break command may optionally take a
parameter. A plain break terminates
only the innermost loop in which it is embedded,
but a break N breaks out of
N levels of loop. Example 10-21. Breaking out of multiple loop levels #!/bin/bash
# break-levels.sh: Breaking out of loops.
# "break N" breaks out of N level loops.
for outerloop in 1 2 3 4 5
do
echo -n "Group $outerloop: "
# --------------------------------------------------------
for innerloop in 1 2 3 4 5
do
echo -n "$innerloop "
if [ "$innerloop" -eq 3 ]
then
break # Try break 2 to see what happens.
# ("Breaks" out of both inner and outer loops.)
fi
done
# --------------------------------------------------------
echo
done
echo
exit 0 |
The continue command, similar to
break, optionally takes a parameter. A
plain continue cuts short the
current iteration within its loop and begins the next.
A continue N terminates all remaining
iterations at its loop level and continues with the
next iteration at the loop, N levels
above. Example 10-22. Continuing at a higher loop level #!/bin/bash
# The "continue N" command, continuing at the Nth level loop.
for outer in I II III IV V # outer loop
do
echo; echo -n "Group $outer: "
# --------------------------------------------------------------------
for inner in 1 2 3 4 5 6 7 8 9 10 # inner loop
do
if [ "$inner" -eq 7 ]
then
continue 2 # Continue at loop on 2nd level, that is "outer loop".
# Replace above line with a simple "continue"
# to see normal loop behavior.
fi
echo -n "$inner " # 7 8 9 10 will never echo.
done
# --------------------------------------------------------------------
done
echo; echo
# Exercise:
# Come up with a meaningful use for "continue N" in a script.
exit 0 |
Example 10-23. Using "continue N" in an actual task # Albert Reiner gives an example of how to use "continue N":
# ---------------------------------------------------------
# Suppose I have a large number of jobs that need to be run, with
#+ any data that is to be treated in files of a given name pattern in a
#+ directory. There are several machines that access this directory, and
#+ I want to distribute the work over these different boxen. Then I
#+ usually nohup something like the following on every box:
while true
do
for n in .iso.*
do
[ "$n" = ".iso.opts" ] && continue
beta=${n#.iso.}
[ -r .Iso.$beta ] && continue
[ -r .lock.$beta ] && sleep 10 && continue
lockfile -r0 .lock.$beta || continue
echo -n "$beta: " `date`
run-isotherm $beta
date
ls -alF .Iso.$beta
[ -r .Iso.$beta ] && rm -f .lock.$beta
continue 2
done
break
done
# The details, in particular the sleep N, are particular to my
#+ application, but the general pattern is:
while true
do
for job in {pattern}
do
{job already done or running} && continue
{mark job as running, do job, mark job as done}
continue 2
done
break # Or something like `sleep 600' to avoid termination.
done
# This way the script will stop only when there are no more jobs to do
#+ (including jobs that were added during runtime). Through the use
#+ of appropriate lockfiles it can be run on several machines
#+ concurrently without duplication of calculations [which run a couple
#+ of hours in my case, so I really want to avoid this]. Also, as search
#+ always starts again from the beginning, one can encode priorities in
#+ the file names. Of course, one could also do this without `continue 2',
#+ but then one would have to actually check whether or not some job
#+ was done (so that we should immediately look for the next job) or not
#+ (in which case we terminate or sleep for a long time before checking
#+ for a new job). |
 | The continue N construct is
difficult to understand and tricky to use in any meaningful
context. It is probably best avoided. |
10.4. Testing and BranchingThe case and select
constructs are technically not loops, since they do not iterate the
execution of a code block. Like loops, however, they direct
program flow according to conditions at the top or bottom of
the block. Controlling program flow in a code
block - case (in) / esac
The case construct is the shell
scripting analog to switch in C/C++.
It permits branching to one of a number of code blocks,
depending on condition tests. It serves as a kind of
shorthand for multiple if/then/else
statements and is an appropriate tool for creating
menus. case "$variable" in
"$condition1" ) command... ;;
"$condition2" ) command... ;;
esac  | Quoting the variables is not mandatory, since
word splitting does not take place. Each test line ends with a right paren ). Each condition block ends with a double
semicolon ;;. The entire case block terminates with an
esac (case spelled
backwards).
|
Example 10-24. Using case #!/bin/bash
# Testing ranges of characters.
echo; echo "Hit a key, then hit return."
read Keypress
case "$Keypress" in
[[:lower:]] ) echo "Lowercase letter";;
[[:upper:]] ) echo "Uppercase letter";;
[0-9] ) echo "Digit";;
* ) echo "Punctuation, whitespace, or other";;
esac # Allows ranges of characters in [square brackets],
#+ or POSIX ranges in [[double square brackets.
# In the first version of this example,
#+ the tests for lowercase and uppercase characters were
#+ [a-z] and [A-Z].
# This no longer works in certain locales and/or Linux distros.
# POSIX is more portable.
# Thanks to Frank Wang for pointing this out.
# Exercise:
# --------
# As the script stands, it accepts a single keystroke, then terminates.
# Change the script so it accepts repeated input,
#+ reports on each keystroke, and terminates only when "X" is hit.
# Hint: enclose everything in a "while" loop.
exit 0 |
Example 10-25. Creating menus using case #!/bin/bash
# Crude address database
clear # Clear the screen.
echo " Contact List"
echo " ------- ----"
echo "Choose one of the following persons:"
echo
echo "[E]vans, Roland"
echo "[J]ones, Mildred"
echo "[S]mith, Julie"
echo "[Z]ane, Morris"
echo
read person
case "$person" in
# Note variable is quoted.
"E" | "e" )
# Accept upper or lowercase input.
echo
echo "Roland Evans"
echo "4321 Floppy Dr."
echo "Hardscrabble, CO 80753"
echo "(303) 734-9874"
echo "(303) 734-9892 fax"
echo "revans@zzy.net"
echo "Business partner & old friend"
;;
# Note double semicolon to terminate each option.
"J" | "j" )
echo
echo "Mildred Jones"
echo "249 E. 7th St., Apt. 19"
echo "New York, NY 10009"
echo "(212) 533-2814"
echo "(212) 533-9972 fax"
echo "milliej@loisaida.com"
echo "Ex-girlfriend"
echo "Birthday: Feb. 11"
;;
# Add info for Smith & Zane later.
* )
# Default option.
# Empty input (hitting RETURN) fits here, too.
echo
echo "Not yet in database."
;;
esac
echo
# Exercise:
# --------
# Change the script so it accepts multiple inputs,
#+ instead of terminating after displaying just one address.
exit 0 |
An exceptionally clever use of case
involves testing for command-line parameters.
#! /bin/bash
case "$1" in
"") echo "Usage: ${0##*/} <filename>"; exit $E_PARAM;; # No command-line parameters,
# or first parameter empty.
# Note that ${0##*/} is ${var##pattern} param substitution. Net result is $0.
-*) FILENAME=./$1;; # If filename passed as argument ($1) starts with a dash,
#+ replace it with ./$1
#+ so further commands don't interpret it as an option.
* ) FILENAME=$1;; # Otherwise, $1.
esac |
Here is an more straightforward example of
command-line parameter handling:
#! /bin/bash
while [ $# -gt 0 ]; do # Until you run out of parameters . . .
case "$1" in
-d|--debug)
# "-d" or "--debug" parameter?
DEBUG=1
;;
-c|--conf)
CONFFILE="$2"
shift
if [ ! -f $CONFFILE ]; then
echo "Error: Supplied file doesn't exist!"
exit $E_CONFFILE # File not found error.
fi
;;
esac
shift # Check next set of parameters.
done
# From Stefano Falsetto's "Log2Rot" script,
#+ part of his "rottlog" package.
# Used with permission. |
Example 10-26. Using command substitution to generate the
case variable #!/bin/bash
# case-cmd.sh: Using command substitution to generate a "case" variable.
case $( arch ) in # "arch" returns machine architecture.
# Equivalent to 'uname -m' ...
i386 ) echo "80386-based machine";;
i486 ) echo "80486-based machine";;
i586 ) echo "Pentium-based machine";;
i686 ) echo "Pentium2+-based machine";;
* ) echo "Other type of machine";;
esac
exit 0 |
A case construct can filter strings for
globbing patterns. Example 10-27. Simple string matching #!/bin/bash
# match-string.sh: simple string matching
match_string ()
{
MATCH=0
NOMATCH=90
PARAMS=2 # Function requires 2 arguments.
BAD_PARAMS=91
[ $# -eq $PARAMS ] || return $BAD_PARAMS
case "$1" in
"$2") return $MATCH;;
* ) return $NOMATCH;;
esac
}
a=one
b=two
c=three
d=two
match_string $a # wrong number of parameters
echo $? # 91
match_string $a $b # no match
echo $? # 90
match_string $b $d # match
echo $? # 0
exit 0 |
Example 10-28. Checking for alphabetic input #!/bin/bash
# isalpha.sh: Using a "case" structure to filter a string.
SUCCESS=0
FAILURE=-1
isalpha () # Tests whether *first character* of input string is alphabetic.
{
if [ -z "$1" ] # No argument passed?
then
return $FAILURE
fi
case "$1" in
[a-zA-Z]*) return $SUCCESS;; # Begins with a letter?
* ) return $FAILURE;;
esac
} # Compare this with "isalpha ()" function in C.
isalpha2 () # Tests whether *entire string* is alphabetic.
{
[ $# -eq 1 ] || return $FAILURE
case $1 in
*[!a-zA-Z]*|"") return $FAILURE;;
*) return $SUCCESS;;
esac
}
isdigit () # Tests whether *entire string* is numerical.
{ # In other words, tests for integer variable.
[ $# -eq 1 ] || return $FAILURE
case $1 in
*[!0-9]*|"") return $FAILURE;;
*) return $SUCCESS;;
esac
}
check_var () # Front-end to isalpha ().
{
if isalpha "$@"
then
echo "\"$*\" begins with an alpha character."
if isalpha2 "$@"
then # No point in testing if first char is non-alpha.
echo "\"$*\" contains only alpha characters."
else
echo "\"$*\" contains at least one non-alpha character."
fi
else
echo "\"$*\" begins with a non-alpha character."
# Also "non-alpha" if no argument passed.
fi
echo
}
digit_check () # Front-end to isdigit ().
{
if isdigit "$@"
then
echo "\"$*\" contains only digits [0 - 9]."
else
echo "\"$*\" has at least one non-digit character."
fi
echo
}
a=23skidoo
b=H3llo
c=-What?
d=What?
e=`echo $b` # Command substitution.
f=AbcDef
g=27234
h=27a34
i=27.34
check_var $a
check_var $b
check_var $c
check_var $d
check_var $e
check_var $f
check_var # No argument passed, so what happens?
#
digit_check $g
digit_check $h
digit_check $i
exit 0 # Script improved by S.C.
# Exercise:
# --------
# Write an 'isfloat ()' function that tests for floating point numbers.
# Hint: The function duplicates 'isdigit ()',
#+ but adds a test for a mandatory decimal point. |
- select
The select construct, adopted from the Korn
Shell, is yet another tool for building menus. select variable [in list] do command... break done This prompts the user to enter one of the choices presented in the
variable list. Note that select uses the
PS3 prompt (#? ) by default,
but that this may be changed. Example 10-29. Creating menus using select #!/bin/bash
PS3='Choose your favorite vegetable: ' # Sets the prompt string.
echo
select vegetable in "beans" "carrots" "potatoes" "onions" "rutabagas"
do
echo
echo "Your favorite veggie is $vegetable."
echo "Yuck!"
echo
break # What happens if there is no 'break' here?
done
exit 0 |
If in list is
omitted, then select uses the list of command
line arguments ($@) passed to the script or to
the function in which the select construct is
embedded. Compare this to the behavior of a
for variable [in list]
construct with the
in list
omitted.Example 10-30. Creating menus using select in a function #!/bin/bash
PS3='Choose your favorite vegetable: '
echo
choice_of()
{
select vegetable
# [in list] omitted, so 'select' uses arguments passed to function.
do
echo
echo "Your favorite veggie is $vegetable."
echo "Yuck!"
echo
break
done
}
choice_of beans rice carrots radishes tomatoes spinach
# $1 $2 $3 $4 $5 $6
# passed to choice_of() function
exit 0 |
See also Example 34-3.
Chapter 11. Internal Commands and BuiltinsA builtin
is a command contained within the Bash tool
set, literally built in. This is either
for performance reasons -- builtins execute faster than external
commands, which usually require forking off a separate process
-- or because a particular builtin needs direct access to the
shell internals.
A builtin may be a synonym to a system command of the same
name, but Bash reimplements it internally. For example,
the Bash echo command is not the same as
/bin/echo, although their behavior is
almost identical.
#!/bin/bash
echo "This line uses the \"echo\" builtin."
/bin/echo "This line uses the /bin/echo system command." |
A keyword
is a reserved word, token or
operator. Keywords have a special meaning to the shell,
and indeed are the building blocks of the shell's
syntax. As examples, "for",
"while", "do", and
"!" are keywords. Similar to a builtin, a keyword is hard-coded into
Bash, but unlike a builtin, a keyword is
not by itself a command, but part of a larger command structure.
I/O - echo
prints (to stdout) an expression
or variable (see Example 4-1).
An echo requires the
-e option to print escaped characters. See
Example 5-2. Normally, each echo command prints
a terminal newline, but the -n option
suppresses this.  | An echo can be used to feed a
sequence of commands down a pipe. if echo "$VAR" | grep -q txt # if [[ $VAR = *txt* ]]
then
echo "$VAR contains the substring sequence \"txt\""
fi |
|
Be aware that echo `command`
deletes any linefeeds that the output
of command
generates. The $IFS (internal field
separator) variable normally contains
\n (linefeed) as one of its set of
whitespace
characters. Bash therefore splits the output of
command at linefeeds
into arguments to echo. Then
echo outputs these arguments,
separated by spaces. bash$ ls -l /usr/share/apps/kjezz/sounds
-rw-r--r-- 1 root root 1407 Nov 7 2000 reflect.au
-rw-r--r-- 1 root root 362 Nov 7 2000 seconds.au
bash$ echo `ls -l /usr/share/apps/kjezz/sounds`
total 40 -rw-r--r-- 1 root root 716 Nov 7 2000 reflect.au -rw-r--r-- 1 root root 362 Nov 7 2000 seconds.au
|
So, how can we embed a linefeed within an
echoed character string?
# Embedding a linefeed?
echo "Why doesn't this string \n split on two lines?"
# Doesn't split.
# Let's try something else.
echo
echo $"A line of text containing
a linefeed."
# Prints as two distinct lines (embedded linefeed).
# But, is the "$" variable prefix really necessary?
echo
echo "This string splits
on two lines."
# No, the "$" is not needed.
echo
echo "---------------"
echo
echo -n $"Another line of text containing
a linefeed."
# Prints as two distinct lines (embedded linefeed).
# Even the -n option fails to suppress the linefeed here.
echo
echo
echo "---------------"
echo
echo
# However, the following doesn't work as expected.
# Why not? Hint: Assignment to a variable.
string1=$"Yet another line of text containing
a linefeed (maybe)."
echo $string1
# Yet another line of text containing a linefeed (maybe).
# ^
# Linefeed becomes a space.
# Thanks, Steve Parker, for pointing this out. |
 | This command is a shell builtin, and not the same as
/bin/echo, although its behavior is
similar. bash$ type -a echo
echo is a shell builtin
echo is /bin/echo
|
|
- printf
The printf, formatted print, command is an
enhanced echo. It is a limited variant
of the C language printf() library
function, and its syntax is somewhat different. printf format-string... parameter... This is the Bash builtin version
of the /bin/printf or
/usr/bin/printf command. See the
printf manpage (of the system command)
for in-depth coverage.  | Older versions of Bash may not support
printf. |
Example 11-2. printf in action #!/bin/bash
# printf demo
PI=3.14159265358979
DecimalConstant=31373
Message1="Greetings,"
Message2="Earthling."
echo
printf "Pi to 2 decimal places = %1.2f" $PI
echo
printf "Pi to 9 decimal places = %1.9f" $PI # It even rounds off correctly.
printf "\n" # Prints a line feed,
# Equivalent to 'echo' . . .
printf "Constant = \t%d\n" $DecimalConstant # Inserts tab (\t).
printf "%s %s \n" $Message1 $Message2
echo
# ==========================================#
# Simulation of C function, sprintf().
# Loading a variable with a formatted string.
echo
Pi12=$(printf "%1.12f" $PI)
echo "Pi to 12 decimal places = $Pi12"
Msg=`printf "%s %s \n" $Message1 $Message2`
echo $Msg; echo $Msg
# As it happens, the 'sprintf' function can now be accessed
#+ as a loadable module to Bash,
#+ but this is not portable.
exit 0 |
Formatting error messages is a useful application of
printf E_BADDIR=65
var=nonexistent_directory
error()
{
printf "$@" >&2
# Formats positional params passed, and sends them to stderr.
echo
exit $E_BADDIR
}
cd $var || error $"Can't cd to %s." "$var"
# Thanks, S.C. |
- read
"Reads" the value
of a variable from stdin, that
is, interactively fetches input from the keyboard. The
-a option lets read
get array variables (see Example 26-6). Example 11-3. Variable assignment, using read #!/bin/bash
# "Reading" variables.
echo -n "Enter the value of variable 'var1': "
# The -n option to echo suppresses newline.
read var1
# Note no '$' in front of var1, since it is being set.
echo "var1 = $var1"
echo
# A single 'read' statement can set multiple variables.
echo -n "Enter the values of variables 'var2' and 'var3' (separated by a space or tab): "
read var2 var3
echo "var2 = $var2 var3 = $var3"
# If you input only one value, the other variable(s) will remain unset (null).
exit 0 |
A read without an associated variable
assigns its input to the dedicated variable $REPLY. Example 11-4. What happens when read has no
variable #!/bin/bash
# read-novar.sh
echo
# -------------------------- #
echo -n "Enter a value: "
read var
echo "\"var\" = "$var""
# Everything as expected here.
# -------------------------- #
echo
# ------------------------------------------------------------------- #
echo -n "Enter another value: "
read # No variable supplied for 'read', therefore...
#+ Input to 'read' assigned to default variable, $REPLY.
var="$REPLY"
echo "\"var\" = "$var""
# This is equivalent to the first code block.
# ------------------------------------------------------------------- #
echo
exit 0 |
Normally, inputting a \
suppresses a newline during input to
a read. The -r
option causes an inputted \ to be
interpreted literally. Example 11-5. Multi-line input to read #!/bin/bash
echo
echo "Enter a string terminated by a \\, then press <ENTER>."
echo "Then, enter a second string, and again press <ENTER>."
read var1 # The "\" suppresses the newline, when reading $var1.
# first line \
# second line
echo "var1 = $var1"
# var1 = first line second line
# For each line terminated by a "\"
#+ you get a prompt on the next line to continue feeding characters into var1.
echo; echo
echo "Enter another string terminated by a \\ , then press <ENTER>."
read -r var2 # The -r option causes the "\" to be read literally.
# first line \
echo "var2 = $var2"
# var2 = first line \
# Data entry terminates with the first <ENTER>.
echo
exit 0 |
The read command has some interesting
options that permit echoing a prompt and even reading keystrokes
without hitting ENTER. # Read a keypress without hitting ENTER.
read -s -n1 -p "Hit a key " keypress
echo; echo "Keypress was "\"$keypress\""."
# -s option means do not echo input.
# -n N option means accept only N characters of input.
# -p option means echo the following prompt before reading input.
# Using these options is tricky, since they need to be in the correct order. |
The -n option to read
also allows detection of the arrow keys
and certain of the other unusual keys. Example 11-6. Detecting the arrow keys #!/bin/bash
# arrow-detect.sh: Detects the arrow keys, and a few more.
# Thank you, Sandro Magi, for showing me how.
# --------------------------------------------
# Character codes generated by the keypresses.
arrowup='\[A'
arrowdown='\[B'
arrowrt='\[C'
arrowleft='\[D'
insert='\[2'
delete='\[3'
# --------------------------------------------
SUCCESS=0
OTHER=65
echo -n "Press a key... "
# May need to also press ENTER if a key not listed above pressed.
read -n3 key # Read 3 characters.
echo -n "$key" | grep "$arrowup" #Check if character code detected.
if [ "$?" -eq $SUCCESS ]
then
echo "Up-arrow key pressed."
exit $SUCCESS
fi
echo -n "$key" | grep "$arrowdown"
if [ "$?" -eq $SUCCESS ]
then
echo "Down-arrow key pressed."
exit $SUCCESS
fi
echo -n "$key" | grep "$arrowrt"
if [ "$?" -eq $SUCCESS ]
then
echo "Right-arrow key pressed."
exit $SUCCESS
fi
echo -n "$key" | grep "$arrowleft"
if [ "$?" -eq $SUCCESS ]
then
echo "Left-arrow key pressed."
exit $SUCCESS
fi
echo -n "$key" | grep "$insert"
if [ "$?" -eq $SUCCESS ]
then
echo "\"Insert\" key pressed."
exit $SUCCESS
fi
echo -n "$key" | grep "$delete"
if [ "$?" -eq $SUCCESS ]
then
echo "\"Delete\" key pressed."
exit $SUCCESS
fi
echo " Some other key pressed."
exit $OTHER
# Exercises:
# ---------
# 1) Simplify this script by rewriting the multiple "if" tests
#+ as a 'case' construct.
# 2) Add detection of the "Home," "End," "PgUp," and "PgDn" keys. |
 | The -n option to read
will not detect the ENTER (newline)
key. |
The -t option to read
permits timed input (see Example 9-4). The read command may also
"read" its variable value from a file
redirected to
stdin. If the file contains
more than one line, only the first line is assigned
to the variable. If read
has more than one parameter, then each of
these variables gets assigned a successive whitespace-delineated
string. Caution! Example 11-7. Using read with
file redirection #!/bin/bash
read var1 <data-file
echo "var1 = $var1"
# var1 set to the entire first line of the input file "data-file"
read var2 var3 <data-file
echo "var2 = $var2 var3 = $var3"
# Note non-intuitive behavior of "read" here.
# 1) Rewinds back to the beginning of input file.
# 2) Each variable is now set to a corresponding string,
# separated by whitespace, rather than to an entire line of text.
# 3) The final variable gets the remainder of the line.
# 4) If there are more variables to be set than whitespace-terminated strings
# on the first line of the file, then the excess variables remain empty.
echo "------------------------------------------------"
# How to resolve the above problem with a loop:
while read line
do
echo "$line"
done <data-file
# Thanks, Heiner Steven for pointing this out.
echo "------------------------------------------------"
# Use $IFS (Internal Field Separator variable) to split a line of input to
# "read", if you do not want the default to be whitespace.
echo "List of all users:"
OIFS=$IFS; IFS=: # /etc/passwd uses ":" for field separator.
while read name passwd uid gid fullname ignore
do
echo "$name ($fullname)"
done </etc/passwd # I/O redirection.
IFS=$OIFS # Restore original $IFS.
# This code snippet also by Heiner Steven.
# Setting the $IFS variable within the loop itself
#+ eliminates the need for storing the original $IFS
#+ in a temporary variable.
# Thanks, Dim Segebart, for pointing this out.
echo "------------------------------------------------"
echo "List of all users:"
while IFS=: read name passwd uid gid fullname ignore
do
echo "$name ($fullname)"
done </etc/passwd # I/O redirection.
echo
echo "\$IFS still $IFS"
exit 0 |
 | Piping output
to a read, using echo to set variables will fail. Yet, piping the output of cat seems to
work. cat file1 file2 |
while read line
do
echo $line
done |
However, as Bjön Eriksson shows: Example 11-8. Problems reading from a pipe #!/bin/sh
# readpipe.sh
# This example contributed by Bjon Eriksson.
last="(null)"
cat $0 |
while read line
do
echo "{$line}"
last=$line
done
printf "\nAll done, last:$last\n"
exit 0 # End of code.
# (Partial) output of script follows.
# The 'echo' supplies extra brackets.
#############################################
./readpipe.sh
{#!/bin/sh}
{last="(null)"}
{cat $0 |}
{while read line}
{do}
{echo "{$line}"}
{last=$line}
{done}
{printf "nAll done, last:$lastn"}
All done, last:(null)
The variable (last) is set within the subshell but unset outside. |
The gendiff script, usually found in
/usr/bin on many Linux distros, pipes the
output of find to a
while read construct.
find $1 \( -name "*$2" -o -name ".*$2" \) -print |
while read f; do
. . . |
|
Filesystem - cd
The familiar cd change directory
command finds use in scripts where execution of a command
requires being in a specified directory.
(cd /source/directory && tar cf - . ) | (cd /dest/directory && tar xpvf -) |
[from the previously cited
example by Alan Cox]The -P (physical) option to
cd causes it to ignore symbolic
links. cd - changes to $OLDPWD, the previous working
directory.
 | The cd command does not function
as expected when presented with two forward slashes.
The output should, of course, be /.
This is a problem both from the command line and in a script. |
- pwd
Print Working Directory. This gives the user's
(or script's) current directory (see Example 11-9). The effect is identical to
reading the value of the builtin variable $PWD. - pushd, popd, dirs
This command set is a mechanism for bookmarking working directories,
a means of moving back and forth through directories in an orderly
manner. A pushdown stack is used to keep track of directory names.
Options allow various manipulations of the directory stack. pushd
dir-name pushes the path
dir-name onto the directory
stack and simultaneously changes the current working
directory to dir-name popd removes
(pops) the top directory path name off the directory stack
and simultaneously changes the current working directory
to that directory popped from the stack. dirs lists the contents of the directory
stack (compare this with the $DIRSTACK variable).
A successful pushd or
popd will automatically invoke
dirs. Scripts that require various changes to the current
working directory without hard-coding the directory name
changes can make good use of these commands. Note that
the implicit $DIRSTACK array variable,
accessible from within a script, holds the contents of
the directory stack.
Example 11-9. Changing the current working directory #!/bin/bash
dir1=/usr/local
dir2=/var/spool
pushd $dir1
# Will do an automatic 'dirs' (list directory stack to stdout).
echo "Now in directory `pwd`." # Uses back-quoted 'pwd'.
# Now, do some stuff in directory 'dir1'.
pushd $dir2
echo "Now in directory `pwd`."
# Now, do some stuff in directory 'dir2'.
echo "The top entry in the DIRSTACK array is $DIRSTACK."
popd
echo "Now back in directory `pwd`."
# Now, do some more stuff in directory 'dir1'.
popd
echo "Now back in original working directory `pwd`."
exit 0
# What happens if you don't 'popd' -- then exit the script?
# Which directory do you end up in? Why? |
Variables - let
The let command carries out arithmetic
operations on variables. In many cases, it functions as a less
complex version of expr. Example 11-10. Letting "let" do arithmetic. #!/bin/bash
echo
let a=11 # Same as 'a=11'
let a=a+5 # Equivalent to let "a = a + 5"
# (Double quotes and spaces make it more readable.)
echo "11 + 5 = $a" # 16
let "a <<= 3" # Equivalent to let "a = a << 3"
echo "\"\$a\" (=16) left-shifted 3 places = $a"
# 128
let "a /= 4" # Equivalent to let "a = a / 4"
echo "128 / 4 = $a" # 32
let "a -= 5" # Equivalent to let "a = a - 5"
echo "32 - 5 = $a" # 27
let "a *= 10" # Equivalent to let "a = a * 10"
echo "27 * 10 = $a" # 270
let "a %= 8" # Equivalent to let "a = a % 8"
echo "270 modulo 8 = $a (270 / 8 = 33, remainder $a)"
# 6
echo
exit 0 |
- eval
eval arg1 [arg2] ... [argN] Combines the arguments in an expression or list of
expressions and evaluates them. Any
variables contained within the expression are expanded. The
result translates into a command. This can be useful for
code generation from the command line or within a script. bash$ process=xterm
bash$ show_process="eval ps ax | grep $process"
bash$ $show_process
1867 tty1 S 0:02 xterm
2779 tty1 S 0:00 xterm
2886 pts/1 S 0:00 grep xterm
|
Example 11-11. Showing the effect of eval #!/bin/bash
y=`eval ls -l` # Similar to y=`ls -l`
echo $y #+ but linefeeds removed because "echoed" variable is unquoted.
echo
echo "$y" # Linefeeds preserved when variable is quoted.
echo; echo
y=`eval df` # Similar to y=`df`
echo $y #+ but linefeeds removed.
# When LF's not preserved, it may make it easier to parse output,
#+ using utilities such as "awk".
echo
echo "==========================================================="
echo
# Now, showing how to "expand" a variable using "eval" . . .
for i in 1 2 3 4 5; do
eval value=$i
# value=$i has same effect. The "eval" is not necessary here.
# A variable lacking a meta-meaning evaluates to itself --
#+ it can't expand to anything other than its literal self.
echo $value
done
echo
echo "---"
echo
for i in ls df; do
value=eval $i
# value=$i has an entirely different effect here.
# The "eval" evaluates the commands "ls" and "df" . . .
# The terms "ls" and "df" have a meta-meaning,
#+ since they are interpreted as commands,
#+ rather than just character strings.
echo $value
done
exit 0 |
Example 11-12. Forcing a log-off #!/bin/bash
# Killing ppp to force a log-off.
# Script should be run as root user.
killppp="eval kill -9 `ps ax | awk '/ppp/ { print $1 }'`"
# -------- process ID of ppp -------
$killppp # This variable is now a command.
# The following operations must be done as root user.
chmod 666 /dev/ttyS3 # Restore read+write permissions, or else what?
# Since doing a SIGKILL on ppp changed the permissions on the serial port,
#+ we restore permissions to previous state.
rm /var/lock/LCK..ttyS3 # Remove the serial port lock file. Why?
exit 0
# Exercises:
# ---------
# 1) Have script check whether root user is invoking it.
# 2) Do a check on whether the process to be killed
#+ is actually running before attempting to kill it.
# 3) Write an alternate version of this script based on 'fuser':
#+ if [ fuser -s /dev/modem ]; then . . . |
Example 11-13. A version of "rot13" #!/bin/bash
# A version of "rot13" using 'eval'.
# Compare to "rot13.sh" example.
setvar_rot_13() # "rot13" scrambling
{
local varname=$1 varvalue=$2
eval $varname='$(echo "$varvalue" | tr a-z n-za-m)'
}
setvar_rot_13 var "foobar" # Run "foobar" through rot13.
echo $var # sbbone
setvar_rot_13 var "$var" # Run "sbbone" through rot13.
# Back to original variable.
echo $var # foobar
# This example by Stephane Chazelas.
# Modified by document author.
exit 0 |
Rory Winston contributed the following instance of how
useful eval can be. Example 11-14. Using eval to force variable
substitution in a Perl script In the Perl script "test.pl":
...
my $WEBROOT = <WEBROOT_PATH>;
...
To force variable substitution try:
$export WEBROOT_PATH=/usr/local/webroot
$sed 's/<WEBROOT_PATH>/$WEBROOT_PATH/' < test.pl > out
But this just gives:
my $WEBROOT = $WEBROOT_PATH;
However:
$export WEBROOT_PATH=/usr/local/webroot
$eval sed 's%\<WEBROOT_PATH\>%$WEBROOT_PATH%' < test.pl > out
# ====
That works fine, and gives the expected substitution:
my $WEBROOT = /usr/local/webroot;
### Correction applied to original example by Paulo Marcel Coelho Aragao. |
 | The eval command can be
risky, and normally should be avoided when there
exists a reasonable alternative. An eval
$COMMANDS executes the contents of
COMMANDS, which may
contain such unpleasant surprises as rm -rf
*. Running an eval on
unfamiliar code written by persons unknown is living
dangerously. |
- set
The set command changes
the value of internal script variables. One use for
this is to toggle option
flags which help determine the behavior of the
script. Another application for it is to reset the positional parameters that
a script sees as the result of a command (set
`command`). The script can then parse the
fields of the command output. Example 11-15. Using set with positional
parameters #!/bin/bash
# script "set-test"
# Invoke this script with three command line parameters,
# for example, "./set-test one two three".
echo
echo "Positional parameters before set \`uname -a\` :"
echo "Command-line argument #1 = $1"
echo "Command-line argument #2 = $2"
echo "Command-line argument #3 = $3"
set `uname -a` # Sets the positional parameters to the output
# of the command `uname -a`
echo $_ # unknown
# Flags set in script.
echo "Positional parameters after set \`uname -a\` :"
# $1, $2, $3, etc. reinitialized to result of `uname -a`
echo "Field #1 of 'uname -a' = $1"
echo "Field #2 of 'uname -a' = $2"
echo "Field #3 of 'uname -a' = $3"
echo ---
echo $_ # ---
echo
exit 0 |
Invoking set without any options or
arguments simply lists all the environmental and other variables
that have been initialized.
bash$ set
AUTHORCOPY=/home/bozo/posts
BASH=/bin/bash
BASH_VERSION=$'2.05.8(1)-release'
...
XAUTHORITY=/home/bozo/.Xauthority
_=/etc/bashrc
variable22=abc
variable23=xzy
|
Using set with the --
option explicitly assigns the contents of a variable to
the positional parameters. When no variable follows the
--, it unsets
the positional parameters. Example 11-16. Reassigning the positional parameters #!/bin/bash
variable="one two three four five"
set -- $variable
# Sets positional parameters to the contents of "$variable".
first_param=$1
second_param=$2
shift; shift # Shift past first two positional params.
remaining_params="$*"
echo
echo "first parameter = $first_param" # one
echo "second parameter = $second_param" # two
echo "remaining parameters = $remaining_params" # three four five
echo; echo
# Again.
set -- $variable
first_param=$1
second_param=$2
echo "first parameter = $first_param" # one
echo "second parameter = $second_param" # two
# ======================================================
set --
# Unsets positional parameters if no variable specified.
first_param=$1
second_param=$2
echo "first parameter = $first_param" # (null value)
echo "second parameter = $second_param" # (null value)
exit 0 |
See also Example 10-2 and Example 12-51. - unset
The unset command deletes a
shell variable, effectively setting it to
null. Note that this command does
not affect positional parameters. bash$ unset PATH
bash$ echo $PATH
bash$ |
Example 11-17. "Unsetting" a variable #!/bin/bash
# unset.sh: Unsetting a variable.
variable=hello # Initialized.
echo "variable = $variable"
unset variable # Unset.
# Same effect as: variable=
echo "(unset) variable = $variable" # $variable is null.
exit 0 |
- export
The export command makes
available variables to all child processes of the
running script or shell. Unfortunately, there
is no way to export variables back
to the parent process, to the process that called or
invoked the script or shell. One important
use of the export command is in startup files, to initialize
and make accessible environmental
variables to subsequent user processes. Example 11-18. Using export to pass a variable to an
embedded awk script #!/bin/bash
# Yet another version of the "column totaler" script (col-totaler.sh)
#+ that adds up a specified column (of numbers) in the target file.
# This uses the environment to pass a script variable to 'awk' . . .
#+ and places the awk script in a variable.
ARGS=2
E_WRONGARGS=65
if [ $# -ne "$ARGS" ] # Check for proper no. of command line args.
then
echo "Usage: `basename $0` filename column-number"
exit $E_WRONGARGS
fi
filename=$1
column_number=$2
#===== Same as original script, up to this point =====#
export column_number
# Export column number to environment, so it's available for retrieval.
# -----------------------------------------------
awkscript='{ total += $ENVIRON["column_number"] }
END { print total }'
# Yes, a variable can hold an awk script.
# -----------------------------------------------
# Now, run the awk script.
awk "$awkscript" "$filename"
# Thanks, Stephane Chazelas.
exit 0 |
 | It is possible to initialize and export
variables in the same operation, as in export
var1=xxx. However, as Greg Keraunen points out, in certain
situations this may have a different effect than
setting a variable, then exporting it. bash$ export var=(a b); echo ${var[0]}
(a b)
bash$ var=(a b); export var; echo ${var[0]}
a
|
|
- declare, typeset
The declare and
typeset commands specify
and/or restrict properties of variables. - readonly
Same as declare -r,
sets a variable as read-only, or, in effect, as a
constant. Attempts to change the variable fail with
an error message. This is the shell analog of the
C language const
type qualifier. - getopts
This powerful tool parses command-line arguments passed
to the script. This is the Bash analog of the getopt external command and the
getopt library function familiar to
C programmers. It permits passing
and concatenating multiple options
and associated arguments to a script (for
example scriptname -abc -e
/usr/local). The getopts construct uses two implicit
variables. $OPTIND is the argument
pointer (OPTion INDex)
and $OPTARG (OPTion
ARGument) the (optional) argument attached
to an option. A colon following the option name in the
declaration tags that option as having an associated
argument. A getopts construct usually comes
packaged in a while
loop, which processes the options and
arguments one at a time, then increments the implicit
$OPTIND variable to step to the
next.  | The arguments passed from the command line to
the script must be preceded by a
minus (-). It is the
prefixed - that lets
getopts recognize command-line
arguments as options.
In fact, getopts will not process
arguments without the prefixed -,
and will terminate option processing at the first
argument encountered lacking them. The getopts template
differs slightly from the standard while
loop, in that it lacks condition brackets. The getopts construct replaces
the deprecated getopt
external command.
|
while getopts ":abcde:fg" Option
# Initial declaration.
# a, b, c, d, e, f, and g are the options (flags) expected.
# The : after option 'e' shows it will have an argument passed with it.
do
case $Option in
a ) # Do something with variable 'a'.
b ) # Do something with variable 'b'.
...
e) # Do something with 'e', and also with $OPTARG,
# which is the associated argument passed with option 'e'.
...
g ) # Do something with variable 'g'.
esac
done
shift $(($OPTIND - 1))
# Move argument pointer to next.
# All this is not nearly as complicated as it looks <grin>.
|
Example 11-19. Using getopts to read the
options/arguments passed to a script #!/bin/bash
# Exercising getopts and OPTIND
# Script modified 10/09/03 at the suggestion of Bill Gradwohl.
# Here we observe how 'getopts' processes command line arguments to script.
# The arguments are parsed as "options" (flags) and associated arguments.
# Try invoking this script with
# 'scriptname -mn'
# 'scriptname -oq qOption' (qOption can be some arbitrary string.)
# 'scriptname -qXXX -r'
#
# 'scriptname -qr' - Unexpected result, takes "r" as the argument to option "q"
# 'scriptname -q -r' - Unexpected result, same as above
# 'scriptname -mnop -mnop' - Unexpected result
# (OPTIND is unreliable at stating where an option came from).
#
# If an option expects an argument ("flag:"), then it will grab
#+ whatever is next on the command line.
NO_ARGS=0
E_OPTERROR=65
if [ $# -eq "$NO_ARGS" ] # Script invoked with no command-line args?
then
echo "Usage: `basename $0` options (-mnopqrs)"
exit $E_OPTERROR # Exit and explain usage, if no argument(s) given.
fi
# Usage: scriptname -options
# Note: dash (-) necessary
while getopts ":mnopq:rs" Option
do
case $Option in
m ) echo "Scenario #1: option -m- [OPTIND=${OPTIND}]";;
n | o ) echo "Scenario #2: option -$Option- [OPTIND=${OPTIND}]";;
p ) echo "Scenario #3: option -p- [OPTIND=${OPTIND}]";;
q ) echo "Scenario #4: option -q-\
with argument \"$OPTARG\" [OPTIND=${OPTIND}]";;
# Note that option 'q' must have an associated argument,
#+ otherwise it falls through to the default.
r | s ) echo "Scenario #5: option -$Option-";;
* ) echo "Unimplemented option chosen.";; # DEFAULT
esac
done
shift $(($OPTIND - 1))
# Decrements the argument pointer so it points to next argument.
# $1 now references the first non option item supplied on the command line
#+ if one exists.
exit 0
# As Bill Gradwohl states,
# "The getopts mechanism allows one to specify: scriptname -mnop -mnop
#+ but there is no reliable way to differentiate what came from where
#+ by using OPTIND." |
Script Behavior - source, . (dot command)
This command, when invoked from the command line,
executes a script. Within a script, a
source file-name loads the
file file-name. Sourcing a file
(dot-command) imports
code into the script, appending to the script (same effect
as the #include directive in a
C program). The net result is the
same as if the "sourced" lines of code were
physically present in the body of the script. This is useful
in situations when multiple scripts use a common data file
or function library. Example 11-20. "Including" a data file #!/bin/bash
. data-file # Load a data file.
# Same effect as "source data-file", but more portable.
# The file "data-file" must be present in current working directory,
#+ since it is referred to by its 'basename'.
# Now, reference some data from that file.
echo "variable1 (from data-file) = $variable1"
echo "variable3 (from data-file) = $variable3"
let "sum = $variable2 + $variable4"
echo "Sum of variable2 + variable4 (from data-file) = $sum"
echo "message1 (from data-file) is \"$message1\""
# Note: escaped quotes
print_message This is the message-print function in the data-file.
exit 0 |
File data-file for Example 11-20, above. Must be present in same
directory. # This is a data file loaded by a script.
# Files of this type may contain variables, functions, etc.
# It may be loaded with a 'source' or '.' command by a shell script.
# Let's initialize some variables.
variable1=22
variable2=474
variable3=5
variable4=97
message1="Hello, how are you?"
message2="Enough for now. Goodbye."
print_message ()
{
# Echoes any message passed to it.
if [ -z "$1" ]
then
return 1
# Error, if argument missing.
fi
echo
until [ -z "$1" ]
do
# Step through arguments passed to function.
echo -n "$1"
# Echo args one at a time, suppressing line feeds.
echo -n " "
# Insert spaces between words.
shift
# Next one.
done
echo
return 0
} |
If the sourced file is itself
an executable script, then it will run, then
return control to the script that called it.
A sourced executable script may use a
return for this
purpose. Arguments may be (optionally) passed to the
sourced file as positional parameters.
source $filename $arg1 arg2 |
It is even possible for a script to
source itself, though this does not
seem to have any practical applications. Example 11-21. A (useless) script that sources itself #!/bin/bash
# self-source.sh: a script sourcing itself "recursively."
# From "Stupid Script Tricks," Volume II.
MAXPASSCNT=100 # Maximum number of execution passes.
echo -n "$pass_count "
# At first execution pass, this just echoes two blank spaces,
#+ since $pass_count still uninitialized.
let "pass_count += 1"
# Assumes the uninitialized variable $pass_count
#+ can be incremented the first time around.
# This works with Bash and pdksh, but
#+ it relies on non-portable (and possibly dangerous) behavior.
# Better would be to initialize $pass_count to 0 before incrementing.
while [ "$pass_count" -le $MAXPASSCNT ]
do
. $0 # Script "sources" itself, rather than calling itself.
# ./$0 (which would be true recursion) doesn't work here. Why?
done
# What occurs here is not actually recursion,
#+ since the script effectively "expands" itself, i.e.,
#+ generates a new section of code
#+ with each pass through the 'while' loop',
# with each 'source' in line 20.
#
# Of course, the script interprets each newly 'sourced' "#!" line
#+ as a comment, and not as the start of a new script.
echo
exit 0 # The net effect is counting from 1 to 100.
# Very impressive.
# Exercise:
# --------
# Write a script that uses this trick to actually do something useful. |
- exit
Unconditionally terminates a script. The
exit command may optionally take an
integer argument, which is returned to the shell as
the exit status
of the script. It is good practice to end all but the
simplest scripts with an exit 0,
indicating a successful run.  | If a script terminates with an exit
lacking an argument, the exit status of the script is the exit
status of the last command executed in the script, not counting
the exit. This is equivalent to an
exit $?. |
- exec
This shell builtin replaces the current process with
a specified command. Normally, when the shell encounters
a command, it forks off a
child process to actually execute the command. Using the
exec builtin, the shell does not fork,
and the command exec'ed replaces the shell. When used in
a script, therefore, it forces an exit from the script when
the exec'ed command terminates.
Example 11-22. Effects of exec #!/bin/bash
exec echo "Exiting \"$0\"." # Exit from script here.
# ----------------------------------
# The following lines never execute.
echo "This echo will never echo."
exit 99 # This script will not exit here.
# Check exit value after script terminates
#+ with an 'echo $?'.
# It will *not* be 99. |
Example 11-23. A script that exec's itself #!/bin/bash
# self-exec.sh
echo
echo "This line appears ONCE in the script, yet it keeps echoing."
echo "The PID of this instance of the script is still $$."
# Demonstrates that a subshell is not forked off.
echo "==================== Hit Ctl-C to exit ===================="
sleep 1
exec $0 # Spawns another instance of this same script
#+ that replaces the previous one.
echo "This line will never echo!" # Why not?
exit 0 |
An exec also serves to reassign
file descriptors. For example, exec
<zzz-file replaces stdin
with the file zzz-file.  | The -exec option to
find is
not the same as the
exec shell builtin. |
- shopt
This command permits changing shell options on the fly (see
Example 24-1 and Example 24-2). It often
appears in the Bash startup
files, but also has its uses in scripts. Needs
version 2 or later of Bash.
shopt -s cdspell
# Allows minor misspelling of directory names with 'cd'
cd /hpme # Oops! Mistyped '/home'.
pwd # /home
# The shell corrected the misspelling. |
- caller
Putting a caller command
inside a function
echoes to stdout information about
the caller of that function. #!/bin/bash
function1 ()
{
# Inside function1 ().
caller 0 # Tell me about it.
}
function1 # Line 9 of script.
# 9 main test.sh
# ^ Line number that the function was called from.
# ^^^^ Invoked from "main" part of script.
# ^^^^^^^ Name of calling script.
caller 0 # Has no effect because it's not inside a function. |
A caller command can also return
caller information from a script sourced within another
script. Like a function, this is a "subroutine
call." You may find this command useful in debugging.
Commands - true
A command that returns a successful
(zero) exit status, but does
nothing else.
# Endless loop
while true # alias for ":"
do
operation-1
operation-2
...
operation-n
# Need a way to break out of loop or script will hang.
done |
- false
A command that returns an unsuccessful exit status,
but does nothing else. # Testing "false"
if false
then
echo "false evaluates \"true\""
else
echo "false evaluates \"false\""
fi
# false evaluates "false"
# Looping while "false" (null loop)
while false
do
# The following code will not execute.
operation-1
operation-2
...
operation-n
# Nothing happens!
done |
- type [cmd]
Similar to the which external command,
type cmd gives the full path name to
"cmd". Unlike which,
type is a Bash builtin. The useful
-a option to type
identifies keywords
and builtins, and also locates
system commands with identical names. bash$ type '['
[ is a shell builtin
bash$ type -a '['
[ is a shell builtin
[ is /usr/bin/[
|
- hash [cmds]
Record the path name of specified commands -- in the
shell hash table
-- so the shell or script will not need to search
the $PATH on subsequent calls to those
commands. When hash is called with no
arguments, it simply lists the commands that have been hashed.
The -r option resets the hash table. - bind
The bind builtin displays or modifies
readline
key bindings. - help
Gets a short usage summary of a shell builtin. This is
the counterpart to whatis,
but for builtins. bash$ help exit
exit: exit [n]
Exit the shell with a status of N. If N is omitted, the exit status
is that of the last command executed.
|
11.1. Job Control CommandsCertain of the following job control commands take a
"job identifier" as an argument. See the table at end of the chapter. - jobs
Lists the jobs running in the background, giving the job number.
Not as useful as ps.  | It is all too easy to confuse
jobs and
processes. Certain builtins, such as
kill, disown, and
wait accept either a job number or a
process number as an argument. The fg,
bg and jobs
commands accept only a job number. bash$ sleep 100 &
[1] 1384
bash $ jobs
[1]+ Running sleep 100 & |
"1" is the job number (jobs are
maintained by the current shell), and "1384"
is the process number (processes are maintained by
the system). To kill this job/process, either a kill
%1 or a kill 1384 works. Thanks, S.C. |
- disown
Remove job(s) from the shell's table of active jobs. - fg, bg
The fg command switches a job
running in the background into the foreground. The
bg command restarts a suspended job, and
runs it in the background. If no job number is specified,
then the fg or bg
command acts upon the currently running job. - wait
Stop script execution until all jobs running in
background have terminated, or until the job number or
process ID specified as an option terminates. Returns the exit status of waited-for
command. You may use the wait command
to prevent a script from exiting before a background
job finishes executing (this would create a dreaded
orphan process). Example 11-24. Waiting for a process to finish before proceeding #!/bin/bash
ROOT_UID=0 # Only users with $UID 0 have root privileges.
E_NOTROOT=65
E_NOPARAMS=66
if [ "$UID" -ne "$ROOT_UID" ]
then
echo "Must be root to run this script."
# "Run along kid, it's past your bedtime."
exit $E_NOTROOT
fi
if [ -z "$1" ]
then
echo "Usage: `basename $0` find-string"
exit $E_NOPARAMS
fi
echo "Updating 'locate' database..."
echo "This may take a while."
updatedb /usr & # Must be run as root.
wait
# Don't run the rest of the script until 'updatedb' finished.
# You want the the database updated before looking up the file name.
locate $1
# Without the 'wait' command, in the worse case scenario,
#+ the script would exit while 'updatedb' was still running,
#+ leaving it as an orphan process.
exit 0 |
Optionally, wait can take a job
identifier as an argument, for example,
wait%1 or wait
$PPID. See the job
id table.
 | Within a script, running a command in the background
with an ampersand (&) may cause the script
to hang until ENTER is hit. This
seems to occur with commands that write to
stdout. It can be a major annoyance.
#!/bin/bash
# test.sh
ls -l &
echo "Done." |
bash$ ./test.sh
Done.
[bozo@localhost test-scripts]$ total 1
-rwxr-xr-x 1 bozo bozo 34 Oct 11 15:09 test.sh
_
|
Placing a wait after the background
command seems to remedy this.
#!/bin/bash
# test.sh
ls -l &
echo "Done."
wait |
bash$ ./test.sh
Done.
[bozo@localhost test-scripts]$ total 1
-rwxr-xr-x 1 bozo bozo 34 Oct 11 15:09 test.sh
|
Redirecting the
output of the command to a file or even to
/dev/null also takes care of this
problem.
|
- suspend
This has a similar effect to
Control-Z,
but it suspends the shell (the shell's parent process should
resume it at an appropriate time). - logout
Exit a login shell, optionally specifying an exit status. - times
Gives statistics on the system time used in executing commands, in the
following form:
This capability is of very limited value, since it is uncommon to
profile and benchmark shell scripts.- kill
Forcibly terminate a process by sending it an
appropriate terminate signal (see Example 13-6). Example 11-25. A script that kills itself #!/bin/bash
# self-destruct.sh
kill $$ # Script kills its own process here.
# Recall that "$$" is the script's PID.
echo "This line will not echo."
# Instead, the shell sends a "Terminated" message to stdout.
exit 0
# After this script terminates prematurely,
#+ what exit status does it return?
#
# sh self-destruct.sh
# echo $?
# 143
#
# 143 = 128 + 15
# TERM signal |
 | kill -l lists all the
signals. A kill
-9 is a "sure kill", which will
usually terminate a process that stubbornly refuses to
die with a plain kill. Sometimes, a
kill -15 works. A "zombie
process," that is, a child process that has
terminated, but that the parent
process has not (yet) killed, cannot be killed by a
logged-on user -- you can't kill something that is already
dead -- but init will generally clean
it up sooner or later. |
- command
The command COMMAND directive
disables aliases and functions for the command
"COMMAND".  | This is one of three shell directives that
effect script command processing. The others are
builtin and enable. |
- builtin
Invoking builtin
BUILTIN_COMMAND runs the command
"BUILTIN_COMMAND" as a shell builtin, temporarily disabling
both functions and external system commands with the
same name. - enable
This either enables or disables a shell
builtin command. As an example, enable -n
kill disables the shell builtin kill, so that when Bash
subsequently encounters kill, it invokes
/bin/kill. The -a
option to enable lists all the
shell builtins, indicating whether or not they
are enabled. The -f filename
option lets enable load a builtin as a shared library
(DLL) module from a properly compiled object file.
.
- autoload
This is a port to Bash of the
ksh autoloader. With
autoload in place, a function with
an "autoload" declaration will load from an
external file at its first invocation.
This saves system resources. Note that autoload is not a part of the
core Bash installation. It needs to be loaded in with
enable -f (see above).
Table 11-1. Job identifiers Notation | Meaning |
---|
%N | Job number [N] | %S | Invocation (command line) of job begins with string S | %?S | Invocation (command line) of job contains within it string S | %% | "current" job (last job stopped in
foreground or started in background) | %+ | "current" job (last job stopped in
foreground or started in background) | %- | Last job | $! | Last background process |
Chapter 12. External Filters, Programs and Commands
Standard UNIX commands make shell scripts more versatile. The
power of scripts comes from coupling system commands and shell
directives with simple programming constructs.
12.1. Basic CommandsThe first commands a novice learns - ls
The basic file "list" command. It is all too easy
to underestimate the power of this humble command. For
example, using the -R, recursive option,
ls provides a tree-like listing of
a directory structure. Other useful options are
-S, sort listing by file size,
-t, sort by file modification time, and
-i, show file inodes (see Example 12-4). Example 12-1. Using ls to create a table of contents
for burning a CDR disk #!/bin/bash
# ex40.sh (burn-cd.sh)
# Script to automate burning a CDR.
SPEED=2 # May use higher speed if your hardware supports it.
IMAGEFILE=cdimage.iso
CONTENTSFILE=contents
DEVICE=cdrom
# DEVICE="0,0" For older versions of cdrecord
DEFAULTDIR=/opt # This is the directory containing the data to be burned.
# Make sure it exists.
# Exercise: Add a test for this.
# Uses Joerg Schilling's "cdrecord" package:
# http://www.fokus.fhg.de/usr/schilling/cdrecord.html
# If this script invoked as an ordinary user, may need to suid cdrecord
#+ chmod u+s /usr/bin/cdrecord, as root.
# Of course, this creates a security hole, though a relatively minor one.
if [ -z "$1" ]
then
IMAGE_DIRECTORY=$DEFAULTDIR
# Default directory, if not specified on command line.
else
IMAGE_DIRECTORY=$1
fi
# Create a "table of contents" file.
ls -lRF $IMAGE_DIRECTORY > $IMAGE_DIRECTORY/$CONTENTSFILE
# The "l" option gives a "long" file listing.
# The "R" option makes the listing recursive.
# The "F" option marks the file types (directories get a trailing /).
echo "Creating table of contents."
# Create an image file preparatory to burning it onto the CDR.
mkisofs -r -o $IMAGEFILE $IMAGE_DIRECTORY
echo "Creating ISO9660 file system image ($IMAGEFILE)."
# Burn the CDR.
echo "Burning the disk."
echo "Please be patient, this will take a while."
cdrecord -v -isosize speed=$SPEED dev=$DEVICE $IMAGEFILE
exit $? |
- cat, tac
cat, an acronym for
concatenate,
lists a file to stdout. When
combined with redirection (> or
>>), it is commonly used to concatenate
files.
# Uses of 'cat'
cat filename # Lists the file.
cat file.1 file.2 file.3 > file.123 # Combines three files into one. |
The -n option to cat
inserts consecutive numbers before all lines of the
target file(s). The -b option numbers
only the non-blank lines. The -v option
echoes nonprintable characters, using ^
notation. The -s option squeezes multiple
consecutive blank lines into a single blank line.See also Example 12-25 and Example 12-21.  | In a pipe, it may be
more efficient to redirect
the stdin to a file, rather than to
cat the file.
cat filename | tr a-z A-Z
tr a-z A-Z < filename # Same effect, but starts one less process,
#+ and also dispenses with the pipe. |
|
tac, is the inverse of
cat, listing a file backwards from its end. - rev
reverses each line of a file, and outputs to
stdout. This does not have the same effect
as tac, as it preserves the order of
the lines, but flips each one around. bash$ cat file1.txt
This is line 1.
This is line 2.
bash$ tac file1.txt
This is line 2.
This is line 1.
bash$ rev file1.txt
.1 enil si sihT
.2 enil si sihT
|
- cp
This is the file copy command. cp file1
file2 copies file1
to file2, overwriting
file2 if it already exists (see Example 12-6).  | Particularly useful are the -a
archive flag (for copying an entire directory tree)
and the -r and -R
recursive flags. |
- mv
This is the file move command. It
is equivalent to a combination of cp
and rm. It may be used to move multiple
files to a directory, or even to rename a directory. For
some examples of using mv in a script,
see Example 9-18 and Example A-2.  | When used in a non-interactive script,
mv takes the -f
(force) option to bypass user
input. When a directory is moved to a preexisting directory,
it becomes a subdirectory of the destination directory. bash$ mv source_directory target_directory
bash$ ls -lF target_directory
total 1
drwxrwxr-x 2 bozo bozo 1024 May 28 19:20 source_directory/
|
|
- rm
Delete (remove) a file or files. The -f
option forces removal of even readonly files, and is useful
for bypassing user input in a script.  | The rm command will, by
itself, fail to remove filenames beginning with a
dash. bash$ rm -badname
rm: invalid option -- b
Try `rm --help' for more information. |
One way to accomplish this is to preface the filename to be
removed with a dot-slash .
Another method is to precede the filename with a " -- ".
|
 | When used with the recursive flag
-r, this command removes files all the
way down the directory tree from the current directory.
A careless rm -rf * can wipe out a big
chunk of a directory structure. |
- rmdir
Remove directory. The directory must be empty of
all files -- including "invisible"
dotfiles
-- for this command to succeed. - mkdir
Make directory, creates a new directory. For example,
mkdir -p project/programs/December
creates the named directory. The
-p option automatically creates
any necessary parent directories. - chmod
Changes the attributes of an existing file (see Example 11-12). chmod +x filename
# Makes "filename" executable for all users.
chmod u+s filename
# Sets "suid" bit on "filename" permissions.
# An ordinary user may execute "filename" with same privileges as the file's owner.
# (This does not apply to shell scripts.) |
chmod 644 filename
# Makes "filename" readable/writable to owner, readable to
# others
# (octal mode). |
chmod 1777 directory-name
# Gives everyone read, write, and execute permission in directory,
# however also sets the "sticky bit".
# This means that only the owner of the directory,
# owner of the file, and, of course, root
# can delete any particular file in that directory. |
- chattr
Change file attributes. This is analogous to
chmod above, but with different options
and a different invocation syntax, and it works only on
an ext2 filesystem. One particularly interesting chattr
option is i. A chattr +i
filename marks the file
as immutable. The file cannot be modified, linked to,
or deleted , not even by root. This
file attribute can be set or removed only by root. In a
similar fashion, the a option marks the
file as append only. root# chattr +i file1.txt
root# rm file1.txt
rm: remove write-protected regular file `file1.txt'? y
rm: cannot remove `file1.txt': Operation not permitted
|
If a file has the s (secure)
attribute set, then when it is deleted its block is zeroed out
on the disk. If a file has the u (undelete)
attribute set, then when it is deleted, its contents can still
be retrieved (undeleted). If a file has the c (compress)
attribute set, then it will automatically be compressed
on writes to disk, and uncompressed on reads.  | The file attributes set with
chattr do not show in a file listing
(ls -l). |
- ln
Creates links to pre-existings files. A "link"
is a reference to a file, an alternate name for it.
The ln command permits referencing
the linked file by more than one name and is a superior
alternative to aliasing (see Example 4-6). The ln creates only a reference, a
pointer to the file only a few bytes in size.
The ln command is most often used
with the -s, symbolic or
"soft" link flag. Advantages of using the
-s flag are that it permits linking across
file systems or to directories. The syntax of the command is a bit tricky. For example:
ln -s oldfile newfile links the
previously existing oldfile to the
newly created link, newfile.  | If a file named newfile has
previously existed, an error message will
result. |
Links give the ability to invoke a script (or any other type
of executable) with multiple names, and having that script
behave according to how it was invoked. Example 12-2. Hello or Good-bye #!/bin/bash
# hello.sh: Saying "hello" or "goodbye"
#+ depending on how script is invoked.
# Make a link in current working directory ($PWD) to this script:
# ln -s hello.sh goodbye
# Now, try invoking this script both ways:
# ./hello.sh
# ./goodbye
HELLO_CALL=65
GOODBYE_CALL=66
if [ $0 = "./goodbye" ]
then
echo "Good-bye!"
# Some other goodbye-type commands, as appropriate.
exit $GOODBYE_CALL
fi
echo "Hello!"
# Some other hello-type commands, as appropriate.
exit $HELLO_CALL |
- man, info
These commands access the manual and information pages on
system commands and installed utilities. When available, the
info pages usually contain a more detailed
description than do the man pages.
12.2. Complex CommandsCommands for more advanced users - find
-exec COMMAND \; Carries out COMMAND on
each file that find matches. The
command sequence terminates with ; (the
";" is escaped to
make certain the shell passes it to find
literally, without interpreting it as a special character). bash$ find ~/ -name '*.txt'
/home/bozo/.kde/share/apps/karm/karmdata.txt
/home/bozo/misc/irmeyc.txt
/home/bozo/test-scripts/1.txt
|
If COMMAND contains
{}, then find
substitutes the full path name of the selected file for
"{}". find ~/ -name 'core*' -exec rm {} \;
# Removes all core dump files from user's home directory. |
find /home/bozo/projects -mtime 1
# Lists all files in /home/bozo/projects directory tree
#+ that were modified within the last day.
#
# mtime = last modification time of the target file
# ctime = last status change time (via 'chmod' or otherwise)
# atime = last access time
DIR=/home/bozo/junk_files
find "$DIR" -type f -atime +5 -exec rm {} \;
# ^^
# Curly brackets are placeholder for the path name output by "find."
#
# Deletes all files in "/home/bozo/junk_files"
#+ that have not been accessed in at least 5 days.
#
# "-type filetype", where
# f = regular file
# d = directory, etc.
# (The 'find' manpage has a complete listing.) |
find /etc -exec grep '[0-9][0-9]*[.][0-9][0-9]*[.][0-9][0-9]*[.][0-9][0-9]*' {} \;
# Finds all IP addresses (xxx.xxx.xxx.xxx) in /etc directory files.
# There a few extraneous hits. How can they be filtered out?
# Perhaps by:
find /etc -type f -exec cat '{}' \; | tr -c '.[:digit:]' '\n' \
| grep '^[^.][^.]*\.[^.][^.]*\.[^.][^.]*\.[^.][^.]*$'
#
# [:digit:] is one of the character classes
#+ introduced with the POSIX 1003.2 standard.
# Thanks, Stéphane Chazelas. |
 | The -exec option to
find should not be confused with the exec shell builtin. |
Example 12-3. Badname, eliminate file names
in current directory containing bad characters and whitespace. #!/bin/bash
# badname.sh
# Delete filenames in current directory containing bad characters.
for filename in *
do
badname=`echo "$filename" | sed -n /[\+\{\;\"\\\=\?~\(\)\<\>\&\*\|\$]/p`
# badname=`echo "$filename" | sed -n '/[+{;"\=?~()<>&*|$]/p'` also works.
# Deletes files containing these nasties: + { ; " \ = ? ~ ( ) < > & * | $
#
rm $badname 2>/dev/null
# ^^^^^^^^^^^ Error messages deep-sixed.
done
# Now, take care of files containing all manner of whitespace.
find . -name "* *" -exec rm -f {} \;
# The path name of the file that "find" finds replaces the "{}".
# The '\' ensures that the ';' is interpreted literally, as end of command.
exit 0
#---------------------------------------------------------------------
# Commands below this line will not execute because of "exit" command.
# An alternative to the above script:
find . -name '*[+{;"\\=?~()<>&*|$ ]*' -exec rm -f '{}' \;
# (Thanks, S.C.) |
Example 12-4. Deleting a file by its inode
number #!/bin/bash
# idelete.sh: Deleting a file by its inode number.
# This is useful when a filename starts with an illegal character,
#+ such as ? or -.
ARGCOUNT=1 # Filename arg must be passed to script.
E_WRONGARGS=70
E_FILE_NOT_EXIST=71
E_CHANGED_MIND=72
if [ $# -ne "$ARGCOUNT" ]
then
echo "Usage: `basename $0` filename"
exit $E_WRONGARGS
fi
if [ ! -e "$1" ]
then
echo "File \""$1"\" does not exist."
exit $E_FILE_NOT_EXIST
fi
inum=`ls -i | grep "$1" | awk '{print $1}'`
# inum = inode (index node) number of file
# -----------------------------------------------------------------------
# Every file has an inode, a record that holds its physical address info.
# -----------------------------------------------------------------------
echo; echo -n "Are you absolutely sure you want to delete \"$1\" (y/n)? "
# The '-v' option to 'rm' also asks this.
read answer
case "$answer" in
[nN]) echo "Changed your mind, huh?"
exit $E_CHANGED_MIND
;;
*) echo "Deleting file \"$1\".";;
esac
find . -inum $inum -exec rm {} \;
# ^^
# Curly brackets are placeholder
#+ for text output by "find."
echo "File "\"$1"\" deleted!"
exit 0 |
See Example 12-27, Example 3-4,
and Example 10-9 for scripts using
find. Its manpage provides more detail
on this complex and powerful command. - xargs
A filter for feeding arguments to a command, and also
a tool for assembling the commands themselves. It breaks
a data stream into small enough chunks for filters and
commands to process. Consider it as a powerful replacement
for backquotes.
In situations where command
substitution fails with a too
many arguments error,
substituting xargs often
works.
Normally, xargs reads from
stdin or from a pipe, but it can also
be given the output of a file. The default command for xargs is
echo. This means that input
piped to xargs may have linefeeds and
other whitespace characters stripped out.
bash$ ls -l
total 0
-rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file1
-rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file2
bash$ ls -l | xargs
total 0 -rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file1 -rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file2
bash$ find ~/mail -type f | xargs grep "Linux"
./misc:User-Agent: slrn/0.9.8.1 (Linux)
./sent-mail-jul-2005: hosted by the Linux Documentation Project.
./sent-mail-jul-2005: (Linux Documentation Project Site, rtf version)
./sent-mail-jul-2005: Subject: Criticism of Bozo's Windows/Linux article
./sent-mail-jul-2005: while mentioning that the Linux ext2/ext3 filesystem
. . .
|
ls | xargs -p -l gzip gzips every file in current
directory, one at a time, prompting before each
operation.  | An interesting xargs
option is -n NN,
which limits to NN the number
of arguments passed. ls | xargs -n 8 echo lists the files in the
current directory in 8 columns. |
 | Another useful option is
-0, in combination with find
-print0 or grep -lZ. This
allows handling arguments containing whitespace or
quotes. find / -type f -print0 | xargs -0 grep -liwZ GUI | xargs -0 rm -f
grep -rliwZ GUI / | xargs -0 rm -f
Either of the above will remove any file containing "GUI".
(Thanks, S.C.) |
Example 12-5. Logfile: Using xargs to monitor system log #!/bin/bash
# Generates a log file in current directory
# from the tail end of /var/log/messages.
# Note: /var/log/messages must be world readable
# if this script invoked by an ordinary user.
# #root chmod 644 /var/log/messages
LINES=5
( date; uname -a ) >>logfile
# Time and machine name
echo --------------------------------------------------------------------- >>logfile
tail -$LINES /var/log/messages | xargs | fmt -s >>logfile
echo >>logfile
echo >>logfile
exit 0
# Note:
# ----
# As Frank Wang points out,
#+ unmatched quotes (either single or double quotes) in the source file
#+ may give xargs indigestion.
#
# He suggests the following substitution for line 15:
# tail -$LINES /var/log/messages | tr -d "\"'" | xargs | fmt -s >>logfile
# Exercise:
# --------
# Modify this script to track changes in /var/log/messages at intervals
#+ of 20 minutes.
# Hint: Use the "watch" command. |
As in
find, a curly bracket
pair serves as a placeholder for replacement text. Example 12-6. Copying files in current directory to another #!/bin/bash
# copydir.sh
# Copy (verbose) all files in current directory ($PWD)
#+ to directory specified on command line.
E_NOARGS=65
if [ -z "$1" ] # Exit if no argument given.
then
echo "Usage: `basename $0` directory-to-copy-to"
exit $E_NOARGS
fi
ls . | xargs -i -t cp ./{} $1
# ^^ ^^ ^^
# -t is "verbose" (output command line to stderr) option.
# -i is "replace strings" option.
# {} is a placeholder for output text.
# This is similar to the use of a curly bracket pair in "find."
#
# List the files in current directory (ls .),
#+ pass the output of "ls" as arguments to "xargs" (-i -t options),
#+ then copy (cp) these arguments ({}) to new directory ($1).
#
# The net result is the exact equivalent of
#+ cp * $1
#+ unless any of the filenames has embedded "whitespace" characters.
exit 0 |
Example 12-7. Killing processes by name #!/bin/bash
# kill-byname.sh: Killing processes by name.
# Compare this script with kill-process.sh.
# For instance,
#+ try "./kill-byname.sh xterm" --
#+ and watch all the xterms on your desktop disappear.
# Warning:
# -------
# This is a fairly dangerous script.
# Running it carelessly (especially as root)
#+ can cause data loss and other undesirable effects.
E_BADARGS=66
if test -z "$1" # No command line arg supplied?
then
echo "Usage: `basename $0` Process(es)_to_kill"
exit $E_BADARGS
fi
PROCESS_NAME="$1"
ps ax | grep "$PROCESS_NAME" | awk '{print $1}' | xargs -i kill {} 2&>/dev/null
# ^^ ^^
# -----------------------------------------------------------
# Notes:
# -i is the "replace strings" option to xargs.
# The curly brackets are the placeholder for the replacement.
# 2&>/dev/null suppresses unwanted error messages.
# -----------------------------------------------------------
exit $? |
Example 12-8. Word frequency analysis
using xargs #!/bin/bash
# wf2.sh: Crude word frequency analysis on a text file.
# Uses 'xargs' to decompose lines of text into single words.
# Compare this example to the "wf.sh" script later on.
# Check for input file on command line.
ARGS=1
E_BADARGS=65
E_NOFILE=66
if [ $# -ne "$ARGS" ]
# Correct number of arguments passed to script?
then
echo "Usage: `basename $0` filename"
exit $E_BADARGS
fi
if [ ! -f "$1" ] # Check if file exists.
then
echo "File \"$1\" does not exist."
exit $E_NOFILE
fi
########################################################
cat "$1" | xargs -n1 | \
# List the file, one word per line.
tr A-Z a-z | \
# Shift characters to lowercase.
sed -e 's/\.//g' -e 's/\,//g' -e 's/ /\
/g' | \
# Filter out periods and commas, and
#+ change space between words to linefeed,
sort | uniq -c | sort -nr
# Finally prefix occurrence count and sort numerically.
########################################################
# This does the same job as the "wf.sh" example,
#+ but a bit more ponderously, and it runs more slowly (why?).
exit 0 |
- expr
All-purpose expression evaluator:
Concatenates and evaluates the arguments according
to the operation given (arguments must be separated
by spaces). Operations may be arithmetic, comparison,
string, or logical. - expr 3 + 5
returns 8 - expr 5 % 3
returns 2 - expr 1 / 0
returns the error message, expr: division by
zero Illegal arithmetic operations not allowed. - expr 5 \* 3
returns 15 The multiplication operator
must be escaped when used in an arithmetic expression
with expr. - y=`expr $y + 1`
Increment a variable, with the same effect
as let y=y+1 and
y=$(($y+1)). This is an
example of arithmetic
expansion. - z=`expr substr
$string $position $length`
Extract substring of $length characters, starting
at $position.
Example 12-9. Using expr #!/bin/bash
# Demonstrating some of the uses of 'expr'
# =======================================
echo
# Arithmetic Operators
# ---------- ---------
echo "Arithmetic Operators"
echo
a=`expr 5 + 3`
echo "5 + 3 = $a"
a=`expr $a + 1`
echo
echo "a + 1 = $a"
echo "(incrementing a variable)"
a=`expr 5 % 3`
# modulo
echo
echo "5 mod 3 = $a"
echo
echo
# Logical Operators
# ------- ---------
# Returns 1 if true, 0 if false,
#+ opposite of normal Bash convention.
echo "Logical Operators"
echo
x=24
y=25
b=`expr $x = $y` # Test equality.
echo "b = $b" # 0 ( $x -ne $y )
echo
a=3
b=`expr $a \> 10`
echo 'b=`expr $a \> 10`, therefore...'
echo "If a > 10, b = 0 (false)"
echo "b = $b" # 0 ( 3 ! -gt 10 )
echo
b=`expr $a \< 10`
echo "If a < 10, b = 1 (true)"
echo "b = $b" # 1 ( 3 -lt 10 )
echo
# Note escaping of operators.
b=`expr $a \<= 3`
echo "If a <= 3, b = 1 (true)"
echo "b = $b" # 1 ( 3 -le 3 )
# There is also a "\>=" operator (greater than or equal to).
echo
echo
# String Operators
# ------ ---------
echo "String Operators"
echo
a=1234zipper43231
echo "The string being operated upon is \"$a\"."
# length: length of string
b=`expr length $a`
echo "Length of \"$a\" is $b."
# index: position of first character in substring
# that matches a character in string
b=`expr index $a 23`
echo "Numerical position of first \"2\" in \"$a\" is \"$b\"."
# substr: extract substring, starting position & length specified
b=`expr substr $a 2 6`
echo "Substring of \"$a\", starting at position 2,\
and 6 chars long is \"$b\"."
# The default behavior of the 'match' operations is to
#+ search for the specified match at the ***beginning*** of the string.
#
# uses Regular Expressions
b=`expr match "$a" '[0-9]*'` # Numerical count.
echo Number of digits at the beginning of \"$a\" is $b.
b=`expr match "$a" '\([0-9]*\)'` # Note that escaped parentheses
# == == + trigger substring match.
echo "The digits at the beginning of \"$a\" are \"$b\"."
echo
exit 0 |
 | The : operator
can substitute for match. For example,
b=`expr $a : [0-9]*` is the
exact equivalent of b=`expr match $a
[0-9]*` in the above listing. #!/bin/bash
echo
echo "String operations using \"expr \$string : \" construct"
echo "==================================================="
echo
a=1234zipper5FLIPPER43231
echo "The string being operated upon is \"`expr "$a" : '\(.*\)'`\"."
# Escaped parentheses grouping operator. == ==
# ***************************
#+ Escaped parentheses
#+ match a substring
# ***************************
# If no escaped parentheses...
#+ then 'expr' converts the string operand to an integer.
echo "Length of \"$a\" is `expr "$a" : '.*'`." # Length of string
echo "Number of digits at the beginning of \"$a\" is `expr "$a" : '[0-9]*'`."
# ------------------------------------------------------------------------- #
echo
echo "The digits at the beginning of \"$a\" are `expr "$a" : '\([0-9]*\)'`."
# == ==
echo "The first 7 characters of \"$a\" are `expr "$a" : '\(.......\)'`."
# ===== == ==
# Again, escaped parentheses force a substring match.
#
echo "The last 7 characters of \"$a\" are `expr "$a" : '.*\(.......\)'`."
# ==== end of string operator ^^
# (actually means skip over one or more of any characters until specified
#+ substring)
echo
exit 0 |
|
The above script illustrates how
expr uses the escaped
parentheses -- \( ... \) -- grouping operator
in tandem with regular
expression parsing to match a substring.
Here is a another example, this time from "real
life."
# Strip the whitespace from the beginning and end.
LRFDATE=`expr "$LRFDATE" : '[[:space:]]*\(.*\)[[:space:]]*$'`
# From Peter Knowles' "booklistgen.sh" script
#+ for converting files to Sony Librie format.
# (http://booklistgensh.peterknowles.com) |
Perl,
sed, and awk have far superior string
parsing facilities. A short sed or
awk "subroutine" within
a script (see Section 33.2) is an attractive
alternative to expr. See Section 9.2 for more on
using expr in string operations.
12.3. Time / Date CommandsTime/date and timing - date
Simply invoked, date prints the date and
time to stdout. Where this command gets
interesting is in its formatting and parsing options. Example 12-10. Using date #!/bin/bash
# Exercising the 'date' command
echo "The number of days since the year's beginning is `date +%j`."
# Needs a leading '+' to invoke formatting.
# %j gives day of year.
echo "The number of seconds elapsed since 01/01/1970 is `date +%s`."
# %s yields number of seconds since "UNIX epoch" began,
#+ but how is this useful?
prefix=temp
suffix=$(date +%s) # The "+%s" option to 'date' is GNU-specific.
filename=$prefix.$suffix
echo $filename
# It's great for creating "unique" temp filenames,
#+ even better than using $$.
# Read the 'date' man page for more formatting options.
exit 0 |
The -u option gives the UTC (Universal
Coordinated Time). bash$ date
Fri Mar 29 21:07:39 MST 2002
bash$ date -u
Sat Mar 30 04:07:42 UTC 2002
|
The date command has quite a number of
output options. For example %N gives the
nanosecond portion of the current time. One interesting use for
this is to generate six-digit random integers.
date +%N | sed -e 's/000$//' -e 's/^0//'
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
# Strip off leading and trailing zeroes, if present. |
There are many more options (try man date).
date +%j
# Echoes day of the year (days elapsed since January 1).
date +%k%M
# Echoes hour and minute in 24-hour format, as a single digit string.
# The 'TZ' parameter permits overriding the default time zone.
date # Mon Mar 28 21:42:16 MST 2005
TZ=EST date # Mon Mar 28 23:42:16 EST 2005
# Thanks, Frank Kannemann and Pete Sjoberg, for the tip.
SixDaysAgo=$(date --date='6 days ago')
OneMonthAgo=$(date --date='1 month ago') # Four weeks back (not a month).
OneYearAgo=$(date --date='1 year ago') |
See also Example 3-4. - zdump
Time zone dump: echoes the time in a specified time zone. bash$ zdump EST
EST Tue Sep 18 22:09:22 2001 EST
|
- time
Outputs very verbose timing statistics for executing a command. time ls -l / gives something like this:
0.00user 0.01system 0:00.05elapsed 16%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (149major+27minor)pagefaults 0swaps |
See also the very similar times command in the previous
section.  | As of version 2.0
of Bash, time became a shell reserved word,
with slightly altered behavior in a pipeline. |
- touch
Utility for updating access/modification times of a
file to current system time or other specified time,
but also useful for creating a new file. The command
touch zzz will create a new file
of zero length, named zzz, assuming
that zzz did not previously exist.
Time-stamping empty files in this way is useful for
storing date information, for example in keeping track of
modification times on a project.
 | The touch command is
equivalent to : >> newfile
or >> newfile (for ordinary
files). |
- at
The at job control command executes
a given set of commands at a specified time. Superficially,
it resembles cron, however,
at is chiefly useful for one-time execution
of a command set. at 2pm January 15 prompts for a set of
commands to execute at that time. These commands should be
shell-script compatible, since, for all practical
purposes, the user is typing in an executable shell
script a line at a time. Input terminates with a Ctl-D. Using either the -f option or input
redirection (<), at
reads a command list from a file. This file is an
executable shell script, though it should, of course,
be noninteractive. Particularly clever is including the
run-parts command in
the file to execute a different set of scripts. bash$ at 2:30 am Friday < at-jobs.list
job 2 at 2000-10-27 02:30
|
- batch
The batch job control command is similar to
at, but it runs a command list when the system
load drops below .8. Like
at, it can read commands from a file with the
-f option. - cal
Prints a neatly formatted monthly calendar to
stdout. Will do current year or a large
range of past and future years. - sleep
This is the shell equivalent of a wait loop. It pauses for a
specified number of seconds, doing nothing. It can be useful
for timing or in processes running in the background,
checking for a specific event every so often (polling),
as in Example 29-6.
sleep 3 # Pauses 3 seconds. |
 | The sleep command defaults to
seconds, but minute, hours, or days may also be specified.
sleep 3 h # Pauses 3 hours! |
|
 | The watch command may
be a better choice than sleep for running
commands at timed intervals. |
- usleep
Microsleep (the "u"
may be read as the Greek "mu", or micro-
prefix). This is the same as sleep,
above, but "sleeps" in microsecond
intervals. It can be used for fine-grain timing, or for
polling an ongoing process at very frequent intervals.
usleep 30 # Pauses 30 microseconds. |
This command is part of the Red Hat initscripts /
rc-scripts package.  | The usleep command does not
provide particularly accurate timing, and is therefore
unsuitable for critical timing loops. |
- hwclock, clock
The hwclock command accesses or
adjusts the machine's hardware clock. Some
options require root privileges. The
/etc/rc.d/rc.sysinit startup file
uses hwclock to set the system time
from the hardware clock at bootup. The clock command is a synonym for
hwclock.
12.4. Text Processing CommandsCommands affecting text and
text files - sort
File sorter, often used as a filter in a pipe. This
command sorts a text stream or file forwards or backwards,
or according to various keys or character positions. Using
the -m option, it merges presorted input
files. The info page lists its many
capabilities and options. See Example 10-9,
Example 10-10, and Example A-8. - tsort
Topological sort, reading in pairs of
whitespace-separated strings and sorting according to
input patterns. - uniq
This filter removes duplicate lines from a sorted
file. It is often seen in a pipe coupled with
sort.
cat list-1 list-2 list-3 | sort | uniq > final.list
# Concatenates the list files,
# sorts them,
# removes duplicate lines,
# and finally writes the result to an output file. |
The useful -c option prefixes each line of
the input file with its number of occurrences. bash$ cat testfile
This line occurs only once.
This line occurs twice.
This line occurs twice.
This line occurs three times.
This line occurs three times.
This line occurs three times.
bash$ uniq -c testfile
1 This line occurs only once.
2 This line occurs twice.
3 This line occurs three times.
bash$ sort testfile | uniq -c | sort -nr
3 This line occurs three times.
2 This line occurs twice.
1 This line occurs only once.
|
The sort INPUTFILE | uniq -c | sort -nr
command string produces a frequency
of occurrence listing on the
INPUTFILE file (the
-nr options to sort
cause a reverse numerical sort). This template finds
use in analysis of log files and dictionary lists, and
wherever the lexical structure of a document needs to
be examined. Example 12-11. Word Frequency Analysis #!/bin/bash
# wf.sh: Crude word frequency analysis on a text file.
# This is a more efficient version of the "wf2.sh" script.
# Check for input file on command line.
ARGS=1
E_BADARGS=65
E_NOFILE=66
if [ $# -ne "$ARGS" ] # Correct number of arguments passed to script?
then
echo "Usage: `basename $0` filename"
exit $E_BADARGS
fi
if [ ! -f "$1" ] # Check if file exists.
then
echo "File \"$1\" does not exist."
exit $E_NOFILE
fi
########################################################
# main ()
sed -e 's/\.//g' -e 's/\,//g' -e 's/ /\
/g' "$1" | tr 'A-Z' 'a-z' | sort | uniq -c | sort -nr
# =========================
# Frequency of occurrence
# Filter out periods and commas, and
#+ change space between words to linefeed,
#+ then shift characters to lowercase, and
#+ finally prefix occurrence count and sort numerically.
# Arun Giridhar suggests modifying the above to:
# . . . | sort | uniq -c | sort +1 [-f] | sort +0 -nr
# This adds a secondary sort key, so instances of
#+ equal occurrence are sorted alphabetically.
# As he explains it:
# "This is effectively a radix sort, first on the
#+ least significant column
#+ (word or string, optionally case-insensitive)
#+ and last on the most significant column (frequency)."
#
# As Frank Wang explains, the above is equivalent to
#+ . . . | sort | uniq -c | sort +0 -nr
#+ and the following also works:
#+ . . . | sort | uniq -c | sort -k1nr -k
########################################################
exit 0
# Exercises:
# ---------
# 1) Add 'sed' commands to filter out other punctuation,
#+ such as semicolons.
# 2) Modify the script to also filter out multiple spaces and
# other whitespace. |
bash$ cat testfile
This line occurs only once.
This line occurs twice.
This line occurs twice.
This line occurs three times.
This line occurs three times.
This line occurs three times.
bash$ ./wf.sh testfile
6 this
6 occurs
6 line
3 times
3 three
2 twice
1 only
1 once
|
- expand, unexpand
The expand filter converts tabs to
spaces. It is often used in a pipe. The unexpand filter
converts spaces to tabs. This reverses the effect of
expand. - cut
A tool for extracting fields from files. It is similar to the
print $N command set in awk, but more limited. It may be
simpler to use cut in a script than
awk. Particularly important are the
-d (delimiter) and -f
(field specifier) options. Using cut to obtain a listing of the
mounted filesystems:
cut -d ' ' -f1,2 /etc/mtab |
Using cut to list the OS and kernel version:
uname -a | cut -d" " -f1,3,11,12 |
Using cut to extract message headers from
an e-mail folder:
bash$ grep '^Subject:' read-messages | cut -c10-80
Re: Linux suitable for mission-critical apps?
MAKE MILLIONS WORKING AT HOME!!!
Spam complaint
Re: Spam complaint |
Using cut to parse a file:
# List all the users in /etc/passwd.
FILENAME=/etc/passwd
for user in $(cut -d: -f1 $FILENAME)
do
echo $user
done
# Thanks, Oleg Philon for suggesting this. |
cut -d ' ' -f2,3 filename is equivalent to
awk -F'[ ]' '{ print $2, $3 }' filename  | It is even possible to specify a linefeed as a
delimiter. The trick is to actually embed a linefeed
(RETURN) in the command sequence. bash$ cut -d'
' -f3,7,19 testfile
This is line 3 of testfile.
This is line 7 of testfile.
This is line 19 of testfile.
|
Thank you, Jaka Kranjc, for pointing this out. |
See also Example 12-43. - paste
Tool for merging together different files into a single,
multi-column file. In combination with
cut, useful for creating system log
files.
- join
Consider this a special-purpose cousin of
paste. This powerful utility allows
merging two files in a meaningful fashion, which essentially
creates a simple version of a relational database. The join command operates on
exactly two files, but pastes together only those lines
with a common tagged field (usually a numerical label),
and writes the result to stdout.
The files to be joined should be sorted according to the
tagged field for the matchups to work properly. File: 1.data
100 Shoes
200 Laces
300 Socks |
File: 2.data
100 $40.00
200 $1.00
300 $2.00 |
bash$ join 1.data 2.data
File: 1.data 2.data
100 Shoes $40.00
200 Laces $1.00
300 Socks $2.00
|
 | The tagged field appears only once in the
output. |
- head
lists the beginning of a file to
stdout (the default is
10 lines, but this can be changed). It
has a number of interesting options.
Example 12-12. Which files are scripts? #!/bin/bash
# script-detector.sh: Detects scripts within a directory.
TESTCHARS=2 # Test first 2 characters.
SHABANG='#!' # Scripts begin with a "sha-bang."
for file in * # Traverse all the files in current directory.
do
if [[ `head -c$TESTCHARS "$file"` = "$SHABANG" ]]
# head -c2 #!
# The '-c' option to "head" outputs a specified
#+ number of characters, rather than lines (the default).
then
echo "File \"$file\" is a script."
else
echo "File \"$file\" is *not* a script."
fi
done
exit 0
# Exercises:
# ---------
# 1) Modify this script to take as an optional argument
#+ the directory to scan for scripts
#+ (rather than just the current working directory).
#
# 2) As it stands, this script gives "false positives" for
#+ Perl, awk, and other scripting language scripts.
# Correct this. |
Example 12-13. Generating 10-digit random numbers #!/bin/bash
# rnd.sh: Outputs a 10-digit random number
# Script by Stephane Chazelas.
head -c4 /dev/urandom | od -N4 -tu4 | sed -ne '1s/.* //p'
# =================================================================== #
# Analysis
# --------
# head:
# -c4 option takes first 4 bytes.
# od:
# -N4 option limits output to 4 bytes.
# -tu4 option selects unsigned decimal format for output.
# sed:
# -n option, in combination with "p" flag to the "s" command,
# outputs only matched lines.
# The author of this script explains the action of 'sed', as follows.
# head -c4 /dev/urandom | od -N4 -tu4 | sed -ne '1s/.* //p'
# ----------------------------------> |
# Assume output up to "sed" --------> |
# is 0000000 1198195154\n
# sed begins reading characters: 0000000 1198195154\n.
# Here it finds a newline character,
#+ so it is ready to process the first line (0000000 1198195154).
# It looks at its <range><action>s. The first and only one is
# range action
# 1 s/.* //p
# The line number is in the range, so it executes the action:
#+ tries to substitute the longest string ending with a space in the line
# ("0000000 ") with nothing (//), and if it succeeds, prints the result
# ("p" is a flag to the "s" command here, this is different from the "p" command).
# sed is now ready to continue reading its input. (Note that before
#+ continuing, if -n option had not been passed, sed would have printed
#+ the line once again).
# Now, sed reads the remainder of the characters, and finds the end of the file.
# It is now ready to process its 2nd line (which is also numbered '$' as
# it's the last one).
# It sees it is not matched by any <range>, so its job is done.
# In few word this sed commmand means:
# "On the first line only, remove any character up to the right-most space,
#+ then print it."
# A better way to do this would have been:
# sed -e 's/.* //;q'
# Here, two <range><action>s (could have been written
# sed -e 's/.* //' -e q):
# range action
# nothing (matches line) s/.* //
# nothing (matches line) q (quit)
# Here, sed only reads its first line of input.
# It performs both actions, and prints the line (substituted) before quitting
#+ (because of the "q" action) since the "-n" option is not passed.
# =================================================================== #
# An even simpler altenative to the above one-line script would be:
# head -c4 /dev/urandom| od -An -tu4
exit 0 |
See also Example 12-35.- tail
lists the end of a file to stdout
(the default is 10 lines). Commonly used
to keep track of changes to a system logfile, using the
-f option, which outputs lines appended
to the file. Example 12-14. Using tail to monitor the system log #!/bin/bash
filename=sys.log
cat /dev/null > $filename; echo "Creating / cleaning out file."
# Creates file if it does not already exist,
#+ and truncates it to zero length if it does.
# : > filename and > filename also work.
tail /var/log/messages > $filename
# /var/log/messages must have world read permission for this to work.
echo "$filename contains tail end of system log."
exit 0 |
 | To list a specific line of a text file,
pipe the output of
head to tail -1.
For example head -8 database.txt | tail
-1 lists the 8th line of the file
database.txt. To set a variable to a given block of a text file:
var=$(head -$m $filename | tail -$n)
# filename = name of file
# m = from beginning of file, number of lines to end of block
# n = number of lines to set variable to (trim from end of block) |
|
See also Example 12-5, Example 12-35 and
Example 29-6. - grep
A multi-purpose file search tool that uses
Regular Expressions.
It was originally a command/filter in the
venerable ed line editor:
g/re/p -- global -
regular expression - print. grep pattern [file...] Search the target file(s) for
occurrences of pattern, where
pattern may be literal text
or a Regular Expression. bash$ grep '[rst]ystem.$' osinfo.txt
The GPL governs the distribution of the Linux operating system.
|
If no target file(s) specified, grep
works as a filter on stdout, as in
a pipe. bash$ ps ax | grep clock
765 tty1 S 0:00 xclock
901 pts/1 S 0:00 grep clock
|
The -i option causes a case-insensitive
search. The -w option matches only whole
words. The -l option lists only the files in which
matches were found, but not the matching lines. The -r (recursive) option searches files in
the current working directory and all subdirectories below
it. The -n option lists the matching lines,
together with line numbers. bash$ grep -n Linux osinfo.txt
2:This is a file containing information about Linux.
6:The GPL governs the distribution of the Linux operating system.
|
The -v (or --invert-match)
option filters out matches.
grep pattern1 *.txt | grep -v pattern2
# Matches all lines in "*.txt" files containing "pattern1",
# but ***not*** "pattern2". |
The -c (--count)
option gives a numerical count of matches, rather than
actually listing the matches.
grep -c txt *.sgml # (number of occurrences of "txt" in "*.sgml" files)
# grep -cz .
# ^ dot
# means count (-c) zero-separated (-z) items matching "."
# that is, non-empty ones (containing at least 1 character).
#
printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz . # 3
printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz '$' # 5
printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz '^' # 5
#
printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -c '$' # 9
# By default, newline chars (\n) separate items to match.
# Note that the -z option is GNU "grep" specific.
# Thanks, S.C. |
When invoked with more than one target file given,
grep specifies which file contains
matches. bash$ grep Linux osinfo.txt misc.txt
osinfo.txt:This is a file containing information about Linux.
osinfo.txt:The GPL governs the distribution of the Linux operating system.
misc.txt:The Linux operating system is steadily gaining in popularity.
|
 | To force grep to show the filename
when searching only one target file, simply give
/dev/null as the second file. bash$ grep Linux osinfo.txt /dev/null
osinfo.txt:This is a file containing information about Linux.
osinfo.txt:The GPL governs the distribution of the Linux operating system.
|
|
If there is a successful match, grep
returns an exit status
of 0, which makes it useful in a condition test in a
script, especially in combination with the -q
option to suppress output.
SUCCESS=0 # if grep lookup succeeds
word=Linux
filename=data.file
grep -q "$word" "$filename" # The "-q" option causes nothing to echo to stdout.
if [ $? -eq $SUCCESS ]
# if grep -q "$word" "$filename" can replace lines 5 - 7.
then
echo "$word found in $filename"
else
echo "$word not found in $filename"
fi |
Example 29-6 demonstrates how to use
grep to search for a word pattern in
a system logfile. Example 12-15. Emulating "grep" in a script #!/bin/bash
# grp.sh: Very crude reimplementation of 'grep'.
E_BADARGS=65
if [ -z "$1" ] # Check for argument to script.
then
echo "Usage: `basename $0` pattern"
exit $E_BADARGS
fi
echo
for file in * # Traverse all files in $PWD.
do
output=$(sed -n /"$1"/p $file) # Command substitution.
if [ ! -z "$output" ] # What happens if "$output" is not quoted?
then
echo -n "$file: "
echo $output
fi # sed -ne "/$1/s|^|${file}: |p" is equivalent to above.
echo
done
echo
exit 0
# Exercises:
# ---------
# 1) Add newlines to output, if more than one match in any given file.
# 2) Add features. |
How can grep search for two (or
more) separate patterns? What if you want
grep to display all lines in a file
or files that contain both "pattern1"
and "pattern2"? One method is to pipe the result of grep
pattern1 to grep pattern2. For example, given the following file: # Filename: tstfile
This is a sample file.
This is an ordinary text file.
This file does not contain any unusual text.
This file is not unusual.
Here is some text. |
Now, let's search this file for lines containing
both "file" and
"text" . . . bash$ grep file tstfile
# Filename: tstfile
This is a sample file.
This is an ordinary text file.
This file does not contain any unusual text.
This file is not unusual.
bash$ grep file tstfile | grep text
This is an ordinary text file.
This file does not contain any unusual text. |
-- egrep
- extended grep - is the same
as grep -E. This uses a somewhat
different, extended set of Regular
Expressions, which can make the search a bit more
flexible. fgrep - fast grep
- is the same as grep -F. It does
a literal string search (no Regular Expressions), which
usually speeds things up a bit.  | On some Linux distros, egrep and
fgrep are symbolic links to, or aliases for
grep, but invoked with the
-E and -F options,
respectively. |
Example 12-16. Looking up definitions in Webster's 1913 Dictionary #!/bin/bash
# dict-lookup.sh
# This script looks up definitions in the 1913 Webster's Dictionary.
# This Public Domain dictionary is available for download
#+ from various sites, including
#+ Project Gutenberg (http://www.gutenberg.org/etext/247).
#
# Convert it from DOS to UNIX format (only LF at end of line)
#+ before using it with this script.
# Store the file in plain, uncompressed ASCII.
# Set DEFAULT_DICTFILE variable below to path/filename.
E_BADARGS=65
MAXCONTEXTLINES=50 # Maximum number of lines to show.
DEFAULT_DICTFILE="/usr/share/dict/webster1913-dict.txt"
# Default dictionary file pathname.
# Change this as necessary.
# Note:
# ----
# This particular edition of the 1913 Webster's
#+ begins each entry with an uppercase letter
#+ (lowercase for the remaining characters).
# Only the *very first line* of an entry begins this way,
#+ and that's why the search algorithm below works.
if [[ -z $(echo "$1" | sed -n '/^[A-Z]/p') ]]
# Must at least specify word to look up, and
#+ it must start with an uppercase letter.
then
echo "Usage: `basename $0` Word-to-define [dictionary-file]"
echo
echo "Note: Word to look up must start with capital letter,"
echo "with the rest of the word in lowercase."
echo "--------------------------------------------"
echo "Examples: Abandon, Dictionary, Marking, etc."
exit $E_BADARGS
fi
if [ -z "$2" ] # May specify different dictionary
#+ as an argument to this script.
then
dictfile=$DEFAULT_DICTFILE
else
dictfile="$2"
fi
# ---------------------------------------------------------
Definition=$(fgrep -A $MAXCONTEXTLINES "$1 \\" "$dictfile")
# Definitions in form "Word \..."
#
# And, yes, "fgrep" is fast enough
#+ to search even a very large text file.
# Now, snip out just the definition block.
echo "$Definition" |
sed -n '1,/^[A-Z]/p' |
# Print from first line of output
#+ to the first line of the next entry.
sed '$d' | sed '$d'
# Delete last two lines of output
#+ (blank line and first line of next entry).
# ---------------------------------------------------------
exit 0
# Exercises:
# ---------
# 1) Modify the script to accept any type of alphabetic input
# + (uppercase, lowercase, mixed case), and convert it
# + to an acceptable format for processing.
#
# 2) Convert the script to a GUI application,
# + using something like "gdialog" . . .
# The script will then no longer take its argument(s)
# + from the command line.
#
# 3) Modify the script to parse one of the other available
# + Public Domain Dictionaries, such as the U.S. Census Bureau Gazetteer. |
agrep (approximate
grep) extends the capabilities of
grep to approximate matching. The search
string may differ by a specified number of characters
from the resulting matches. This utility is not part of
the core Linux distribution.  | To search compressed files, use
zgrep, zegrep, or
zfgrep. These also work on non-compressed
files, though slower than plain grep,
egrep, fgrep.
They are handy for searching through a mixed set of files,
some compressed, some not. To search bzipped
files, use bzgrep. |
- look
The command look works like
grep, but does a lookup on
a "dictionary", a sorted word list.
By default, look searches for a match
in /usr/dict/words, but a different
dictionary file may be specified. Example 12-17. Checking words in a list for validity #!/bin/bash
# lookup: Does a dictionary lookup on each word in a data file.
file=words.data # Data file from which to read words to test.
echo
while [ "$word" != end ] # Last word in data file.
do
read word # From data file, because of redirection at end of loop.
look $word > /dev/null # Don't want to display lines in dictionary file.
lookup=$? # Exit status of 'look' command.
if [ "$lookup" -eq 0 ]
then
echo "\"$word\" is valid."
else
echo "\"$word\" is invalid."
fi
done <"$file" # Redirects stdin to $file, so "reads" come from there.
echo
exit 0
# ----------------------------------------------------------------
# Code below line will not execute because of "exit" command above.
# Stephane Chazelas proposes the following, more concise alternative:
while read word && [[ $word != end ]]
do if look "$word" > /dev/null
then echo "\"$word\" is valid."
else echo "\"$word\" is invalid."
fi
done <"$file"
exit 0 |
- sed, awk
Scripting languages especially suited for parsing text
files and command output. May be embedded singly or in
combination in pipes and shell scripts. - sed
Non-interactive "stream editor", permits using
many ex commands in batch mode. It
finds many uses in shell scripts. - awk
Programmable file extractor and formatter, good for
manipulating and/or extracting fields (columns) in
structured text files. Its syntax is similar to C. - wc
wc gives a "word count" on a file or I/O stream:
bash $ wc /usr/share/doc/sed-4.1.2/README
13 70 447 README
[13 lines 70 words 447 characters] |
wc -w gives only the word count. wc -l gives only the line count. wc -c gives only the byte count. wc -m gives only the character count. wc -L gives only the length of the longest line. Using wc to count how many
.txt files are in current working directory:
$ ls *.txt | wc -l
# Will work as long as none of the "*.txt" files have a linefeed in their name.
# Alternative ways of doing this are:
# find . -maxdepth 1 -name \*.txt -print0 | grep -cz .
# (shopt -s nullglob; set -- *.txt; echo $#)
# Thanks, S.C. |
Using wc to total up the size of all the
files whose names begin with letters in the range d - h
bash$ wc [d-h]* | grep total | awk '{print $3}'
71832
|
Using wc to count the instances of the
word "Linux" in the main source file for
this book.
bash$ grep Linux abs-book.sgml | wc -l
50
|
See also Example 12-35 and Example 16-8. Certain commands include some of the
functionality of wc as options.
... | grep foo | wc -l
# This frequently used construct can be more concisely rendered.
... | grep -c foo
# Just use the "-c" (or "--count") option of grep.
# Thanks, S.C. |
- tr
character translation filter.  | Must use quoting and/or
brackets, as appropriate. Quotes prevent the
shell from reinterpreting the special characters in
tr command sequences. Brackets should be
quoted to prevent expansion by the shell. |
Either tr "A-Z" "*" <filename
or tr A-Z \* <filename changes
all the uppercase letters in filename
to asterisks (writes to stdout).
On some systems this may not work, but tr A-Z
'[**]' will. The -d option deletes a range of
characters.
echo "abcdef" # abcdef
echo "abcdef" | tr -d b-d # aef
tr -d 0-9 <filename
# Deletes all digits from the file "filename". |
The --squeeze-repeats (or
-s) option deletes all but the
first instance of a string of consecutive characters.
This option is useful for removing excess whitespace.
bash$ echo "XXXXX" | tr --squeeze-repeats 'X'
X |
The -c "complement"
option inverts the character set to
match. With this option, tr acts only
upon those characters not matching
the specified set. bash$ echo "acfdeb123" | tr -c b-d +
+c+d+b++++ |
Note that tr recognizes POSIX character classes.
bash$ echo "abcd2ef1" | tr '[:alpha:]' -
----2--1
|
Example 12-18. toupper: Transforms a file to all uppercase. #!/bin/bash
# Changes a file to all uppercase.
E_BADARGS=65
if [ -z "$1" ] # Standard check for command line arg.
then
echo "Usage: `basename $0` filename"
exit $E_BADARGS
fi
tr a-z A-Z <"$1"
# Same effect as above, but using POSIX character set notation:
# tr '[:lower:]' '[:upper:]' <"$1"
# Thanks, S.C.
exit 0
# Exercise:
# Rewrite this script to give the option of changing a file
#+ to *either* upper or lowercase. |
Example 12-19. lowercase: Changes all filenames in working directory to lowercase. #!/bin/bash
#
# Changes every filename in working directory to all lowercase.
#
# Inspired by a script of John Dubois,
#+ which was translated into Bash by Chet Ramey,
#+ and considerably simplified by the author of the ABS Guide.
for filename in * # Traverse all files in directory.
do
fname=`basename $filename`
n=`echo $fname | tr A-Z a-z` # Change name to lowercase.
if [ "$fname" != "$n" ] # Rename only files not already lowercase.
then
mv $fname $n
fi
done
exit $?
# Code below this line will not execute because of "exit".
#--------------------------------------------------------#
# To run it, delete script above line.
# The above script will not work on filenames containing blanks or newlines.
# Stephane Chazelas therefore suggests the following alternative:
for filename in * # Not necessary to use basename,
# since "*" won't return any file containing "/".
do n=`echo "$filename/" | tr '[:upper:]' '[:lower:]'`
# POSIX char set notation.
# Slash added so that trailing newlines are not
# removed by command substitution.
# Variable substitution:
n=${n%/} # Removes trailing slash, added above, from filename.
[[ $filename == $n ]] || mv "$filename" "$n"
# Checks if filename already lowercase.
done
exit $? |
Example 12-20. Du: DOS to UNIX text file conversion. #!/bin/bash
# Du.sh: DOS to UNIX text file converter.
E_WRONGARGS=65
if [ -z "$1" ]
then
echo "Usage: `basename $0` filename-to-convert"
exit $E_WRONGARGS
fi
NEWFILENAME=$1.unx
CR='\015' # Carriage return.
# 015 is octal ASCII code for CR.
# Lines in a DOS text file end in CR-LF.
# Lines in a UNIX text file end in LF only.
tr -d $CR < $1 > $NEWFILENAME
# Delete CR's and write to new file.
echo "Original DOS text file is \"$1\"."
echo "Converted UNIX text file is \"$NEWFILENAME\"."
exit 0
# Exercise:
# --------
# Change the above script to convert from UNIX to DOS. |
Example 12-21. rot13: rot13, ultra-weak encryption. #!/bin/bash
# rot13.sh: Classic rot13 algorithm,
# encryption that might fool a 3-year old.
# Usage: ./rot13.sh filename
# or ./rot13.sh <filename
# or ./rot13.sh and supply keyboard input (stdin)
cat "$@" | tr 'a-zA-Z' 'n-za-mN-ZA-M' # "a" goes to "n", "b" to "o", etc.
# The 'cat "$@"' construction
#+ permits getting input either from stdin or from files.
exit 0 |
Example 12-22. Generating "Crypto-Quote" Puzzles #!/bin/bash
# crypto-quote.sh: Encrypt quotes
# Will encrypt famous quotes in a simple monoalphabetic substitution.
# The result is similar to the "Crypto Quote" puzzles
#+ seen in the Op Ed pages of the Sunday paper.
key=ETAOINSHRDLUBCFGJMQPVWZYXK
# The "key" is nothing more than a scrambled alphabet.
# Changing the "key" changes the encryption.
# The 'cat "$@"' construction gets input either from stdin or from files.
# If using stdin, terminate input with a Control-D.
# Otherwise, specify filename as command-line parameter.
cat "$@" | tr "a-z" "A-Z" | tr "A-Z" "$key"
# | to uppercase | encrypt
# Will work on lowercase, uppercase, or mixed-case quotes.
# Passes non-alphabetic characters through unchanged.
# Try this script with something like:
# "Nothing so needs reforming as other people's habits."
# --Mark Twain
#
# Output is:
# "CFPHRCS QF CIIOQ MINFMBRCS EQ FPHIM GIFGUI'Q HETRPQ."
# --BEML PZERC
# To reverse the encryption:
# cat "$@" | tr "$key" "A-Z"
# This simple-minded cipher can be broken by an average 12-year old
#+ using only pencil and paper.
exit 0
# Exercise:
# --------
# Modify the script so that it will either encrypt or decrypt,
#+ depending on command-line argument(s). |
- fold
A filter that wraps lines of input to a specified width.
This is especially useful with the -s
option, which breaks lines at word spaces (see Example 12-23 and Example A-1). - fmt
Simple-minded file formatter, used as a filter in a
pipe to "wrap" long lines of text
output. Example 12-23. Formatted file listing. #!/bin/bash
WIDTH=40 # 40 columns wide.
b=`ls /usr/local/bin` # Get a file listing...
echo $b | fmt -w $WIDTH
# Could also have been done by
# echo $b | fold - -s -w $WIDTH
exit 0 |
See also Example 12-5. - col
This deceptively named filter removes reverse line feeds
from an input stream. It also attempts to replace
whitespace with equivalent tabs. The chief use of
col is in filtering the output
from certain text processing utilities, such as
groff and tbl. - column
Column formatter. This filter transforms list-type
text output into a "pretty-printed" table
by inserting tabs at appropriate places. Example 12-24. Using column to format a directory
listing #!/bin/bash
# This is a slight modification of the example file in the "column" man page.
(printf "PERMISSIONS LINKS OWNER GROUP SIZE MONTH DAY HH:MM PROG-NAME\n" \
; ls -l | sed 1d) | column -t
# The "sed 1d" in the pipe deletes the first line of output,
#+ which would be "total N",
#+ where "N" is the total number of files found by "ls -l".
# The -t option to "column" pretty-prints a table.
exit 0 |
- colrm
Column removal filter. This removes columns (characters)
from a file and writes the file, lacking the range of
specified columns, back to stdout.
colrm 2 4 <filename removes the
second through fourth characters from each line of the
text file filename.  | If the file contains tabs or nonprintable
characters, this may cause unpredictable
behavior. In such cases, consider using
expand and
unexpand in a pipe preceding
colrm. |
- nl
Line numbering filter. nl filename
lists filename to
stdout, but inserts consecutive
numbers at the beginning of each non-blank line. If
filename omitted, operates on
stdin. The output of nl is very similar to
cat -n, however, by default
nl does not list blank lines. Example 12-25. nl: A self-numbering script. #!/bin/bash
# line-number.sh
# This script echoes itself twice to stdout with its lines numbered.
# 'nl' sees this as line 4 since it does not number blank lines.
# 'cat -n' sees the above line as number 6.
nl `basename $0`
echo; echo # Now, let's try it with 'cat -n'
cat -n `basename $0`
# The difference is that 'cat -n' numbers the blank lines.
# Note that 'nl -ba' will also do so.
exit 0
# ----------------------------------------------------------------- |
- pr
Print formatting filter. This will paginate files
(or stdout) into sections suitable for
hard copy printing or viewing on screen. Various options
permit row and column manipulation, joining lines, setting
margins, numbering lines, adding page headers, and merging
files, among other things. The pr
command combines much of the functionality of
nl, paste,
fold, column, and
expand. pr -o 5 --width=65 fileZZZ | more
gives a nice paginated listing to screen of
fileZZZ with margins set at 5 and
65. A particularly useful option is -d,
forcing double-spacing (same effect as sed
-G). - gettext
The GNU gettext package is a set of
utilities for localizing
and translating the text output of programs into foreign
languages. While originally intended for C programs, it
now supports quite a number of programming and scripting
languages. The gettext
program works on shell scripts. See
the info page. - msgfmt
A program for generating binary
message catalogs. It is used for localization. - iconv
A utility for converting file(s) to a different encoding
(character set). Its chief use is for localization. # Convert a string from UTF-8 to UTF-16 and print to the BookList
function write_utf8_string {
STRING=$1
BOOKLIST=$2
echo -n "$STRING" | iconv -f UTF8 -t UTF16 | cut -b 3- | tr -d \\n >> "$BOOKLIST"
}
# From Peter Knowles' "booklistgen.sh" script
#+ for converting files to Sony Librie format.
# (http://booklistgensh.peterknowles.com) |
- recode
Consider this a fancier version of
iconv, above. This very versatile utility
for converting a file to a different encoding is not part
of the standard Linux installation. - TeX, gs
TeX and Postscript
are text markup languages used for preparing copy for
printing or formatted video display. TeX is Donald Knuth's elaborate
typsetting system. It is often convenient to write a
shell script encapsulating all the options and arguments
passed to one of these markup languages. Ghostscript
(gs) is a GPL-ed Postscript
interpreter. - enscript
Utility for converting plain text file to PostScript For example, enscript filename.txt -p filename.ps
produces the PostScript output file
filename.ps. - groff, tbl, eqn
Yet another text markup and display formatting language
is groff. This is the enhanced GNU version
of the venerable UNIX roff/troff display
and typesetting package. Manpages
use groff. The tbl table processing utility
is considered part of groff, as its
function is to convert table markup into
groff commands. The eqn equation processing utility
is likewise part of groff, and
its function is to convert equation markup into
groff commands. Example 12-26. manview: Viewing formatted manpages
#!/bin/bash
# manview.sh: Formats the source of a man page for viewing.
# This script is useful when writing man page source.
# It lets you look at the intermediate results on the fly
#+ while working on it.
E_WRONGARGS=65
if [ -z "$1" ]
then
echo "Usage: `basename $0` filename"
exit $E_WRONGARGS
fi
# ---------------------------
groff -Tascii -man $1 | less
# From the man page for groff.
# ---------------------------
# If the man page includes tables and/or equations,
#+ then the above code will barf.
# The following line can handle such cases.
#
# gtbl < "$1" | geqn -Tlatin1 | groff -Tlatin1 -mtty-char -man
#
# Thanks, S.C.
exit 0 |
- lex, yacc
The lex lexical analyzer produces
programs for pattern matching. This has been replaced
by the nonproprietary flex on Linux
systems. The yacc utility creates a
parser based on a set of specifications. This has been
replaced by the nonproprietary bison
on Linux systems.
12.5. File and Archiving CommandsArchiving - tar
The standard UNIX archiving utility.
Originally a
Tape ARchiving program, it has
developed into a general purpose package that can handle
all manner of archiving with all types of destination
devices, ranging from tape drives to regular files to even
stdout (see Example 3-4). GNU
tar has been patched to accept various compression
filters, such as tar czvf archive_name.tar.gz
*, which recursively archives and gzips all files in a directory
tree except dotfiles
in the current working directory ($PWD).
Some useful tar options:
-c create (a new
archive) -x extract (files from
existing archive) --delete delete (files
from existing archive)  | This option will not work on magnetic tape
devices. |
-r append (files to
existing archive) -A append
(tar files to
existing archive) -t list (contents of
existing archive) -u update archive -d compare archive with
specified filesystem -z gzip the archive (compress or uncompress, depending on whether
combined with the -c or
-x) option -j
bzip2 the
archive
 | It may be difficult to recover data from a
corrupted gzipped tar
archive. When archiving important files, make multiple
backups. |
- shar
Shell archiving utility. The files in a shell archive
are concatenated without compression, and the
resultant archive is essentially a shell script,
complete with #!/bin/sh header,
and containing all the necessary unarchiving
commands. Shar archives
still show up in Internet newsgroups, but otherwise
shar has been pretty well replaced by
tar/gzip. The
unshar command unpacks
shar archives. - ar
Creation and manipulation utility for archives, mainly
used for binary object file libraries. - rpm
The Red Hat Package Manager, or
rpm utility provides a wrapper for
source or binary archives. It includes commands for
installing and checking the integrity of packages, among
other things. A simple rpm -i package_name.rpm
usually suffices to install a package, though there are many
more options available.  | rpm -qf identifies which package a
file originates from. bash$ rpm -qf /bin/ls
coreutils-5.2.1-31
|
|
 | rpm -qa gives a
complete list of all installed rpm packages
on a given system. An rpm -qa package_name
lists only the package(s) corresponding to
package_name. bash$ rpm -qa
redhat-logos-1.1.3-1
glibc-2.2.4-13
cracklib-2.7-12
dosfstools-2.7-1
gdbm-1.8.0-10
ksymoops-2.4.1-1
mktemp-1.5-11
perl-5.6.0-17
reiserfs-utils-3.x.0j-2
...
bash$ rpm -qa docbook-utils
docbook-utils-0.6.9-2
bash$ rpm -qa docbook | grep docbook
docbook-dtd31-sgml-1.0-10
docbook-style-dsssl-1.64-3
docbook-dtd30-sgml-1.0-10
docbook-dtd40-sgml-1.0-11
docbook-utils-pdf-0.6.9-2
docbook-dtd41-sgml-1.0-10
docbook-utils-0.6.9-2
|
|
- cpio
This specialized archiving copy command
(copy
input and output)
is rarely seen any more, having been supplanted by
tar/gzip. It still
has its uses, such as moving a directory tree. Example 12-27. Using cpio to move a directory tree #!/bin/bash
# Copying a directory tree using 'cpio.'
# Advantages of using 'cpio':
# Speed of copying. It's faster than 'tar' with pipes.
# Well suited for copying special files (named pipes, etc.)
#+ that 'cp' may choke on.
ARGS=2
E_BADARGS=65
if [ $# -ne "$ARGS" ]
then
echo "Usage: `basename $0` source destination"
exit $E_BADARGS
fi
source=$1
destination=$2
find "$source" -depth | cpio -admvp "$destination"
# ^^^^^ ^^^^^
# Read the 'find' and 'cpio' man page to decipher these options.
# Exercise:
# --------
# Add code to check the exit status ($?) of the 'find | cpio' pipe
#+ and output appropriate error messages if anything went wrong.
exit 0 |
- rpm2cpio
This command extracts a
cpio archive from an rpm one. Example 12-28. Unpacking an rpm archive #!/bin/bash
# de-rpm.sh: Unpack an 'rpm' archive
: ${1?"Usage: `basename $0` target-file"}
# Must specify 'rpm' archive name as an argument.
TEMPFILE=$$.cpio # Tempfile with "unique" name.
# $$ is process ID of script.
rpm2cpio < $1 > $TEMPFILE # Converts rpm archive into cpio archive.
cpio --make-directories -F $TEMPFILE -i # Unpacks cpio archive.
rm -f $TEMPFILE # Deletes cpio archive.
exit 0
# Exercise:
# Add check for whether 1) "target-file" exists and
#+ 2) it is really an rpm archive.
# Hint: parse output of 'file' command. |
Compression - gzip
The standard GNU/UNIX compression utility, replacing
the inferior and proprietary
compress. The corresponding decompression
command is gunzip, which is the equivalent of
gzip -d. The zcat filter decompresses a
gzipped file to
stdout, as possible input to a pipe or
redirection. This is, in effect, a cat
command that works on compressed files (including files
processed with the older compress
utility). The zcat command is equivalent to
gzip -dc.  | On some commercial UNIX systems, zcat
is a synonym for uncompress -c,
and will not work on gzipped
files. |
See also Example 7-7. - bzip2
An alternate compression utility, usually more efficient
(but slower) than gzip, especially on
large files. The corresponding decompression command is
bunzip2.  | Newer versions of tar have been patched with
bzip2 support. |
- compress, uncompress
This is an older, proprietary compression
utility found in commercial UNIX distributions. The
more efficient gzip has largely
replaced it. Linux distributions generally include a
compress workalike for compatibility,
although gunzip can unarchive files
treated with compress.  | The znew command transforms
compressed files into
gzipped ones. |
- sq
Yet another compression utility, a filter that works
only on sorted ASCII word lists. It uses the standard
invocation syntax for a filter, sq < input-file >
output-file. Fast, but not nearly as efficient
as gzip. The corresponding
uncompression filter is unsq, invoked
like sq.  | The output of sq may be
piped to gzip for further
compression. |
- zip, unzip
Cross-platform file archiving and compression utility
compatible with DOS pkzip.exe.
"Zipped" archives seem to be a more
acceptable medium of exchange on the Internet than
"tarballs". - unarc, unarj, unrar
These Linux utilities permit unpacking archives
compressed with the DOS arc.exe,
arj.exe, and
rar.exe programs.
File Information - file
A utility for identifying file types. The command
file file-name will return a
file specification for file-name,
such as ascii text or
data. It references
the magic numbers
found in /usr/share/magic,
/etc/magic, or
/usr/lib/magic, depending on the
Linux/UNIX distribution. The -f option causes
file to run in batch mode, to read from
a designated file a list of filenames to analyze. The
-z option, when used on a compressed
target file, forces an attempt to analyze the uncompressed
file type. bash$ file test.tar.gz
test.tar.gz: gzip compressed data, deflated, last modified: Sun Sep 16 13:34:51 2001, os: Unix
bash file -z test.tar.gz
test.tar.gz: GNU tar archive (gzip compressed data, deflated, last modified: Sun Sep 16 13:34:51 2001, os: Unix)
|
# Find sh and Bash scripts in a given directory:
DIRECTORY=/usr/local/bin
KEYWORD=Bourne
# Bourne and Bourne-Again shell scripts
file $DIRECTORY/* | fgrep $KEYWORD
# Output:
# /usr/local/bin/burn-cd: Bourne-Again shell script text executable
# /usr/local/bin/burnit: Bourne-Again shell script text executable
# /usr/local/bin/cassette.sh: Bourne shell script text executable
# /usr/local/bin/copy-cd: Bourne-Again shell script text executable
# . . . |
Example 12-29. Stripping comments from C program files #!/bin/bash
# strip-comment.sh: Strips out the comments (/* COMMENT */) in a C program.
E_NOARGS=0
E_ARGERROR=66
E_WRONG_FILE_TYPE=67
if [ $# -eq "$E_NOARGS" ]
then
echo "Usage: `basename $0` C-program-file" >&2 # Error message to stderr.
exit $E_ARGERROR
fi
# Test for correct file type.
type=`file $1 | awk '{ print $2, $3, $4, $5 }'`
# "file $1" echoes file type . . .
# Then awk removes the first field of this, the filename . . .
# Then the result is fed into the variable "type".
correct_type="ASCII C program text"
if [ "$type" != "$correct_type" ]
then
echo
echo "This script works on C program files only."
echo
exit $E_WRONG_FILE_TYPE
fi
# Rather cryptic sed script:
#--------
sed '
/^\/\*/d
/.*\*\//d
' $1
#--------
# Easy to understand if you take several hours to learn sed fundamentals.
# Need to add one more line to the sed script to deal with
#+ case where line of code has a comment following it on same line.
# This is left as a non-trivial exercise.
# Also, the above code deletes non-comment lines with a "*/" --
#+ not a desirable result.
exit 0
# ----------------------------------------------------------------
# Code below this line will not execute because of 'exit 0' above.
# Stephane Chazelas suggests the following alternative:
usage() {
echo "Usage: `basename $0` C-program-file" >&2
exit 1
}
WEIRD=`echo -n -e '\377'` # or WEIRD=$'\377'
[[ $# -eq 1 ]] || usage
case `file "$1"` in
*"C program text"*) sed -e "s%/\*%${WEIRD}%g;s%\*/%${WEIRD}%g" "$1" \
| tr '\377\n' '\n\377' \
| sed -ne 'p;n' \
| tr -d '\n' | tr '\377' '\n';;
*) usage;;
esac
# This is still fooled by things like:
# printf("/*");
# or
# /* /* buggy embedded comment */
#
# To handle all special cases (comments in strings, comments in string
#+ where there is a \", \\" ...) the only way is to write a C parser
#+ (using lex or yacc perhaps?).
exit 0 |
- which
which command-xxx gives the full path
to "command-xxx". This is useful for finding
out whether a particular command or utility is installed
on the system. $bash which rm
- whereis
Similar to which, above,
whereis command-xxx gives the
full path to "command-xxx", but also to its
manpage. $bash whereis rm
rm: /bin/rm /usr/share/man/man1/rm.1.bz2 |
- whatis
whatis filexxx looks up
"filexxx" in the
whatis database. This is useful
for identifying system commands and important configuration
files. Consider it a simplified man
command. $bash whatis whatis
whatis (1) - search the whatis database for complete words |
Example 12-30. Exploring /usr/X11R6/bin #!/bin/bash
# What are all those mysterious binaries in /usr/X11R6/bin?
DIRECTORY="/usr/X11R6/bin"
# Try also "/bin", "/usr/bin", "/usr/local/bin", etc.
for file in $DIRECTORY/*
do
whatis `basename $file` # Echoes info about the binary.
done
exit 0
# You may wish to redirect output of this script, like so:
# ./what.sh >>whatis.db
# or view it a page at a time on stdout,
# ./what.sh | less |
See also Example 10-3. - vdir
Show a detailed directory listing. The effect is similar to
ls -l. This is one of the GNU fileutils. bash$ vdir
total 10
-rw-r--r-- 1 bozo bozo 4034 Jul 18 22:04 data1.xrolo
-rw-r--r-- 1 bozo bozo 4602 May 25 13:58 data1.xrolo.bak
-rw-r--r-- 1 bozo bozo 877 Dec 17 2000 employment.xrolo
bash ls -l
total 10
-rw-r--r-- 1 bozo bozo 4034 Jul 18 22:04 data1.xrolo
-rw-r--r-- 1 bozo bozo 4602 May 25 13:58 data1.xrolo.bak
-rw-r--r-- 1 bozo bozo 877 Dec 17 2000 employment.xrolo
|
- locate, slocate
The locate command searches for files using a
database stored for just that purpose. The
slocate command is the secure version of
locate (which may be aliased to
slocate). $bash locate hickson
/usr/lib/xephem/catalogs/hickson.edb |
- readlink
Disclose the file that a symbolic link points to. bash$ readlink /usr/bin/awk
../../bin/gawk
|
- strings
Use the strings command to find
printable strings in a binary or data file. It will list
sequences of printable characters found in the target
file. This might be handy for a quick 'n dirty examination
of a core dump or for looking at an unknown graphic image
file (strings image-file | more might
show something like JFIF,
which would identify the file as a jpeg
graphic). In a script, you would probably
parse the output of strings
with grep or sed. See Example 10-7
and Example 10-9. Example 12-31. An "improved" strings
command #!/bin/bash
# wstrings.sh: "word-strings" (enhanced "strings" command)
#
# This script filters the output of "strings" by checking it
#+ against a standard word list file.
# This effectively eliminates gibberish and noise,
#+ and outputs only recognized words.
# ===========================================================
# Standard Check for Script Argument(s)
ARGS=1
E_BADARGS=65
E_NOFILE=66
if [ $# -ne $ARGS ]
then
echo "Usage: `basename $0` filename"
exit $E_BADARGS
fi
if [ ! -f "$1" ] # Check if file exists.
then
echo "File \"$1\" does not exist."
exit $E_NOFILE
fi
# ===========================================================
MINSTRLEN=3 # Minimum string length.
WORDFILE=/usr/share/dict/linux.words # Dictionary file.
# May specify a different
#+ word list file
#+ of one-word-per-line format.
wlist=`strings "$1" | tr A-Z a-z | tr '[:space:]' Z | \
tr -cs '[:alpha:]' Z | tr -s '\173-\377' Z | tr Z ' '`
# Translate output of 'strings' command with multiple passes of 'tr'.
# "tr A-Z a-z" converts to lowercase.
# "tr '[:space:]'" converts whitespace characters to Z's.
# "tr -cs '[:alpha:]' Z" converts non-alphabetic characters to Z's,
#+ and squeezes multiple consecutive Z's.
# "tr -s '\173-\377' Z" converts all characters past 'z' to Z's
#+ and squeezes multiple consecutive Z's,
#+ which gets rid of all the weird characters that the previous
#+ translation failed to deal with.
# Finally, "tr Z ' '" converts all those Z's to whitespace,
#+ which will be seen as word separators in the loop below.
# ****************************************************************
# Note the technique of feeding the output of 'tr' back to itself,
#+ but with different arguments and/or options on each pass.
# ****************************************************************
for word in $wlist # Important:
# $wlist must not be quoted here.
# "$wlist" does not work.
# Why not?
do
strlen=${#word} # String length.
if [ "$strlen" -lt "$MINSTRLEN" ] # Skip over short strings.
then
continue
fi
grep -Fw $word "$WORDFILE" # Match whole words only.
# ^^^ # "Fixed strings" and
#+ "whole words" options.
done
exit $? |
Comparison - diff, patch
diff: flexible file comparison
utility. It compares the target files line-by-line
sequentially. In some applications, such as comparing
word dictionaries, it may be helpful to filter the
files through sort
and uniq before piping them
to diff. diff file-1
file-2 outputs the lines in the files that
differ, with carets showing which file each particular
line belongs to. The --side-by-side option to
diff outputs each compared file, line by
line, in separate columns, with non-matching lines marked. The
-c and -u options likewise
make the output of the command easier to interpret. There are available various fancy frontends for
diff, such as sdiff,
wdiff, xdiff, and
mgdiff.  | The diff command returns an exit
status of 0 if the compared files are identical, and 1 if
they differ. This permits use of diff
in a test construct within a shell script (see
below). |
A common use for diff is generating
difference files to be used with patch
The -e option outputs files suitable
for ed or ex
scripts. patch: flexible versioning
utility. Given a difference file generated by
diff, patch can
upgrade a previous version of a package to a newer version.
It is much more convenient to distribute a relatively
small "diff" file than the entire body of a
newly revised package. Kernel "patches" have
become the preferred method of distributing the frequent
releases of the Linux kernel. patch -p1 <patch-file
# Takes all the changes listed in 'patch-file'
# and applies them to the files referenced therein.
# This upgrades to a newer version of the package. |
Patching the kernel: cd /usr/src
gzip -cd patchXX.gz | patch -p0
# Upgrading kernel source using 'patch'.
# From the Linux kernel docs "README",
# by anonymous author (Alan Cox?). |
 | The diff command can also
recursively compare directories (for the filenames
present). bash$ diff -r ~/notes1 ~/notes2
Only in /home/bozo/notes1: file02
Only in /home/bozo/notes1: file03
Only in /home/bozo/notes2: file04
|
|
 | Use zdiff to compare
gzipped files. |
- diff3
An extended version of diff that compares
three files at a time. This command returns an exit value
of 0 upon successful execution, but unfortunately this gives
no information about the results of the comparison. bash$ diff3 file-1 file-2 file-3
====
1:1c
This is line 1 of "file-1".
2:1c
This is line 1 of "file-2".
3:1c
This is line 1 of "file-3"
|
- sdiff
Compare and/or edit two files in order to merge
them into an output file. Because of its interactive nature,
this command would find little use in a script. - cmp
The cmp command is a simpler version of
diff, above. Whereas diff
reports the differences between two files,
cmp merely shows at what point they
differ.  | Like diff, cmp
returns an exit status of 0 if the compared files are
identical, and 1 if they differ. This permits use in a test
construct within a shell script. |
Example 12-32. Using cmp to compare two files
within a script. #!/bin/bash
ARGS=2 # Two args to script expected.
E_BADARGS=65
E_UNREADABLE=66
if [ $# -ne "$ARGS" ]
then
echo "Usage: `basename $0` file1 file2"
exit $E_BADARGS
fi
if [[ ! -r "$1" || ! -r "$2" ]]
then
echo "Both files to be compared must exist and be readable."
exit $E_UNREADABLE
fi
cmp $1 $2 &> /dev/null # /dev/null buries the output of the "cmp" command.
# cmp -s $1 $2 has same result ("-s" silent flag to "cmp")
# Thank you Anders Gustavsson for pointing this out.
#
# Also works with 'diff', i.e., diff $1 $2 &> /dev/null
if [ $? -eq 0 ] # Test exit status of "cmp" command.
then
echo "File \"$1\" is identical to file \"$2\"."
else
echo "File \"$1\" differs from file \"$2\"."
fi
exit 0 |
 | Use zcmp on
gzipped files. |
- comm
Versatile file comparison utility. The files must be
sorted for this to be useful. comm
-options
first-file
second-file comm file-1 file-2 outputs three columns:
column 1 = lines unique to file-1 column 2 = lines unique to file-2 column 3 = lines common to both.
The options allow suppressing output of one or more columns.
Utilities - basename
Strips the path information from a file name, printing
only the file name. The construction basename
$0 lets the script know its name, that is, the name it
was invoked by. This can be used for "usage" messages if,
for example a script is called with missing arguments:
echo "Usage: `basename $0` arg1 arg2 ... argn" |
- dirname
Strips the basename from
a filename, printing only the path information.  | basename and dirname
can operate on any arbitrary string. The argument
does not need to refer to an existing file, or
even be a filename for that matter (see Example A-7). |
Example 12-33. basename and dirname #!/bin/bash
a=/home/bozo/daily-journal.txt
echo "Basename of /home/bozo/daily-journal.txt = `basename $a`"
echo "Dirname of /home/bozo/daily-journal.txt = `dirname $a`"
echo
echo "My own home is `basename ~/`." # `basename ~` also works.
echo "The home of my home is `dirname ~/`." # `dirname ~` also works.
exit 0 |
- split, csplit
These are utilities for splitting a file into smaller
chunks. They are usually used for splitting up large files
in order to back them up on floppies or preparatory to
e-mailing or uploading them. The csplit command splits a file
according to context, the split occuring
where patterns are matched. - sum, cksum, md5sum, sha1sum
These are utilities for generating checksums. A
checksum is a number mathematically
calculated from the contents of a file, for the purpose
of checking its integrity. A script might refer to a list
of checksums for security purposes, such as ensuring
that the contents of key system files have not been
altered or corrupted. For security applications, use the
md5sum (message
digest 5
checksum) command, or better yet,
the newer sha1sum (Secure Hash
Algorithm). bash$ cksum /boot/vmlinuz
1670054224 804083 /boot/vmlinuz
bash$ echo -n "Top Secret" | cksum
3391003827 10
bash$ md5sum /boot/vmlinuz
0f43eccea8f09e0a0b2b5cf1dcf333ba /boot/vmlinuz
bash$ echo -n "Top Secret" | md5sum
8babc97a6f62a4649716f4df8d61728f -
|
 | The cksum command shows the size,
in bytes, of its target, whether file or
stdout. The md5sum and
sha1sum commands display a
dash when they receive their input from
stdout. |
Example 12-34. Checking file integrity #!/bin/bash
# file-integrity.sh: Checking whether files in a given directory
# have been tampered with.
E_DIR_NOMATCH=70
E_BAD_DBFILE=71
dbfile=File_record.md5
# Filename for storing records (database file).
set_up_database ()
{
echo ""$directory"" > "$dbfile"
# Write directory name to first line of file.
md5sum "$directory"/* >> "$dbfile"
# Append md5 checksums and filenames.
}
check_database ()
{
local n=0
local filename
local checksum
# ------------------------------------------- #
# This file check should be unnecessary,
#+ but better safe than sorry.
if [ ! -r "$dbfile" ]
then
echo "Unable to read checksum database file!"
exit $E_BAD_DBFILE
fi
# ------------------------------------------- #
while read record[n]
do
directory_checked="${record[0]}"
if [ "$directory_checked" != "$directory" ]
then
echo "Directories do not match up!"
# Tried to use file for a different directory.
exit $E_DIR_NOMATCH
fi
if [ "$n" -gt 0 ] # Not directory name.
then
filename[n]=$( echo ${record[$n]} | awk '{ print $2 }' )
# md5sum writes records backwards,
#+ checksum first, then filename.
checksum[n]=$( md5sum "${filename[n]}" )
if [ "${record[n]}" = "${checksum[n]}" ]
then
echo "${filename[n]} unchanged."
elif [ "`basename ${filename[n]}`" != "$dbfile" ]
# Skip over checksum database file,
#+ as it will change with each invocation of script.
# ---
# This unfortunately means that when running
#+ this script on $PWD, tampering with the
#+ checksum database file will not be detected.
# Exercise: Fix this.
then
echo "${filename[n]} : CHECKSUM ERROR!"
# File has been changed since last checked.
fi
fi
let "n+=1"
done <"$dbfile" # Read from checksum database file.
}
# =================================================== #
# main ()
if [ -z "$1" ]
then
directory="$PWD" # If not specified,
else #+ use current working directory.
directory="$1"
fi
clear # Clear screen.
echo " Running file integrity check on $directory"
echo
# ------------------------------------------------------------------ #
if [ ! -r "$dbfile" ] # Need to create database file?
then
echo "Setting up database file, \""$directory"/"$dbfile"\"."; echo
set_up_database
fi
# ------------------------------------------------------------------ #
check_database # Do the actual work.
echo
# You may wish to redirect the stdout of this script to a file,
#+ especially if the directory checked has many files in it.
exit 0
# For a much more thorough file integrity check,
#+ consider the "Tripwire" package,
#+ http://sourceforge.net/projects/tripwire/.
|
Also see Example A-19 and Example 33-14 for creative uses of
the md5sum command.  | There have been reports that the 128-bit
md5sum can be cracked, so the more secure
160-bit sha1sum is a welcome new addition
to the checksum toolkit.
bash$ md5sum testfile
e181e2c8720c60522c4c4c981108e367 testfile
bash$ sha1sum testfile
5d7425a9c08a66c3177f1e31286fa40986ffc996 testfile
|
|
- shred
Securely erase a file by overwriting it multiple times with
random bit patterns before deleting it. This command has
the same effect as Example 12-55, but does it
in a more thorough and elegant manner. This is one of the GNU fileutils.  | Advanced forensic technology may still be able to
recover the contents of a file, even after application of
shred. |
Encoding and Encryption - uuencode
This utility encodes binary files into ASCII characters, making them
suitable for transmission in the body of an e-mail message or in a
newsgroup posting. - uudecode
This reverses the encoding, decoding uuencoded files back into the
original binaries. Example 12-35. Uudecoding encoded files #!/bin/bash
# Uudecodes all uuencoded files in current working directory.
lines=35 # Allow 35 lines for the header (very generous).
for File in * # Test all the files in $PWD.
do
search1=`head -$lines $File | grep begin | wc -w`
search2=`tail -$lines $File | grep end | wc -w`
# Uuencoded files have a "begin" near the beginning,
#+ and an "end" near the end.
if [ "$search1" -gt 0 ]
then
if [ "$search2" -gt 0 ]
then
echo "uudecoding - $File -"
uudecode $File
fi
fi
done
# Note that running this script upon itself fools it
#+ into thinking it is a uuencoded file,
#+ because it contains both "begin" and "end".
# Exercise:
# --------
# Modify this script to check each file for a newsgroup header,
#+ and skip to next if not found.
exit 0 |
 | The fold -s command
may be useful (possibly in a pipe) to process long uudecoded
text messages downloaded from Usenet newsgroups. |
- mimencode, mmencode
The mimencode and
mmencode commands process
multimedia-encoded e-mail attachments. Although
mail user agents (such as
pine or kmail)
normally handle this automatically, these particular
utilities permit manipulating such attachments manually
from the command line or in a batch by means of a shell
script. - crypt
At one time, this was the standard UNIX file encryption
utility.
Politically motivated government regulations
prohibiting the export of encryption software resulted
in the disappearance of crypt
from much of the UNIX world, and it is still
missing from most Linux distributions. Fortunately,
programmers have come up with a number of decent
alternatives to it, among them the author's very own cruft
(see Example A-4).
Miscellaneous - mktemp
Create a temporary file
with a "unique" filename. When invoked
from the command line without additional arguments,
it creates a zero-length file in the /tmp directory. bash$ mktemp
/tmp/tmp.zzsvql3154
|
PREFIX=filename
tempfile=`mktemp $PREFIX.XXXXXX`
# ^^^^^^ Need at least 6 placeholders
#+ in the filename template.
# If no filename template supplied,
#+ "tmp.XXXXXXXXXX" is the default.
echo "tempfile name = $tempfile"
# tempfile name = filename.QA2ZpY
# or something similar...
# Creates a file of that name in the current working directory
#+ with 600 file permissions.
# A "umask 177" is therefore unnecessary,
# but it's good programming practice anyhow. |
- make
Utility for building and compiling binary packages.
This can also be used for any set of operations that is
triggered by incremental changes in source files.
The make command checks a
Makefile, a list of file dependencies and
operations to be carried out. - install
Special purpose file copying command, similar to
cp, but capable of setting permissions
and attributes of the copied files. This command seems
tailormade for installing software packages, and as such it
shows up frequently in Makefiles
(in the make install :
section). It could likewise find use in installation
scripts. - dos2unix
This utility, written by Benjamin Lin and collaborators,
converts DOS-formatted text files (lines terminated by
CR-LF) to UNIX format (lines terminated by LF only),
and vice-versa. - ptx
The ptx [targetfile] command
outputs a permuted index (cross-reference list) of the
targetfile. This may be further filtered and formatted in a
pipe, if necessary. - more, less
Pagers that display a text file or stream to
stdout, one screenful at a time.
These may be used to filter the output of
stdout . . . or of a script. An interesting application of more
is to "test drive" a command sequence,
to forestall potentially unpleasant consequences.
ls /home/bozo | awk '{print "rm -rf " $1}' | more
# ^^^^
# Testing the effect of the following (disastrous) command line:
# ls /home/bozo | awk '{print "rm -rf " $1}' | sh
# Hand off to the shell to execute . . . ^^ |
12.6. Communications CommandsCertain of the following commands find use in chasing spammers, as well as in
network data transfer and analysis. Information and Statistics - host
Searches for information about an Internet host by name or
IP address, using DNS. bash$ host surfacemail.com
surfacemail.com. has address 202.92.42.236
|
- ipcalc
Displays IP information for a host.
With the -h option,
ipcalc does a reverse DNS lookup, finding
the name of the host (server) from the IP address. bash$ ipcalc -h 202.92.42.236
HOSTNAME=surfacemail.com
|
- nslookup
Do an Internet "name server lookup"
on a host by IP address. This is essentially equivalent
to ipcalc -h or dig -x
. The command may be run either interactively
or noninteractively, i.e., from within a script. The nslookup command has allegedly
been "deprecated," but it still has its
uses. bash$ nslookup -sil 66.97.104.180
nslookup kuhleersparnis.ch
Server: 135.116.137.2
Address: 135.116.137.2#53
Non-authoritative answer:
Name: kuhleersparnis.ch
|
- dig
Domain Information
Groper. Similar to
nslookup, dig does
an Internet "name server lookup" on a host.
May be run either interactively or noninteractively, i.e.,
from within a script. Some interesting options to dig are
+time=N for setting a query timeout to
N seconds, +nofail for
continuing to query servers until a reply is received, and
-x for doing a reverse address lookup. Compare the output of dig -x with
ipcalc -h and
nslookup. bash$ dig -x 81.9.6.2
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 11649
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0
;; QUESTION SECTION:
;2.6.9.81.in-addr.arpa. IN PTR
;; AUTHORITY SECTION:
6.9.81.in-addr.arpa. 3600 IN SOA ns.eltel.net. noc.eltel.net.
2002031705 900 600 86400 3600
;; Query time: 537 msec
;; SERVER: 135.116.137.2#53(135.116.137.2)
;; WHEN: Wed Jun 26 08:35:24 2002
;; MSG SIZE rcvd: 91
|
Example 12-36. Finding out where to report a spammer #!/bin/bash
# spam-lookup.sh: Look up abuse contact to report a spammer.
# Thanks, Michael Zick.
# Check for command-line arg.
ARGCOUNT=1
E_WRONGARGS=65
if [ $# -ne "$ARGCOUNT" ]
then
echo "Usage: `basename $0` domain-name"
exit $E_WRONGARGS
fi
dig +short $1.contacts.abuse.net -c in -t txt
# Also try:
# dig +nssearch $1
# Tries to find "authoritative name servers" and display SOA records.
# The following also works:
# whois -h whois.abuse.net $1
# ^^ ^^^^^^^^^^^^^^^ Specify host.
# Can even lookup multiple spammers with this, i.e."
# whois -h whois.abuse.net $spamdomain1 $spamdomain2 . . .
# Exercise:
# --------
# Expand the functionality of this script
#+ so that it automatically e-mails a notification
#+ to the responsible ISP's contact address(es).
# Hint: use the "mail" command.
exit $?
# spam-lookup.sh chinatietong.com
# A known spam domain.
# "crnet_mgr@chinatietong.com"
# "crnet_tec@chinatietong.com"
# "postmaster@chinatietong.com"
# For a more elaborate version of this script,
#+ see the SpamViz home page, http://www.spamviz.net/index.html. |
Example 12-37. Analyzing a spam domain #! /bin/bash
# is-spammer.sh: Identifying spam domains
# $Id: is-spammer, v 1.4 2004/09/01 19:37:52 mszick Exp $
# Above line is RCS ID info.
#
# This is a simplified version of the "is_spammer.bash
#+ script in the Contributed Scripts appendix.
# is-spammer <domain.name>
# Uses an external program: 'dig'
# Tested with version: 9.2.4rc5
# Uses functions.
# Uses IFS to parse strings by assignment into arrays.
# And even does something useful: checks e-mail blacklists.
# Use the domain.name(s) from the text body:
# http://www.good_stuff.spammer.biz/just_ignore_everything_else
# ^^^^^^^^^^^
# Or the domain.name(s) from any e-mail address:
# Really_Good_Offer@spammer.biz
#
# as the only argument to this script.
#(PS: have your Inet connection running)
#
# So, to invoke this script in the above two instances:
# is-spammer.sh spammer.biz
# Whitespace == :Space:Tab:Line Feed:Carriage Return:
WSP_IFS=$'\x20'$'\x09'$'\x0A'$'\x0D'
# No Whitespace == Line Feed:Carriage Return
No_WSP=$'\x0A'$'\x0D'
# Field separator for dotted decimal ip addresses
ADR_IFS=${No_WSP}'.'
# Get the dns text resource record.
# get_txt <error_code> <list_query>
get_txt() {
# Parse $1 by assignment at the dots.
local -a dns
IFS=$ADR_IFS
dns=( $1 )
IFS=$WSP_IFS
if [ "${dns[0]}" == '127' ]
then
# See if there is a reason.
echo $(dig +short $2 -t txt)
fi
}
# Get the dns address resource record.
# chk_adr <rev_dns> <list_server>
chk_adr() {
local reply
local server
local reason
server=${1}${2}
reply=$( dig +short ${server} )
# If reply might be an error code . . .
if [ ${#reply} -gt 6 ]
then
reason=$(get_txt ${reply} ${server} )
reason=${reason:-${reply}}
fi
echo ${reason:-' not blacklisted.'}
}
# Need to get the IP address from the name.
echo 'Get address of: '$1
ip_adr=$(dig +short $1)
dns_reply=${ip_adr:-' no answer '}
echo ' Found address: '${dns_reply}
# A valid reply is at least 4 digits plus 3 dots.
if [ ${#ip_adr} -gt 6 ]
then
echo
declare query
# Parse by assignment at the dots.
declare -a dns
IFS=$ADR_IFS
dns=( ${ip_adr} )
IFS=$WSP_IFS
# Reorder octets into dns query order.
rev_dns="${dns[3]}"'.'"${dns[2]}"'.'"${dns[1]}"'.'"${dns[0]}"'.'
# See: http://www.spamhaus.org (Conservative, well maintained)
echo -n 'spamhaus.org says: '
echo $(chk_adr ${rev_dns} 'sbl-xbl.spamhaus.org')
# See: http://ordb.org (Open mail relays)
echo -n ' ordb.org says: '
echo $(chk_adr ${rev_dns} 'relays.ordb.org')
# See: http://www.spamcop.net/ (You can report spammers here)
echo -n ' spamcop.net says: '
echo $(chk_adr ${rev_dns} 'bl.spamcop.net')
# # # other blacklist operations # # #
# See: http://cbl.abuseat.org.
echo -n ' abuseat.org says: '
echo $(chk_adr ${rev_dns} 'cbl.abuseat.org')
# See: http://dsbl.org/usage (Various mail relays)
echo
echo 'Distributed Server Listings'
echo -n ' list.dsbl.org says: '
echo $(chk_adr ${rev_dns} 'list.dsbl.org')
echo -n ' multihop.dsbl.org says: '
echo $(chk_adr ${rev_dns} 'multihop.dsbl.org')
echo -n 'unconfirmed.dsbl.org says: '
echo $(chk_adr ${rev_dns} 'unconfirmed.dsbl.org')
else
echo
echo 'Could not use that address.'
fi
exit 0
# Exercises:
# --------
# 1) Check arguments to script,
# and exit with appropriate error message if necessary.
# 2) Check if on-line at invocation of script,
# and exit with appropriate error message if necessary.
# 3) Substitute generic variables for "hard-coded" BHL domains.
# 4) Set a time-out for the script using the "+time=" option
to the 'dig' command. |
For a much more elaborate version of the above script, see
Example A-27. - traceroute
Trace the route taken by packets sent to a remote host. This
command works within a LAN, WAN, or over the
Internet. The remote host may be specified by an IP
address. The output of this command may be filtered
by grep or sed in a pipe. bash$ traceroute 81.9.6.2
traceroute to 81.9.6.2 (81.9.6.2), 30 hops max, 38 byte packets
1 tc43.xjbnnbrb.com (136.30.178.8) 191.303 ms 179.400 ms 179.767 ms
2 or0.xjbnnbrb.com (136.30.178.1) 179.536 ms 179.534 ms 169.685 ms
3 192.168.11.101 (192.168.11.101) 189.471 ms 189.556 ms *
...
|
- ping
Broadcast an "ICMP ECHO_REQUEST" packet to
another machine, either on a local or remote network. This
is a diagnostic tool for testing network connections,
and it should be used with caution. A successful ping returns an exit status of
0. This can be tested for in a
script. bash$ ping localhost
PING localhost.localdomain (127.0.0.1) from 127.0.0.1 : 56(84) bytes of data.
64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=0 ttl=255 time=709 usec
64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=1 ttl=255 time=286 usec
--- localhost.localdomain ping statistics ---
2 packets transmitted, 2 packets received, 0% packet loss
round-trip min/avg/max/mdev = 0.286/0.497/0.709/0.212 ms
|
- whois
Perform a DNS (Domain Name System) lookup.
The -h option permits specifying which
particular whois server to query. See
Example 4-6 and Example 12-36. - finger
Retrieve information about users on a
network. Optionally, this command can display
a user's ~/.plan,
~/.project, and
~/.forward files, if present. bash$ finger
Login Name Tty Idle Login Time Office Office Phone
bozo Bozo Bozeman tty1 8 Jun 25 16:59
bozo Bozo Bozeman ttyp0 Jun 25 16:59
bozo Bozo Bozeman ttyp1 Jun 25 17:07
bash$ finger bozo
Login: bozo Name: Bozo Bozeman
Directory: /home/bozo Shell: /bin/bash
Office: 2355 Clown St., 543-1234
On since Fri Aug 31 20:13 (MST) on tty1 1 hour 38 minutes idle
On since Fri Aug 31 20:13 (MST) on pts/0 12 seconds idle
On since Fri Aug 31 20:13 (MST) on pts/1
On since Fri Aug 31 20:31 (MST) on pts/2 1 hour 16 minutes idle
No mail.
No Plan.
|
Out of security considerations, many networks disable
finger and its associated daemon.
- chfn
Change information disclosed by the
finger command. - vrfy
Verify an Internet e-mail address.
Remote Host Access - sx, rx
The sx and rx
command set serves to transfer files to and from a remote
host using the xmodem protocol. These
are generally part of a communications package, such as
minicom. - sz, rz
The sz and rz
command set serves to transfer files to and from a remote
host using the zmodem protocol.
Zmodem has certain advantages over
xmodem, such as faster transmission
rate and resumption of interrupted file transfers.
Like sx and rx,
these are generally part of a communications package. - ftp
Utility and protocol for uploading / downloading
files to or from a remote host. An ftp session can be automated
in a script (see Example 17-6, Example A-4, and Example A-13). - uucp, uux, cu
uucp: UNIX to UNIX
copy. This is a
communications package for transferring files between UNIX
servers. A shell script is an effective way to handle a
uucp command sequence. Since the advent of the Internet and e-mail,
uucp seems to have faded into obscurity,
but it still exists and remains perfectly workable in
situations where an Internet connection is not available
or appropriate. The advantage of uucp
is that it is fault-tolerant, so even if there is a service
interruption the copy operation will resume where it left
off when the connection is restored. --- uux: UNIX to UNIX execute.
Execute a command on a remote system. This
command is part of the uucp package. --- cu: Call Up
a remote system and connect as a simple terminal.
It is a sort of dumbed-down version of telnet. This command is part
of the uucp package. - telnet
Utility and protocol for connecting to a remote host.  | The telnet protocol contains security holes and
should therefore probably be avoided. |
- wget
The wget utility
non-interactively retrieves or
downloads files from a Web or ftp site. It works well in a
script. wget -p http://www.xyz23.com/file01.html
# The -p or --page-requisite option causes wget to fetch all files
#+ required to display the specified page.
wget -r ftp://ftp.xyz24.net/~bozo/project_files/ -O $SAVEFILE
# The -r option recursively follows and retrieves all links
#+ on the specified site. |
Example 12-38. Getting a stock quote #!/bin/bash
# quote-fetch.sh: Download a stock quote.
E_NOPARAMS=66
if [ -z "$1" ] # Must specify a stock (symbol) to fetch.
then echo "Usage: `basename $0` stock-symbol"
exit $E_NOPARAMS
fi
stock_symbol=$1
file_suffix=.html
# Fetches an HTML file, so name it appropriately.
URL='http://finance.yahoo.com/q?s='
# Yahoo finance board, with stock query suffix.
# -----------------------------------------------------------
wget -O ${stock_symbol}${file_suffix} "${URL}${stock_symbol}"
# -----------------------------------------------------------
# To look up stuff on http://search.yahoo.com:
# -----------------------------------------------------------
# URL="http://search.yahoo.com/search?fr=ush-news&p=${query}"
# wget -O "$savefilename" "${URL}"
# -----------------------------------------------------------
# Saves a list of relevant URLs.
exit $?
# Exercises:
# ---------
#
# 1) Add a test to ensure the user running the script is on-line.
# (Hint: parse the output of 'ps -ax' for "ppp" or "connect."
#
# 2) Modify this script to fetch the local weather report,
#+ taking the user's zip code as an argument. |
See also Example A-29 and Example A-30. - lynx
The lynx Web and file browser
can be used inside a script (with the
-dump option) to retrieve a file from a Web or
ftp site non-interactively.
lynx -dump http://www.xyz23.com/file01.html >$SAVEFILE |
With the -traversal option,
lynx starts at the HTTP URL specified
as an argument, then "crawls" through all
links located on that particular server. Used together
with the -crawl option, outputs page text
to a log file. - rlogin
Remote login, initates a
session on a remote host. This command has security issues,
so use ssh instead. - rsh
Remote shell, executes
command(s) on a remote host. This has security issues,
so use ssh instead. - rcp
Remote copy, copies files
between two different networked machines. - rsync
Remote synchronize, updates
(synchronizes) files
between two different networked machines. bash$ rsync -a ~/sourcedir/*txt /node1/subdirectory/
|
Example 12-39. Updating FC4 #!/bin/bash
# fc4upd.sh
# Script author: Frank Wang.
# Slight stylistic modifications by ABS Guide author.
# Used in ABS Guide with permission.
# Download Fedora 4 update from mirror site using rsync.
# Only download latest package if multiple versions exist,
#+ to save space.
URL=rsync://distro.ibiblio.org/fedora-linux-core/updates/
# URL=rsync://ftp.kddilabs.jp/fedora/core/updates/
# URL=rsync://rsync.planetmirror.com/fedora-linux-core/updates/
DEST=${1:-/var/www/html/fedora/updates/}
LOG=/tmp/repo-update-$(/bin/date +%Y-%m-%d).txt
PID_FILE=/var/run/${0##*/}.pid
E_RETURN=65 # Something unexpected happened.
# General rsync options
# -r: recursive download
# -t: reserve time
# -v: verbose
OPTS="-rtv --delete-excluded --delete-after --partial"
# rsync include pattern
# Leading slash causes absolute path name match.
INCLUDE=(
"/4/i386/kde-i18n-Chinese*"
# ^ ^
# Quoting is necessary to prevent globbing.
)
# rsync exclude pattern
# Temporarily comment out unwanted pkgs using "#" . . .
EXCLUDE=(
/1
/2
/3
/testing
/4/SRPMS
/4/ppc
/4/x86_64
/4/i386/debug
"/4/i386/kde-i18n-*"
"/4/i386/openoffice.org-langpack-*"
"/4/i386/*i586.rpm"
"/4/i386/GFS-*"
"/4/i386/cman-*"
"/4/i386/dlm-*"
"/4/i386/gnbd-*"
"/4/i386/kernel-smp*"
# "/4/i386/kernel-xen*"
# "/4/i386/xen-*"
)
init () {
# Let pipe command return possible rsync error, e.g., stalled network.
set -o pipefail
TMP=${TMPDIR:-/tmp}/${0##*/}.$$ # Store refined download list.
trap "{
rm -f $TMP 2>/dev/null
}" EXIT # Clear temporary file on exit.
}
check_pid () {
# Check if process exists.
if [ -s "$PID_FILE" ]; then
echo "PID file exists. Checking ..."
PID=$(/bin/egrep -o "^[[:digit:]]+" $PID_FILE)
if /bin/ps --pid $PID &>/dev/null; then
echo "Process $PID found. ${0##*/} seems to be running!"
/usr/bin/logger -t ${0##*/} \
"Process $PID found. ${0##*/} seems to be running!"
exit $E_RETURN
fi
echo "Process $PID not found. Start new process . . ."
fi
}
# Set overall file update range starting from root or $URL,
#+ according to above patterns.
set_range () {
include=
exclude=
for p in "${INCLUDE[@]}"; do
include="$include --include \"$p\""
done
for p in "${EXCLUDE[@]}"; do
exclude="$exclude --exclude \"$p\""
done
}
# Retrieve and refine rsync update list.
get_list () {
echo $$ > $PID_FILE || {
echo "Can't write to pid file $PID_FILE"
exit $E_RETURN
}
echo -n "Retrieving and refining update list . . ."
# Retrieve list -- 'eval' is needed to run rsync as a single command.
# $3 and $4 is the date and time of file creation.
# $5 is the full package name.
previous=
pre_file=
pre_date=0
eval /bin/nice /usr/bin/rsync \
-r $include $exclude $URL | \
egrep '^dr.x|^-r' | \
awk '{print $3, $4, $5}' | \
sort -k3 | \
{ while read line; do
# Get seconds since epoch, to filter out obsolete pkgs.
cur_date=$(date -d "$(echo $line | awk '{print $1, $2}')" +%s)
# echo $cur_date
# Get file name.
cur_file=$(echo $line | awk '{print $3}')
# echo $cur_file
# Get rpm pkg name from file name, if possible.
if [[ $cur_file == *rpm ]]; then
pkg_name=$(echo $cur_file | sed -r -e \
's/(^([^_-]+[_-])+)[[:digit:]]+\..*[_-].*$/\1/')
else
pkg_name=
fi
# echo $pkg_name
if [ -z "$pkg_name" ]; then # If not a rpm file,
echo $cur_file >> $TMP #+ then append to download list.
elif [ "$pkg_name" != "$previous" ]; then # A new pkg found.
echo $pre_file >> $TMP # Output latest file.
previous=$pkg_name # Save current.
pre_date=$cur_date
pre_file=$cur_file
elif [ "$cur_date" -gt "$pre_date" ]; then # If same pkg, but newer,
pre_date=$cur_date #+ then update latest pointer.
pre_file=$cur_file
fi
done
echo $pre_file >> $TMP # TMP contains ALL
#+ of refined list now.
# echo "subshell=$BASH_SUBSHELL"
} # Bracket required here to let final "echo $pre_file >> $TMP"
# Remained in the same subshell ( 1 ) with the entire loop.
RET=$? # Get return code of the pipe command.
[ "$RET" -ne 0 ] && {
echo "List retrieving failed with code $RET"
exit $E_RETURN
}
echo "done"; echo
}
# Real rsync download part.
get_file () {
echo "Downloading..."
/bin/nice /usr/bin/rsync \
$OPTS \
--filter "merge,+/ $TMP" \
--exclude '*' \
$URL $DEST \
| /usr/bin/tee $LOG
RET=$?
# --filter merge,+/ is crucial for the intention.
# + modifier means include and / means absolute path.
# Then sorted list in $TMP will contain ascending dir name and
#+ prevent the following --exclude '*' from "shortcutting the circuit."
echo "Done"
rm -f $PID_FILE 2>/dev/null
return $RET
}
# -------
# Main
init
check_pid
set_range
get_list
get_file
RET=$?
# -------
if [ "$RET" -eq 0 ]; then
/usr/bin/logger -t ${0##*/} "Fedora update mirrored successfully."
else
/usr/bin/logger -t ${0##*/} "Fedora update mirrored with failure code: $RET"
fi
exit $RET |
Using rcp, rsync,
and similar utilities with security implications in a
shell script may not be advisable. Consider, instead,
using ssh, scp,
or an expect script. - ssh
Secure shell, logs onto
a remote host and executes commands there. This
secure replacement for telnet,
rlogin, rcp, and
rsh uses identity authentication
and encryption. See its manpage
for details. Example 12-40. Using ssh #!/bin/bash
# remote.bash: Using ssh.
# This example by Michael Zick.
# Used with permission.
# Presumptions:
# ------------
# fd-2 isn't being captured ( '2>/dev/null' ).
# ssh/sshd presumes stderr ('2') will display to user.
#
# sshd is running on your machine.
# For any 'standard' distribution, it probably is,
#+ and without any funky ssh-keygen having been done.
# Try ssh to your machine from the command line:
#
# $ ssh $HOSTNAME
# Without extra set-up you'll be asked for your password.
# enter password
# when done, $ exit
#
# Did that work? If so, you're ready for more fun.
# Try ssh to your machine as 'root':
#
# $ ssh -l root $HOSTNAME
# When asked for password, enter root's, not yours.
# Last login: Tue Aug 10 20:25:49 2004 from localhost.localdomain
# Enter 'exit' when done.
# The above gives you an interactive shell.
# It is possible for sshd to be set up in a 'single command' mode,
#+ but that is beyond the scope of this example.
# The only thing to note is that the following will work in
#+ 'single command' mode.
# A basic, write stdout (local) command.
ls -l
# Now the same basic command on a remote machine.
# Pass a different 'USERNAME' 'HOSTNAME' if desired:
USER=${USERNAME:-$(whoami)}
HOST=${HOSTNAME:-$(hostname)}
# Now excute the above command line on the remote host,
#+ with all transmissions encrypted.
ssh -l ${USER} ${HOST} " ls -l "
# The expected result is a listing of your username's home
#+ directory on the remote machine.
# To see any difference, run this script from somewhere
#+ other than your home directory.
# In other words, the Bash command is passed as a quoted line
#+ to the remote shell, which executes it on the remote machine.
# In this case, sshd does ' bash -c "ls -l" ' on your behalf.
# For information on topics such as not having to enter a
#+ password/passphrase for every command line, see
#+ man ssh
#+ man ssh-keygen
#+ man sshd_config.
exit 0 |
 | Within a loop, ssh may cause
unexpected behavior. According to a Usenet post in the comp.unix shell archives,
ssh inherits the loop's
stdin. To remedy this, pass
ssh either the -n
or -f option. Thanks, Jason Bechtel, for pointing this out. |
- scp
Secure copy, similar in
function to rcp, copies files between
two different networked machines, but does so using
authentication, and with a security level similar to
ssh.
Local Network - write
This is a utility for terminal-to-terminal communication.
It allows sending lines from your terminal (console or
xterm) to that of another user. The
mesg command may, of course,
be used to disable write access to a terminal Since write is interactive, it
would not normally find use in a script. - netconfig
A command-line utility for configuring a network adapter
(using DHCP). This command is native to Red Hat centric Linux
distros.
Mail - mail
Send or read e-mail messages. This stripped-down command-line mail client
works fine as a command embedded in a script. Example 12-41. A script that mails itself #!/bin/sh
# self-mailer.sh: Self-mailing script
adr=${1:-`whoami`} # Default to current user, if not specified.
# Typing 'self-mailer.sh wiseguy@superdupergenius.com'
#+ sends this script to that addressee.
# Just 'self-mailer.sh' (no argument) sends the script
#+ to the person invoking it, for example, bozo@localhost.localdomain.
#
# For more on the ${parameter:-default} construct,
#+ see the "Parameter Substitution" section
#+ of the "Variables Revisited" chapter.
# ============================================================================
cat $0 | mail -s "Script \"`basename $0`\" has mailed itself to you." "$adr"
# ============================================================================
# --------------------------------------------
# Greetings from the self-mailing script.
# A mischievous person has run this script,
#+ which has caused it to mail itself to you.
# Apparently, some people have nothing better
#+ to do with their time.
# --------------------------------------------
echo "At `date`, script \"`basename $0`\" mailed to "$adr"."
exit 0 |
- mailto
Similar to the mail command,
mailto sends e-mail messages
from the command line or in a script. However,
mailto also permits sending MIME
(multimedia) messages. - vacation
This utility automatically replies to e-mails that
the intended recipient is on vacation and temporarily
unavailable. This runs on a network, in conjunction with
sendmail, and is not applicable to a
dial-up POPmail account.
12.7. Terminal Control CommandsCommand affecting the console
or terminal - tput
Initialize terminal and/or fetch information about it from
terminfo data. Various options permit
certain terminal operations. tput clear
is the equivalent of clear,
below. tput reset is the equivalent
of reset, below. tput
sgr0 also resets the terminal, but without
clearing the screen. bash$ tput longname
xterm terminal emulator (XFree86 4.0 Window System)
|
Issuing a tput cup X Y moves
the cursor to the (X,Y) coordinates in the current
terminal. A clear to erase the terminal
screen would normally precede this. Note that stty offers
a more powerful command set for controlling a terminal. - infocmp
This command prints out extensive information about the
current terminal. It references the
terminfo database. bash$ infocmp
# Reconstructed via infocmp from file:
/usr/share/terminfo/r/rxvt
rxvt|rxvt terminal emulator (X Window System),
am, bce, eo, km, mir, msgr, xenl, xon,
colors#8, cols#80, it#8, lines#24, pairs#64,
acsc=``aaffggjjkkllmmnnooppqqrrssttuuvvwwxxyyzz{{||}}~~,
bel=^G, blink=\E[5m, bold=\E[1m,
civis=\E[?25l,
clear=\E[H\E[2J, cnorm=\E[?25h, cr=^M,
...
|
- reset
Reset terminal parameters and clear text screen. As with
clear, the cursor and prompt reappear in the
upper lefthand corner of the terminal. - clear
The clear command simply clears
the text screen at the console or in an
xterm. The prompt and cursor
reappear at the upper lefthand corner of the screen or
xterm window. This command may be used either at the command
line or in a script. See Example 10-25. - script
This utility records (saves to a file) all the user keystrokes at
the command line in a console or an xterm window. This, in effect,
creates a record of a session.
12.8. Math Commands"Doing the
numbers" - factor
Decompose an integer into prime factors. bash$ factor 27417
27417: 3 13 19 37
|
- bc
Bash can't handle floating point calculations, and
it lacks operators for certain important mathematical
functions. Fortunately, bc comes to
the rescue. Not just a versatile, arbitrary precision calculation
utility, bc offers many of the facilities of
a programming language. bc has a syntax vaguely resembling C. Since it is a fairly well-behaved UNIX utility, and may
therefore be used in a pipe,
bc comes in handy in scripts. Here is a simple template for using
bc to calculate a script
variable. This uses command
substitution. variable=$(echo "OPTIONS; OPERATIONS" | bc)
|
Example 12-42. Monthly Payment on a Mortgage #!/bin/bash
# monthlypmt.sh: Calculates monthly payment on a mortgage.
# This is a modification of code in the "mcalc" (mortgage calculator) package,
#+ by Jeff Schmidt and Mendel Cooper (yours truly, the author of this document).
# http://www.ibiblio.org/pub/Linux/apps/financial/mcalc-1.6.tar.gz [15k]
echo
echo "Given the principal, interest rate, and term of a mortgage,"
echo "calculate the monthly payment."
bottom=1.0
echo
echo -n "Enter principal (no commas) "
read principal
echo -n "Enter interest rate (percent) " # If 12%, enter "12", not ".12".
read interest_r
echo -n "Enter term (months) "
read term
interest_r=$(echo "scale=9; $interest_r/100.0" | bc) # Convert to decimal.
# "scale" determines how many decimal places.
interest_rate=$(echo "scale=9; $interest_r/12 + 1.0" | bc)
top=$(echo "scale=9; $principal*$interest_rate^$term" | bc)
echo; echo "Please be patient. This may take a while."
let "months = $term - 1"
# ====================================================================
for ((x=$months; x > 0; x--))
do
bot=$(echo "scale=9; $interest_rate^$x" | bc)
bottom=$(echo "scale=9; $bottom+$bot" | bc)
# bottom = $(($bottom + $bot"))
done
# ====================================================================
# --------------------------------------------------------------------
# Rick Boivie pointed out a more efficient implementation
#+ of the above loop, which decreases computation time by 2/3.
# for ((x=1; x <= $months; x++))
# do
# bottom=$(echo "scale=9; $bottom * $interest_rate + 1" | bc)
# done
# And then he came up with an even more efficient alternative,
#+ one that cuts down the run time by about 95%!
# bottom=`{
# echo "scale=9; bottom=$bottom; interest_rate=$interest_rate"
# for ((x=1; x <= $months; x++))
# do
# echo 'bottom = bottom * interest_rate + 1'
# done
# echo 'bottom'
# } | bc` # Embeds a 'for loop' within command substitution.
# --------------------------------------------------------------------------
# On the other hand, Frank Wang suggests:
# bottom=$(echo "scale=9; ($interest_rate^$term-1)/($interest_rate-1)" | bc)
# Because . . .
# The algorithm behind the loop
#+ is actually a sum of geometric proportion series.
# The sum formula is e0(1-q^n)/(1-q),
#+ where e0 is the first element and q=e(n+1)/e(n)
#+ and n is the number of elements.
# --------------------------------------------------------------------------
# let "payment = $top/$bottom"
payment=$(echo "scale=2; $top/$bottom" | bc)
# Use two decimal places for dollars and cents.
echo
echo "monthly payment = \$$payment" # Echo a dollar sign in front of amount.
echo
exit 0
# Exercises:
# 1) Filter input to permit commas in principal amount.
# 2) Filter input to permit interest to be entered as percent or decimal.
# 3) If you are really ambitious,
# expand this script to print complete amortization tables. |
Example 12-43. Base Conversion #!/bin/bash
##########################################################################
# Shellscript: base.sh - print number to different bases (Bourne Shell)
# Author : Heiner Steven (heiner.steven@odn.de)
# Date : 07-03-95
# Category : Desktop
# $Id: base.sh,v 1.2 2000/02/06 19:55:35 heiner Exp $
# ==> Above line is RCS ID info.
##########################################################################
# Description
#
# Changes
# 21-03-95 stv fixed error occuring with 0xb as input (0.2)
##########################################################################
# ==> Used in this document with the script author's permission.
# ==> Comments added by document author.
NOARGS=65
PN=`basename "$0"` # Program name
VER=`echo '$Revision: 1.2 $' | cut -d' ' -f2` # ==> VER=1.2
Usage () {
echo "$PN - print number to different bases, $VER (stv '95)
usage: $PN [number ...]
If no number is given, the numbers are read from standard input.
A number may be
binary (base 2) starting with 0b (i.e. 0b1100)
octal (base 8) starting with 0 (i.e. 014)
hexadecimal (base 16) starting with 0x (i.e. 0xc)
decimal otherwise (i.e. 12)" >&2
exit $NOARGS
} # ==> Function to print usage message.
Msg () {
for i # ==> in [list] missing.
do echo "$PN: $i" >&2
done
}
Fatal () { Msg "$@"; exit 66; }
PrintBases () {
# Determine base of the number
for i # ==> in [list] missing...
do # ==> so operates on command line arg(s).
case "$i" in
0b*) ibase=2;; # binary
0x*|[a-f]*|[A-F]*) ibase=16;; # hexadecimal
0*) ibase=8;; # octal
[1-9]*) ibase=10;; # decimal
*)
Msg "illegal number $i - ignored"
continue;;
esac
# Remove prefix, convert hex digits to uppercase (bc needs this)
number=`echo "$i" | sed -e 's:^0[bBxX]::' | tr '[a-f]' '[A-F]'`
# ==> Uses ":" as sed separator, rather than "/".
# Convert number to decimal
dec=`echo "ibase=$ibase; $number" | bc` # ==> 'bc' is calculator utility.
case "$dec" in
[0-9]*) ;; # number ok
*) continue;; # error: ignore
esac
# Print all conversions in one line.
# ==> 'here document' feeds command list to 'bc'.
echo `bc <<!
obase=16; "hex="; $dec
obase=10; "dec="; $dec
obase=8; "oct="; $dec
obase=2; "bin="; $dec
!
` | sed -e 's: : :g'
done
}
while [ $# -gt 0 ]
# ==> Is a "while loop" really necessary here,
# ==>+ since all the cases either break out of the loop
# ==>+ or terminate the script.
# ==> (Thanks, Paulo Marcel Coelho Aragao.)
do
case "$1" in
--) shift; break;;
-h) Usage;; # ==> Help message.
-*) Usage;;
*) break;; # first number
esac # ==> More error checking for illegal input might be useful.
shift
done
if [ $# -gt 0 ]
then
PrintBases "$@"
else # read from stdin
while read line
do
PrintBases $line
done
fi
exit 0 |
An alternate method of invoking bc
involves using a here
document embedded within a command substitution
block. This is especially appropriate when a script
needs to pass a list of options and commands to
bc. variable=`bc << LIMIT_STRING
options
statements
operations
LIMIT_STRING
`
...or...
variable=$(bc << LIMIT_STRING
options
statements
operations
LIMIT_STRING
) |
Example 12-44. Invoking bc using a "here
document" #!/bin/bash
# Invoking 'bc' using command substitution
# in combination with a 'here document'.
var1=`bc << EOF
18.33 * 19.78
EOF
`
echo $var1 # 362.56
# $( ... ) notation also works.
v1=23.53
v2=17.881
v3=83.501
v4=171.63
var2=$(bc << EOF
scale = 4
a = ( $v1 + $v2 )
b = ( $v3 * $v4 )
a * b + 15.35
EOF
)
echo $var2 # 593487.8452
var3=$(bc -l << EOF
scale = 9
s ( 1.7 )
EOF
)
# Returns the sine of 1.7 radians.
# The "-l" option calls the 'bc' math library.
echo $var3 # .991664810
# Now, try it in a function...
hyp= # Declare global variable.
hypotenuse () # Calculate hypotenuse of a right triangle.
{
hyp=$(bc -l << EOF
scale = 9
sqrt ( $1 * $1 + $2 * $2 )
EOF
)
# Unfortunately, can't return floating point values from a Bash function.
}
hypotenuse 3.68 7.31
echo "hypotenuse = $hyp" # 8.184039344
exit 0 |
Example 12-45. Calculating PI #!/bin/bash
# cannon.sh: Approximating PI by firing cannonballs.
# This is a very simple instance of a "Monte Carlo" simulation:
#+ a mathematical model of a real-life event,
#+ using pseudorandom numbers to emulate random chance.
# Consider a perfectly square plot of land, 10000 units on a side.
# This land has a perfectly circular lake in its center,
#+ with a diameter of 10000 units.
# The plot is actually mostly water, except for land in the four corners.
# (Think of it as a square with an inscribed circle.)
#
# We will fire iron cannonballs from an old-style cannon
#+ at the square.
# All the shots impact somewhere on the square,
#+ either in the lake or on the dry corners.
# Since the lake takes up most of the area,
#+ most of the shots will SPLASH! into the water.
# Just a few shots will THUD! into solid ground
#+ in the four corners of the square.
#
# If we take enough random, unaimed shots at the square,
#+ Then the ratio of SPLASHES to total shots will approximate
#+ the value of PI/4.
#
# The reason for this is that the cannon is actually shooting
#+ only at the upper right-hand quadrant of the square,
#+ i.e., Quadrant I of the Cartesian coordinate plane.
# (The previous explanation was a simplification.)
#
# Theoretically, the more shots taken, the better the fit.
# However, a shell script, as opposed to a compiled language
#+ with floating-point math built in, requires a few compromises.
# This tends to lower the accuracy of the simulation, of course.
DIMENSION=10000 # Length of each side of the plot.
# Also sets ceiling for random integers generated.
MAXSHOTS=1000 # Fire this many shots.
# 10000 or more would be better, but would take too long.
PMULTIPLIER=4.0 # Scaling factor to approximate PI.
get_random ()
{
SEED=$(head -1 /dev/urandom | od -N 1 | awk '{ print $2 }')
RANDOM=$SEED # From "seeding-random.sh"
#+ example script.
let "rnum = $RANDOM % $DIMENSION" # Range less than 10000.
echo $rnum
}
distance= # Declare global variable.
hypotenuse () # Calculate hypotenuse of a right triangle.
{ # From "alt-bc.sh" example.
distance=$(bc -l << EOF
scale = 0
sqrt ( $1 * $1 + $2 * $2 )
EOF
)
# Setting "scale" to zero rounds down result to integer value,
#+ a necessary compromise in this script.
# This diminshes the accuracy of the simulation, unfortunately.
}
# main() {
# Initialize variables.
shots=0
splashes=0
thuds=0
Pi=0
while [ "$shots" -lt "$MAXSHOTS" ] # Main loop.
do
xCoord=$(get_random) # Get random X and Y coords.
yCoord=$(get_random)
hypotenuse $xCoord $yCoord # Hypotenuse of right-triangle =
#+ distance.
((shots++))
printf "#%4d " $shots
printf "Xc = %4d " $xCoord
printf "Yc = %4d " $yCoord
printf "Distance = %5d " $distance # Distance from
#+ center of lake --
# the "origin" --
#+ coordinate (0,0).
if [ "$distance" -le "$DIMENSION" ]
then
echo -n "SPLASH! "
((splashes++))
else
echo -n "THUD! "
((thuds++))
fi
Pi=$(echo "scale=9; $PMULTIPLIER*$splashes/$shots" | bc)
# Multiply ratio by 4.0.
echo -n "PI ~ $Pi"
echo
done
echo
echo "After $shots shots, PI looks like approximately $Pi."
# Tends to run a bit high . . .
# Probably due to round-off error and imperfect randomness of $RANDOM.
echo
# }
exit 0
# One might well wonder whether a shell script is appropriate for
#+ an application as complex and computation-intensive as a simulation.
#
# There are at least two justifications.
# 1) As a proof of concept: to show it can be done.
# 2) To prototype and test the algorithms before rewriting
#+ it in a compiled high-level language. |
- dc
The dc (desk
calculator) utility is stack-oriented
and uses RPN ("Reverse Polish Notation"). Like
bc, it has much of the power of a
programming language. Most persons avoid dc, since it
requires non-intuitive RPN input. Yet, it has its uses. Example 12-46. Converting a decimal number to hexadecimal #!/bin/bash
# hexconvert.sh: Convert a decimal number to hexadecimal.
E_NOARGS=65 # Command-line arg missing.
BASE=16 # Hexadecimal.
if [ -z "$1" ]
then
echo "Usage: $0 number"
exit $E_NOARGS
# Need a command line argument.
fi
# Exercise: add argument validity checking.
hexcvt ()
{
if [ -z "$1" ]
then
echo 0
return # "Return" 0 if no arg passed to function.
fi
echo ""$1" "$BASE" o p" | dc
# "o" sets radix (numerical base) of output.
# "p" prints the top of stack.
# See 'man dc' for other options.
return
}
hexcvt "$1"
exit 0 |
Studying the info page for
dc gives some insight into its
intricacies. However, there seems to be a small,
select group of dc wizards who
delight in showing off their mastery of this powerful,
but arcane utility. bash$ echo "16i[q]sa[ln0=aln100%Pln100/snlbx]sbA0D68736142snlbxq" | dc"
Bash
|
Example 12-47. Factoring #!/bin/bash
# factr.sh: Factor a number
MIN=2 # Will not work for number smaller than this.
E_NOARGS=65
E_TOOSMALL=66
if [ -z $1 ]
then
echo "Usage: $0 number"
exit $E_NOARGS
fi
if [ "$1" -lt "$MIN" ]
then
echo "Number to factor must be $MIN or greater."
exit $E_TOOSMALL
fi
# Exercise: Add type checking (to reject non-integer arg).
echo "Factors of $1:"
# ---------------------------------------------------------------------------------
echo "$1[p]s2[lip/dli%0=1dvsr]s12sid2%0=13sidvsr[dli%0=1lrli2+dsi!>.]ds.xd1<2" | dc
# ---------------------------------------------------------------------------------
# Above line of code written by Michel Charpentier <charpov@cs.unh.edu>.
# Used with permission (thanks).
exit 0 |
- awk
Yet another way of doing floating point math in
a script is using awk's
built-in math functions in a shell
wrapper. Example 12-48. Calculating the hypotenuse of a triangle #!/bin/bash
# hypotenuse.sh: Returns the "hypotenuse" of a right triangle.
# ( square root of sum of squares of the "legs")
ARGS=2 # Script needs sides of triangle passed.
E_BADARGS=65 # Wrong number of arguments.
if [ $# -ne "$ARGS" ] # Test number of arguments to script.
then
echo "Usage: `basename $0` side_1 side_2"
exit $E_BADARGS
fi
AWKSCRIPT=' { printf( "%3.7f\n", sqrt($1*$1 + $2*$2) ) } '
# command(s) / parameters passed to awk
# Now, pipe the parameters to awk.
echo -n "Hypotenuse of $1 and $2 = "
echo $1 $2 | awk "$AWKSCRIPT"
exit 0 |
12.9. Miscellaneous CommandsCommand that fit in no
special category - jot, seq
These utilities emit a sequence of integers, with a
user-selected increment. The normal separator character between each integer is a
newline, but this can be changed with the -s
option. bash$ seq 5
1
2
3
4
5
bash$ seq -s : 5
1:2:3:4:5
|
Both jot and seq
come in handy in a for
loop. Example 12-49. Using seq to generate loop arguments #!/bin/bash
# Using "seq"
echo
for a in `seq 80` # or for a in $( seq 80 )
# Same as for a in 1 2 3 4 5 ... 80 (saves much typing!).
# May also use 'jot' (if present on system).
do
echo -n "$a "
done # 1 2 3 4 5 ... 80
# Example of using the output of a command to generate
# the [list] in a "for" loop.
echo; echo
COUNT=80 # Yes, 'seq' may also take a replaceable parameter.
for a in `seq $COUNT` # or for a in $( seq $COUNT )
do
echo -n "$a "
done # 1 2 3 4 5 ... 80
echo; echo
BEGIN=75
END=80
for a in `seq $BEGIN $END`
# Giving "seq" two arguments starts the count at the first one,
#+ and continues until it reaches the second.
do
echo -n "$a "
done # 75 76 77 78 79 80
echo; echo
BEGIN=45
INTERVAL=5
END=80
for a in `seq $BEGIN $INTERVAL $END`
# Giving "seq" three arguments starts the count at the first one,
#+ uses the second for a step interval,
#+ and continues until it reaches the third.
do
echo -n "$a "
done # 45 50 55 60 65 70 75 80
echo; echo
exit 0 |
A simpler example:
# Create a set of 10 files,
#+ named file.1, file.2 . . . file.10.
COUNT=10
PREFIX=file
for filename in `seq $COUNT`
do
touch $PREFIX.$filename
# Or, can do other operations,
#+ such as rm, grep, etc.
done |
Example 12-50. Letter Count" #!/bin/bash
# letter-count.sh: Counting letter occurrences in a text file.
# Written by Stefano Palmeri.
# Used in ABS Guide with permission.
# Slightly modified by document author.
MINARGS=2 # Script requires at least two arguments.
E_BADARGS=65
FILE=$1
let LETTERS=$#-1 # How many letters specified (as command-line args).
# (Subtract 1 from number of command line args.)
show_help(){
echo
echo Usage: `basename $0` file letters
echo Note: `basename $0` arguments are case sensitive.
echo Example: `basename $0` foobar.txt G n U L i N U x.
echo
}
# Checks number of arguments.
if [ $# -lt $MINARGS ]; then
echo
echo "Not enough arguments."
echo
show_help
exit $E_BADARGS
fi
# Checks if file exists.
if [ ! -f $FILE ]; then
echo "File \"$FILE\" does not exist."
exit $E_BADARGS
fi
# Counts letter occurrences .
for n in `seq $LETTERS`; do
shift
if [[ `echo -n "$1" | wc -c` -eq 1 ]]; then # Checks arg.
echo "$1" -\> `cat $FILE | tr -cd "$1" | wc -c` # Counting.
else
echo "$1 is not a single char."
fi
done
exit $?
# This script has exactly the same functionality as letter-count2.sh,
#+ but executes faster.
# Why? |
- getopt
The getopt command
parses command-line options preceded by a dash. This external command
corresponds to the getopts
Bash builtin. Using getopt permits
handling long options by means of the -l
flag, and this also allows parameter reshuffling. Example 12-51. Using getopt to parse command-line
options #!/bin/bash
# Using getopt.
# Try the following when invoking this script:
# sh ex33a.sh -a
# sh ex33a.sh -abc
# sh ex33a.sh -a -b -c
# sh ex33a.sh -d
# sh ex33a.sh -dXYZ
# sh ex33a.sh -d XYZ
# sh ex33a.sh -abcd
# sh ex33a.sh -abcdZ
# sh ex33a.sh -z
# sh ex33a.sh a
# Explain the results of each of the above.
E_OPTERR=65
if [ "$#" -eq 0 ]
then # Script needs at least one command-line argument.
echo "Usage $0 -[options a,b,c]"
exit $E_OPTERR
fi
set -- `getopt "abcd:" "$@"`
# Sets positional parameters to command-line arguments.
# What happens if you use "$*" instead of "$@"?
while [ ! -z "$1" ]
do
case "$1" in
-a) echo "Option \"a\"";;
-b) echo "Option \"b\"";;
-c) echo "Option \"c\"";;
-d) echo "Option \"d\" $2";;
*) break;;
esac
shift
done
# It is usually better to use the 'getopts' builtin in a script,
#+ rather than 'getopt'.
# See "ex33.sh".
exit 0 |
See Example 9-12 for a simplified emulation
of getopt. - run-parts
The run-parts command
executes all the scripts in a target directory, sequentially
in ASCII-sorted filename order. Of course, the scripts
need to have execute permission. The cron daemon invokes
run-parts to run the scripts in
the /etc/cron.*
directories. - yes
In its default behavior the yes
command feeds a continuous string of the character
y followed
by a line feed to stdout. A
control-c
terminates the run. A different output string
may be specified, as in yes different
string, which would continually output
different string to
stdout. One might well ask the purpose
of this. From the command line or in a script, the output
of yes can be redirected or piped into a
program expecting user input. In effect, this becomes a sort
of poor man's version of expect. yes | fsck /dev/hda1 runs
fsck non-interactively (careful!). yes | rm -r dirname has same effect as
rm -rf dirname (careful!).  | Caution advised when piping yes
to a potentially dangerous system command, such
as fsck or fdisk. It may have unintended
side-effects. |
- banner
Prints arguments as a large vertical banner to
stdout, using an ASCII character
(default '#'). This may be redirected to a printer for
hardcopy. - printenv
Show all the environmental
variables set for a particular user. bash$ printenv | grep HOME
HOME=/home/bozo
|
- lp
The lp and lpr
commands send file(s) to the print queue, to be printed as
hard copy.
These commands trace the origin of their names to the
line printers of another era. bash$ lp file1.txt
or bash lp
<file1.txt It is often useful to pipe the formatted output from
pr to lp. bash$ pr -options file1.txt | lp
Formatting packages, such as groff and
Ghostscript may send their output
directly to lp. bash$ groff -Tascii file.tr | lp
bash$ gs -options | lp file.ps
Related commands are lpq, for viewing
the print queue, and lprm, for removing
jobs from the print queue. - tee
[UNIX borrows an idea from the plumbing trade.] This is a redirection operator, but with a difference. Like the
plumber's "tee," it permits "siponing
off" to a file the output of a command
or commands within a pipe, but without affecting the result. This is
useful for printing an ongoing process to a file or paper, perhaps to
keep track of it for debugging purposes. (redirection)
|----> to file
|
==========================|====================
command ---> command ---> |tee ---> command ---> ---> output of pipe
===============================================
|
cat listfile* | sort | tee check.file | uniq > result.file |
(The file check.file contains
the concatenated sorted "listfiles",
before the duplicate lines are removed by uniq.)- mkfifo
This obscure command
creates a named pipe, a temporary
first-in-first-out buffer for
transferring data between processes.
Typically, one process writes to the FIFO, and the other
reads from it. See Example A-15. - pathchk
This command checks the validity of a filename. If the
filename exceeds the maximum allowable length (255
characters) or one or more of the directories in
its path is not searchable, then an error message
results. Unfortunately, pathchk does
not return a recognizable error code, and it is therefore
pretty much useless in a script. Consider instead the
file test operators. - dd
This is the somewhat obscure and much feared "data
duplicator" command. Originally a utility
for exchanging data on magnetic tapes between UNIX
minicomputers and IBM mainframes, this command still
has its uses. The dd command simply
copies a file (or stdin/stdout), but
with conversions. Possible conversions are ASCII/EBCDIC,
upper/lower case, swapping of byte pairs between input
and output, and skipping and/or truncating the head or
tail of the input file. A dd --help
lists the conversion and other options that this powerful
utility takes. # Converting a file to all uppercase:
dd if=$filename conv=ucase > $filename.uppercase
# lcase # For lower case conversion |
Example 12-52. A script that copies itself #!/bin/bash
# self-copy.sh
# This script copies itself.
file_subscript=copy
dd if=$0 of=$0.$file_subscript 2>/dev/null
# Suppress messages from dd: ^^^^^^^^^^^
exit $? |
Example 12-53. Exercising dd #!/bin/bash
# exercising-dd.sh
# Script by Stephane Chazelas.
# Somewhat modified by document author.
input_file=$0 # This script.
output_file=log.txt
n=3
p=5
dd if=$input_file of=$output_file bs=1 skip=$((n-1)) count=$((p-n+1)) 2> /dev/null
# Extracts characters n to p from this script.
# -------------------------------------------------------
echo -n "hello world" | dd cbs=1 conv=unblock 2> /dev/null
# Echoes "hello world" vertically.
exit 0 |
To demonstrate just how versatile dd is,
let's use it to capture keystrokes. Example 12-54. Capturing Keystrokes #!/bin/bash
# dd-keypress.sh: Capture keystrokes without needing to press ENTER.
keypresses=4 # Number of keypresses to capture.
old_tty_setting=$(stty -g) # Save old terminal settings.
echo "Press $keypresses keys."
stty -icanon -echo # Disable canonical mode.
# Disable local echo.
keys=$(dd bs=1 count=$keypresses 2> /dev/null)
# 'dd' uses stdin, if "if" (input file) not specified.
stty "$old_tty_setting" # Restore old terminal settings.
echo "You pressed the \"$keys\" keys."
# Thanks, Stephane Chazelas, for showing the way.
exit 0 |
The dd command can do random access on a
data stream.
echo -n . | dd bs=1 seek=4 of=file conv=notrunc
# The "conv=notrunc" option means that the output file will not be truncated.
# Thanks, S.C. |
The dd command can copy raw data
and disk images to and from devices, such as floppies and
tape drives (Example A-5). A common use is
creating boot floppies. dd if=kernel-image of=/dev/fd0H1440
Similarly, dd can copy the entire
contents of a floppy, even one formatted with a
"foreign" OS, to the hard drive as an
image file. dd if=/dev/fd0 of=/home/bozo/projects/floppy.img
Other applications of dd include
initializing temporary swap files (Example 28-2)
and ramdisks (Example 28-3). It can even do a
low-level copy of an entire hard drive partition, although
this is not necessarily recommended. People (with presumably nothing better to do with
their time) are constantly thinking of interesting
applications of dd. Example 12-55. Securely deleting a file #!/bin/bash
# blot-out.sh: Erase "all" traces of a file.
# This script overwrites a target file alternately
#+ with random bytes, then zeros before finally deleting it.
# After that, even examining the raw disk sectors by conventional methods
#+ will not reveal the original file data.
PASSES=7 # Number of file-shredding passes.
# Increasing this slows script execution,
#+ especially on large target files.
BLOCKSIZE=1 # I/O with /dev/urandom requires unit block size,
#+ otherwise you get weird results.
E_BADARGS=70 # Various error exit codes.
E_NOT_FOUND=71
E_CHANGED_MIND=72
if [ -z "$1" ] # No filename specified.
then
echo "Usage: `basename $0` filename"
exit $E_BADARGS
fi
file=$1
if [ ! -e "$file" ]
then
echo "File \"$file\" not found."
exit $E_NOT_FOUND
fi
echo; echo -n "Are you absolutely sure you want to blot out \"$file\" (y/n)? "
read answer
case "$answer" in
[nN]) echo "Changed your mind, huh?"
exit $E_CHANGED_MIND
;;
*) echo "Blotting out file \"$file\".";;
esac
flength=$(ls -l "$file" | awk '{print $5}') # Field 5 is file length.
pass_count=1
chmod u+w "$file" # Allow overwriting/deleting the file.
echo
while [ "$pass_count" -le "$PASSES" ]
do
echo "Pass #$pass_count"
sync # Flush buffers.
dd if=/dev/urandom of=$file bs=$BLOCKSIZE count=$flength
# Fill with random bytes.
sync # Flush buffers again.
dd if=/dev/zero of=$file bs=$BLOCKSIZE count=$flength
# Fill with zeros.
sync # Flush buffers yet again.
let "pass_count += 1"
echo
done
rm -f $file # Finally, delete scrambled and shredded file.
sync # Flush buffers a final time.
echo "File \"$file\" blotted out and deleted."; echo
exit 0
# This is a fairly secure, if inefficient and slow method
#+ of thoroughly "shredding" a file.
# The "shred" command, part of the GNU "fileutils" package,
#+ does the same thing, although more efficiently.
# The file cannot not be "undeleted" or retrieved by normal methods.
# However . . .
#+ this simple method would *not* likely withstand
#+ sophisticated forensic analysis.
# This script may not play well with a journaled file system.
# Exercise (difficult): Fix it so it does.
# Tom Vier's "wipe" file-deletion package does a much more thorough job
#+ of file shredding than this simple script.
# http://www.ibiblio.org/pub/Linux/utils/file/wipe-2.0.0.tar.bz2
# For an in-depth analysis on the topic of file deletion and security,
#+ see Peter Gutmann's paper,
#+ "Secure Deletion of Data From Magnetic and Solid-State Memory".
# http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html |
- od
The od, or octal
dump filter converts input (or files) to octal
(base-8) or other bases. This is useful for viewing or
processing binary data files or otherwise unreadable system
device files, such as /dev/urandom,
and as a filter for binary data. See Example 9-28 and Example 12-13. - hexdump
Performs a hexadecimal, octal, decimal, or ASCII
dump of a binary file. This command is the rough equivalent
of od, above, but not nearly as
useful. - objdump
Displays information about an object file or binary
executable in either hexadecimal form or as a disassembled
listing (with the -d option). bash$ objdump -d /bin/ls
/bin/ls: file format elf32-i386
Disassembly of section .init:
080490bc <.init>:
80490bc: 55 push %ebp
80490bd: 89 e5 mov %esp,%ebp
. . .
|
- mcookie
This command generates a "magic cookie", a
128-bit (32-character) pseudorandom hexadecimal number,
normally used as an authorization "signature"
by the X server. This also available for use in a script as
a "quick 'n dirty" random number.
Of course, a script could use md5 for the same purpose.
# Generate md5 checksum on the script itself.
random001=`md5sum $0 | awk '{print $1}'`
# Uses 'awk' to strip off the filename. |
The mcookie command gives yet another way
to generate a "unique" filename. Example 12-56. Filename generator #!/bin/bash
# tempfile-name.sh: temp filename generator
BASE_STR=`mcookie` # 32-character magic cookie.
POS=11 # Arbitrary position in magic cookie string.
LEN=5 # Get $LEN consecutive characters.
prefix=temp # This is, after all, a "temp" file.
# For more "uniqueness," generate the filename prefix
#+ using the same method as the suffix, below.
suffix=${BASE_STR:POS:LEN}
# Extract a 5-character string, starting at position 11.
temp_filename=$prefix.$suffix
# Construct the filename.
echo "Temp filename = "$temp_filename""
# sh tempfile-name.sh
# Temp filename = temp.e19ea
# Compare this method of generating "unique" filenames
#+ with the 'date' method in ex51.sh.
exit 0 |
- units
This utility converts between different units of measure.
While normally invoked in interactive mode,
units may find use in a script. Example 12-57. Converting meters to miles #!/bin/bash
# unit-conversion.sh
convert_units () # Takes as arguments the units to convert.
{
cf=$(units "$1" "$2" | sed --silent -e '1p' | awk '{print $2}')
# Strip off everything except the actual conversion factor.
echo "$cf"
}
Unit1=miles
Unit2=meters
cfactor=`convert_units $Unit1 $Unit2`
quantity=3.73
result=$(echo $quantity*$cfactor | bc)
echo "There are $result $Unit2 in $quantity $Unit1."
# What happens if you pass incompatible units,
#+ such as "acres" and "miles" to the function?
exit 0 |
- m4
A hidden treasure, m4 is a
powerful macro processing filter,
virtually a complete language. Although
originally written as a pre-processor for
RatFor, m4
turned out to be useful as a stand-alone utility. In
fact, m4 combines some of the
functionality of eval,
tr, and awk, in addition to its extensive
macro expansion facilities. The April, 2002 issue of Linux Journal
has a very nice article on m4 and
its uses. Example 12-58. Using m4 #!/bin/bash
# m4.sh: Using the m4 macro processor
# Strings
string=abcdA01
echo "len($string)" | m4 # 7
echo "substr($string,4)" | m4 # A01
echo "regexp($string,[0-1][0-1],\&Z)" | m4 # 01Z
# Arithmetic
echo "incr(22)" | m4 # 23
echo "eval(99 / 3)" | m4 # 33
exit 0 |
- doexec
The doexec command enables passing
an arbitrary list of arguments to a binary
executable. In particular, passing
argv[0] (which corresponds to $0 in a script) lets the
executable be invoked by various names, and it can then
carry out different sets of actions, according to the name
by which it was called. What this amounts to is roundabout
way of passing options to an executable. For example, the /usr/local/bin directory might
contain a binary called "aaa". Invoking
doexec /usr/local/bin/aaa list
would list all those files
in the current working directory beginning with an
"a", while invoking (the same executable
with) doexec /usr/local/bin/aaa delete
would delete those files.  | The various behaviors of the executable
must be defined within the code of the executable itself,
analogous to something like the following in a shell script:
case `basename $0` in
"name1" ) do_something;;
"name2" ) do_something_else;;
"name3" ) do_yet_another_thing;;
* ) bail_out;;
esac |
|
- dialog
The dialog family of tools
provide a method of calling interactive
"dialog" boxes from a script. The more
elaborate variations of dialog --
gdialog, Xdialog,
and kdialog -- actually invoke X-Windows
widgets. See Example 33-19. - sox
The sox, or
"sound
exchange" command plays and
performs transformations on sound files. In fact,
the /usr/bin/play executable
(now deprecated) is nothing but a shell wrapper for
sox. For example, sox soundfile.wav
soundfile.au changes a WAV sound file into a
(Sun audio format) AU sound file. Shell scripts are ideally suited for batch processing
sox operations on
sound files. For examples, see the Linux Radio
Timeshift HOWTO and the MP3do
Project.
Chapter 13. System and Administrative CommandsThe startup and shutdown scripts in
/etc/rc.d illustrate the uses
(and usefulness) of many of these comands. These are usually
invoked by root and used for system maintenance or emergency
filesystem repairs. Use with caution, as some of these commands
may damage your system if misused. Users and Groups - users
Show all logged on users. This is the approximate
equivalent of who -q. - groups
Lists the current user and the groups she belongs to.
This corresponds to the $GROUPS internal variable,
but gives the group names, rather than the numbers. bash$ groups
bozita cdrom cdwriter audio xgrp
bash$ echo $GROUPS
501 |
- chown, chgrp
The chown command changes the
ownership of a file or files. This command is a useful
method that root can use to
shift file ownership from one user to another. An ordinary
user may not change the ownership of files, not even her
own files.
The chgrp command changes the
group ownership of a file or
files. You must be owner of the file(s) as well as a member
of the destination group (or root)
to use this operation.
chgrp --recursive dunderheads *.data
# The "dunderheads" group will now own all the "*.data" files
#+ all the way down the $PWD directory tree (that's what "recursive" means). |
- useradd, userdel
The useradd administrative command
adds a user account to the system and creates a home
directory for that particular user, if so specified. The
corresponding userdel command removes
a user account from the system
and deletes associated files.  | The adduser command is a synonym
for useradd and is usually a symbolic link to
it. |
- usermod
Modify a user account. Changes may be made to the password,
group membership, expiration date, and other attributes of
a given user's account. With this command, a user's password
may be locked, which has the effect of disabling the
account. - groupmod
Modify a given group. The group name and/or ID number may be
changed using this command. - id
The id command lists the real and
effective user IDs and the group IDs of the user
associated with the current process. This is the
counterpart to the $UID,
$EUID, and $GROUPS internal Bash
variables. bash$ id
uid=501(bozo) gid=501(bozo) groups=501(bozo),22(cdrom),80(cdwriter),81(audio)
bash$ echo $UID
501 |
 | The id command shows the
effective IDs only when they differ
from the real ones. |
Also see Example 9-5. - who
Show all users logged on to the system. bash$ who
bozo tty1 Apr 27 17:45
bozo pts/0 Apr 27 17:46
bozo pts/1 Apr 27 17:47
bozo pts/2 Apr 27 17:49
|
The -m gives detailed information about
only the current user. Passing any two arguments to
who is the equivalent of who
-m, as in who am i or who
The Man. bash$ who -m
localhost.localdomain!bozo pts/2 Apr 27 17:49
|
whoami is similar to who
-m, but only lists the user name.
- w
Show all logged on users and the processes belonging to them. This is
an extended version of who. The output of w
may be piped to grep to find a specific user and/or process. bash$ w | grep startx
bozo tty1 - 4:22pm 6:41 4.47s 0.45s startx |
- logname
Show current user's login name (as found in
/var/run/utmp). This is a
near-equivalent to whoami,
above. bash$ logname
bozo
bash$ whoami
bozo |
However... bash$ su
Password: ......
bash# whoami
root
bash# logname
bozo |
 | While logname prints the name
of the logged in user, whoami gives the
name of the user attached to the current process. As we have
just seen, sometimes these are not the same. |
- su
Runs a program or script as a
substitute user.
su rjones starts a shell as user
rjones. A naked su
defaults to root. See Example A-15. - sudo
Runs a command as root (or another user). This may
be used in a script, thus permitting a regular user to
run the script. #!/bin/bash
# Some commands.
sudo cp /root/secretfile /home/bozo/secret
# Some more commands. |
The file /etc/sudoers holds
the names of users permitted to invoke
sudo. - passwd
Sets, changes, or manages a user's password. The passwd command can be used in
a script, but should not be. Example 13-1. Setting a new password #!/bin/bash
# setnew-password.sh: For demonstration purposes only.
# Not a good idea to actually run this script.
# This script must be run as root.
ROOT_UID=0 # Root has $UID 0.
E_WRONG_USER=65 # Not root?
E_NOSUCHUSER=70
SUCCESS=0
if [ "$UID" -ne "$ROOT_UID" ]
then
echo; echo "Only root can run this script."; echo
exit $E_WRONG_USER
else
echo
echo "You should know better than to run this script, root."
echo "Even root users get the blues... "
echo
fi
username=bozo
NEWPASSWORD=security_violation
# Check if bozo lives here.
grep -q "$username" /etc/passwd
if [ $? -ne $SUCCESS ]
then
echo "User $username does not exist."
echo "No password changed."
exit $E_NOSUCHUSER
fi
echo "$NEWPASSWORD" | passwd --stdin "$username"
# The '--stdin' option to 'passwd' permits
#+ getting a new password from stdin (or a pipe).
echo; echo "User $username's password changed!"
# Using the 'passwd' command in a script is dangerous.
exit 0 |
The passwd command's -l,
-u, and -d options permit
locking, unlocking, and deleting a user's password. Only
root may use these options. - ac
Show users' logged in time, as read from
/var/log/wtmp. This is one of the GNU
accounting utilities. - last
List last logged in users, as read from
/var/log/wtmp. This command can also
show remote logins. For example, to show the last few times the system
rebooted: bash$ last reboot
reboot system boot 2.6.9-1.667 Fri Feb 4 18:18 (00:02)
reboot system boot 2.6.9-1.667 Fri Feb 4 15:20 (01:27)
reboot system boot 2.6.9-1.667 Fri Feb 4 12:56 (00:49)
reboot system boot 2.6.9-1.667 Thu Feb 3 21:08 (02:17)
. . .
wtmp begins Tue Feb 1 12:50:09 2005 |
- newgrp
Change user's group ID without logging out. This permits
access to the new group's files. Since users may be
members of multiple groups simultaneously, this command
finds little use.
Terminals - tty
Echoes the name of the current user's terminal.
Note that each separate xterm
window counts as a different terminal. - stty
Shows and/or changes terminal settings. This complex
command, used in a script, can control terminal behavior
and the way output displays. See the info page, and study
it carefully. Example 13-2. Setting an erase character #!/bin/bash
# erase.sh: Using "stty" to set an erase character when reading input.
echo -n "What is your name? "
read name # Try to backspace
#+ to erase characters of input.
# Problems?
echo "Your name is $name."
stty erase '#' # Set "hashmark" (#) as erase character.
echo -n "What is your name? "
read name # Use # to erase last character typed.
echo "Your name is $name."
# Warning: Even after the script exits, the new key value remains set.
exit 0 |
Example 13-3. secret password:
Turning off terminal echoing #!/bin/bash
# secret-pw.sh: secret password
echo
echo -n "Enter password "
read passwd
echo "password is $passwd"
echo -n "If someone had been looking over your shoulder, "
echo "your password would have been compromised."
echo && echo # Two line-feeds in an "and list."
stty -echo # Turns off screen echo.
echo -n "Enter password again "
read passwd
echo
echo "password is $passwd"
echo
stty echo # Restores screen echo.
exit 0
# Do an 'info stty' for more on this useful-but-tricky command. |
A creative use of stty is detecting a
user keypress (without hitting
ENTER). Example 13-4. Keypress detection #!/bin/bash
# keypress.sh: Detect a user keypress ("hot keys").
echo
old_tty_settings=$(stty -g) # Save old settings (why?).
stty -icanon
Keypress=$(head -c1) # or $(dd bs=1 count=1 2> /dev/null)
# on non-GNU systems
echo
echo "Key pressed was \""$Keypress"\"."
echo
stty "$old_tty_settings" # Restore old settings.
# Thanks, Stephane Chazelas.
exit 0 |
Also see Example 9-3. - setterm
Set certain terminal attributes. This command writes
to its terminal's stdout a string that
changes the behavior of that terminal. bash$ setterm -cursor off
bash$
|
The setterm command can be used within a
script to change the appearance of text written to
stdout, although there are certainly
better tools available
for this purpose. setterm -bold on
echo bold hello
setterm -bold off
echo normal hello |
- tset
Show or initialize terminal settings.
This is a less capable version of
stty. bash$ tset -r
Terminal type is xterm-xfree86.
Kill is control-U (^U).
Interrupt is control-C (^C).
|
- setserial
Set or display serial port parameters. This command must be
run by root user and is usually found in a system setup
script. # From /etc/pcmcia/serial script:
IRQ=`setserial /dev/$DEVICE | sed -e 's/.*IRQ: //'`
setserial /dev/$DEVICE irq 0 ; setserial /dev/$DEVICE irq $IRQ |
- getty, agetty
The initialization process for a terminal uses
getty or agetty
to set it up for login by a user. These commands are not
used within user shell scripts. Their scripting counterpart
is stty. - mesg
Enables or disables write access to the current user's
terminal. Disabling access would prevent another user
on the network to write
to the terminal.  | It can be very annoying to have a message
about ordering pizza suddenly appear in the middle of
the text file you are editing. On a multi-user network,
you might therefore wish to disable write access to your
terminal when you need to avoid interruptions. |
- wall
This is an acronym for "write all", i.e., sending
a message to all users at every terminal logged into the
network. It is primarily a system administrator's tool,
useful, for example, when warning everyone that the
system will shortly go down due to a problem (see Example 17-1). bash$ wall System going down for maintenance in 5 minutes!
Broadcast message from bozo (pts/1) Sun Jul 8 13:53:27 2001...
System going down for maintenance in 5 minutes!
|
 | If write access to a particular terminal has been
disabled with mesg, then
wall cannot send a message to
it. |
Information and Statistics - uname
Output system specifications (OS, kernel version,
etc.) to stdout. Invoked with the
-a option, gives verbose system info
(see Example 12-5). The -s
option shows only the OS type. bash$ uname -a
Linux localhost.localdomain 2.2.15-2.5.0 #1 Sat Feb 5 00:13:43 EST 2000 i686 unknown
bash$ uname -s
Linux |
- arch
Show system architecture.
Equivalent to uname -m. See Example 10-26. bash$ arch
i686
bash$ uname -m
i686 |
- lastcomm
Gives information about previous commands, as stored
in the /var/account/pacct file. Command
name and user name can be specified by options. This is
one of the GNU accounting utilities. - lastlog
List the last login time of all system users. This
references the /var/log/lastlog
file. bash$ lastlog
root tty1 Fri Dec 7 18:43:21 -0700 2001
bin **Never logged in**
daemon **Never logged in**
...
bozo tty1 Sat Dec 8 21:14:29 -0700 2001
bash$ lastlog | grep root
root tty1 Fri Dec 7 18:43:21 -0700 2001
|
 | This command will fail if the user invoking
it does not have read permission for the
/var/log/lastlog file. |
- lsof
List open files. This command outputs a detailed
table of all currently open files and gives information
about their owner, size, the processes associated with
them, and more. Of course, lsof may
be piped to grep and/or
awk to parse and analyze
its results. bash$ lsof
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
init 1 root mem REG 3,5 30748 30303 /sbin/init
init 1 root mem REG 3,5 73120 8069 /lib/ld-2.1.3.so
init 1 root mem REG 3,5 931668 8075 /lib/libc-2.1.3.so
cardmgr 213 root mem REG 3,5 36956 30357 /sbin/cardmgr
...
|
- strace
Diagnostic and debugging tool for tracing system
calls and signals. The simplest way of invoking it is
strace COMMAND. bash$ strace df
execve("/bin/df", ["df"], [/* 45 vars */]) = 0
uname({sys="Linux", node="bozo.localdomain", ...}) = 0
brk(0) = 0x804f5e4
...
|
This is the Linux equivalent of
the Solaris truss command. - nmap
Network port scanner. This command scans a server to
locate open ports and the services associated with those
ports. It is an important security tool for locking down
a network against hacking attempts. #!/bin/bash
SERVER=$HOST # localhost.localdomain (127.0.0.1).
PORT_NUMBER=25 # SMTP port.
nmap $SERVER | grep -w "$PORT_NUMBER" # Is that particular port open?
# grep -w matches whole words only,
#+ so this wouldn't match port 1025, for example.
exit 0
# 25/tcp open smtp |
- nc
The nc (netcat)
utility is a complete toolkit for connecting to and
listening to TCP and UDP ports. It is useful as a diagnostic
and testing tool and as a component in simple script-based HTTP
clients and servers. bash$ nc localhost.localdomain 25
220 localhost.localdomain ESMTP Sendmail 8.13.1/8.13.1; Thu, 31 Mar 2005 15:41:35 -0700 |
Example 13-5. Checking a remote server for identd #! /bin/sh
## Duplicate DaveG's ident-scan thingie using netcat. Oooh, he'll be p*ssed.
## Args: target port [port port port ...]
## Hose stdout _and_ stderr together.
##
## Advantages: runs slower than ident-scan, giving remote inetd less cause
##+ for alarm, and only hits the few known daemon ports you specify.
## Disadvantages: requires numeric-only port args, the output sleazitude,
##+ and won't work for r-services when coming from high source ports.
# Script author: Hobbit <hobbit@avian.org>
# Used in ABS Guide with permission.
# ---------------------------------------------------
E_BADARGS=65 # Need at least two args.
TWO_WINKS=2 # How long to sleep.
THREE_WINKS=3
IDPORT=113 # Authentication "tap ident" port.
RAND1=999
RAND2=31337
TIMEOUT0=9
TIMEOUT1=8
TIMEOUT2=4
# ---------------------------------------------------
case "${2}" in
"" ) echo "Need HOST and at least one PORT." ; exit $E_BADARGS ;;
esac
# Ping 'em once and see if they *are* running identd.
nc -z -w $TIMEOUT0 "$1" $IDPORT || { echo "Oops, $1 isn't running identd." ; exit 0 ; }
# -z scans for listening daemons.
# -w $TIMEOUT = How long to try to connect.
# Generate a randomish base port.
RP=`expr $$ % $RAND1 + $RAND2`
TRG="$1"
shift
while test "$1" ; do
nc -v -w $TIMEOUT1 -p ${RP} "$TRG" ${1} < /dev/null > /dev/null &
PROC=$!
sleep $THREE_WINKS
echo "${1},${RP}" | nc -w $TIMEOUT2 -r "$TRG" $IDPORT 2>&1
sleep $TWO_WINKS
# Does this look like a lamer script or what . . . ?
# ABS Guide author comments: "It ain't really all that bad,
#+ rather clever, actually."
kill -HUP $PROC
RP=`expr ${RP} + 1`
shift
done
exit $?
# Notes:
# -----
# Try commenting out line 30 and running this script
#+ with "localhost.localdomain 25" as arguments.
# For more of Hobbit's 'nc' example scripts,
#+ look in the documentation:
#+ the /usr/share/doc/nc-X.XX/scripts directory. |
And, of course, there's Dr. Andrew Tridgell's notorious
one-line script in the BitKeeper Affair:
echo clone | nc thunk.org 5000 > e2fsprogs.dat |
- free
Shows memory and cache usage in tabular form. The
output of this command lends itself to parsing, using
grep, awk or Perl. The
procinfo command shows all the
information that free does, and much
more. bash$ free
total used free shared buffers cached
Mem: 30504 28624 1880 15820 1608 16376
-/+ buffers/cache: 10640 19864
Swap: 68540 3128 65412 |
To show unused RAM memory: bash$ free | grep Mem | awk '{ print $4 }'
1880 |
- procinfo
Extract and list information and statistics from the
/proc
pseudo-filesystem. This gives a very extensive and
detailed listing. bash$ procinfo | grep Bootup
Bootup: Wed Mar 21 15:15:50 2001 Load average: 0.04 0.21 0.34 3/47 6829 |
- lsdev
List devices, that is, show installed hardware. bash$ lsdev
Device DMA IRQ I/O Ports
------------------------------------------------
cascade 4 2
dma 0080-008f
dma1 0000-001f
dma2 00c0-00df
fpu 00f0-00ff
ide0 14 01f0-01f7 03f6-03f6
...
|
- du
Show (disk) file usage, recursively. Defaults to current
working directory, unless otherwise specified. bash$ du -ach
1.0k ./wi.sh
1.0k ./tst.sh
1.0k ./random.file
6.0k .
6.0k total |
- df
Shows filesystem usage in tabular form. bash$ df
Filesystem 1k-blocks Used Available Use% Mounted on
/dev/hda5 273262 92607 166547 36% /
/dev/hda8 222525 123951 87085 59% /home
/dev/hda7 1408796 1075744 261488 80% /usr |
- dmesg
Lists all system bootup messages to
stdout. Handy for debugging and
ascertaining which device drivers were installed
and which system interrupts in use. The output
of dmesg may, of course, be
parsed with grep,
sed, or awk from within a script. bash$ dmesg | grep hda
Kernel command line: ro root=/dev/hda2
hda: IBM-DLGA-23080, ATA DISK drive
hda: 6015744 sectors (3080 MB) w/96KiB Cache, CHS=746/128/63
hda: hda1 hda2 hda3 < hda5 hda6 hda7 > hda4
|
- stat
Gives detailed and verbose statistics
on a given file (even a directory or device file) or set
of files. bash$ stat test.cru
File: "test.cru"
Size: 49970 Allocated Blocks: 100 Filetype: Regular File
Mode: (0664/-rw-rw-r--) Uid: ( 501/ bozo) Gid: ( 501/ bozo)
Device: 3,8 Inode: 18185 Links: 1
Access: Sat Jun 2 16:40:24 2001
Modify: Sat Jun 2 16:40:24 2001
Change: Sat Jun 2 16:40:24 2001
|
If the target file does not exist, stat
returns an error message. bash$ stat nonexistent-file
nonexistent-file: No such file or directory
|
- vmstat
Display virtual memory statistics. bash$ vmstat
procs memory swap io system cpu
r b w swpd free buff cache si so bi bo in cs us sy id
0 0 0 0 11040 2636 38952 0 0 33 7 271 88 8 3 89
|
- netstat
Show current network statistics and information,
such as routing tables and active connections. This utility
accesses information in /proc/net
(Chapter 27). See Example 27-3. netstat -r is equivalent to route. bash$ netstat
Active Internet connections (w/o servers)
Proto Recv-Q Send-Q Local Address Foreign Address State
Active UNIX domain sockets (w/o servers)
Proto RefCnt Flags Type State I-Node Path
unix 11 [ ] DGRAM 906 /dev/log
unix 3 [ ] STREAM CONNECTED 4514 /tmp/.X11-unix/X0
unix 3 [ ] STREAM CONNECTED 4513
. . . |
- uptime
Shows how long the system has been running, along with
associated statistics. bash$ uptime
10:28pm up 1:57, 3 users, load average: 0.17, 0.34, 0.27 |
 | A load average of 1 or less
indicates that the system handles processes immediately. A load
average greater than 1 means that processes are being queued. When
the load average gets above 3, then system performance is
significantly degraded. |
- hostname
Lists the system's host name. This command sets the host
name in an /etc/rc.d
setup script (/etc/rc.d/rc.sysinit
or similar). It is equivalent to uname
-n, and a counterpart to the $HOSTNAME internal
variable. bash$ hostname
localhost.localdomain
bash$ echo $HOSTNAME
localhost.localdomain |
Similar to the hostname command are the
domainname,
dnsdomainname,
nisdomainname, and
ypdomainname commands. Use these to
display or set the system DNS or NIS/YP domain name. Various
options to hostname also perform these
functions. - hostid
Echo a 32-bit hexadecimal numerical identifier for the
host machine.
 | This command allegedly fetches a "unique"
serial number for a particular system. Certain
product registration procedures use this number
to brand a particular user license. Unfortunately,
hostid only returns the machine
network address in hexadecimal, with pairs of bytes
transposed. The network address of a typical non-networked Linux
machine, is found in /etc/hosts. bash$ cat /etc/hosts
127.0.0.1 localhost.localdomain localhost |
As it happens, transposing the bytes of
127.0.0.1, we get
0.127.1.0, which translates in
hex to 007f0100, the exact equivalent
of what hostid returns, above. There
exist only a few million other Linux machines with this
identical hostid. |
- sar
Invoking sar (System Activity Reporter)
gives a very detailed rundown on system statistics. The
Santa Cruz Operation ("Old" SCO) released
sar as Open Source in June, 1999. This command is not part of the base Linux distribution,
but may be obtained as part of the sysstat utilities package, written by Sebastien
Godard. bash$ sar
Linux 2.4.9 (brooks.seringas.fr) 09/26/03
10:30:00 CPU %user %nice %system %iowait %idle
10:40:00 all 2.21 10.90 65.48 0.00 21.41
10:50:00 all 3.36 0.00 72.36 0.00 24.28
11:00:00 all 1.12 0.00 80.77 0.00 18.11
Average: all 2.23 3.63 72.87 0.00 21.27
14:32:30 LINUX RESTART
15:00:00 CPU %user %nice %system %iowait %idle
15:10:00 all 8.59 2.40 17.47 0.00 71.54
15:20:00 all 4.07 1.00 11.95 0.00 82.98
15:30:00 all 0.79 2.94 7.56 0.00 88.71
Average: all 6.33 1.70 14.71 0.00 77.26
|
- readelf
Show information and statistics about a designated
elf binary. This is part of the
binutils package. bash$ readelf -h /bin/bash
ELF Header:
Magic: 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00
Class: ELF32
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: EXEC (Executable file)
. . . |
- size
The size [/path/to/binary] command
gives the segment sizes of a binary executable or archive file.
This is mainly of use to programmers. bash$ size /bin/bash
text data bss dec hex filename
495971 22496 17392 535859 82d33 /bin/bash
|
System Logs - logger
Appends a user-generated message to the system log
(/var/log/messages). You do not have
to be root to invoke logger.
logger Experiencing instability in network connection at 23:10, 05/21.
# Now, do a 'tail /var/log/messages'. |
By embedding a logger command in a script,
it is possible to write debugging information to
/var/log/messages.
logger -t $0 -i Logging at line "$LINENO".
# The "-t" option specifies the tag for the logger entry.
# The "-i" option records the process ID.
# tail /var/log/message
# ...
# Jul 7 20:48:58 localhost ./test.sh[1712]: Logging at line 3. |
- logrotate
This utility manages the system log files, rotating,
compressing, deleting, and/or e-mailing them, as appropriate.
This keeps the /var/log
from getting cluttered with old log files.
Usually cron runs
logrotate on a daily basis. Adding an appropriate entry to
/etc/logrotate.conf makes it possible
to manage personal log files, as well as system-wide
ones.  | Stefano Falsetto has created rottlog,
which he considers to be an improved version of
logrotate. |
Job Control - ps
Process
Statistics: lists currently
executing processes by owner and PID (process ID). This
is usually invoked with ax options,
and may be piped to grep
or sed to search for a
specific process (see Example 11-12 and Example 27-2). bash$ ps ax | grep sendmail
295 ? S 0:00 sendmail: accepting connections on port 25 |
To display system processes in graphical "tree"
format: ps afjx or
ps ax --forest. - pstree
Lists currently executing processes in
"tree" format. The -p option
shows the PIDs, as well as the process names. - top
Continuously updated display of most cpu-intensive
processes. The -b option displays in text
mode, so that the output may be parsed or accessed from
a script. bash$ top -b
8:30pm up 3 min, 3 users, load average: 0.49, 0.32, 0.13
45 processes: 44 sleeping, 1 running, 0 zombie, 0 stopped
CPU states: 13.6% user, 7.3% system, 0.0% nice, 78.9% idle
Mem: 78396K av, 65468K used, 12928K free, 0K shrd, 2352K buff
Swap: 157208K av, 0K used, 157208K free 37244K cached
PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
848 bozo 17 0 996 996 800 R 5.6 1.2 0:00 top
1 root 8 0 512 512 444 S 0.0 0.6 0:04 init
2 root 9 0 0 0 0 SW 0.0 0.0 0:00 keventd
...
|
- nice
Run a background job with an altered
priority. Priorities run from 19 (lowest) to -20
(highest). Only root may set the
negative (higher) priorities. Related commands are
renice, snice,
and skill. - nohup
Keeps a command running even after user logs off.
The command will run as a foreground process unless followed
by &. If you use nohup
within a script, consider coupling it with a wait to avoid creating an orphan
or zombie process. - pidof
Identifies process ID (PID) of a
running job. Since job control commands, such as kill and renice
act on the PID of a process (not
its name), it is sometimes necessary to identify that
PID. The pidof
command is the approximate counterpart to the $PPID internal variable.
Example 13-6. pidof helps kill a process #!/bin/bash
# kill-process.sh
NOPROCESS=2
process=xxxyyyzzz # Use nonexistent process.
# For demo purposes only...
# ... don't want to actually kill any actual process with this script.
#
# If, for example, you wanted to use this script to logoff the Internet,
# process=pppd
t=`pidof $process` # Find pid (process id) of $process.
# The pid is needed by 'kill' (can't 'kill' by program name).
if [ -z "$t" ] # If process not present, 'pidof' returns null.
then
echo "Process $process was not running."
echo "Nothing killed."
exit $NOPROCESS
fi
kill $t # May need 'kill -9' for stubborn process.
# Need a check here to see if process allowed itself to be killed.
# Perhaps another " t=`pidof $process` " or ...
# This entire script could be replaced by
# kill $(pidof -x process_name)
# but it would not be as instructive.
exit 0 |
- fuser
Identifies the processes (by PID) that are accessing
a given file, set of files, or directory. May also be
invoked with the -k option, which kills
those processes. This has interesting implications for
system security, especially in scripts preventing
unauthorized users from accessing system services. bash$ fuser -u /usr/bin/vim
/usr/bin/vim: 3207e(bozo)
bash$ fuser -u /dev/null
/dev/null: 3009(bozo) 3010(bozo) 3197(bozo) 3199(bozo)
|
One important application for fuser is
when physically inserting or removing storage media, such
as CD ROM disks or USB flash drives. Sometimes trying
a umount fails with a
device is busy error message. This
means that some user(s) and/or process(es) are accessing
the device. An fuser -um /dev/device_name
will clear up the mystery, so you can kill any relevant
processes. bash$ umount /mnt/usbdrive
umount: /mnt/usbdrive: device is busy
bash$ fuser -um /dev/usbdrive
/mnt/usbdrive: 1772c(bozo)
bash$ kill -9 1772
bash$ umount /mnt/usbdrive
|
The fuser command, invoked with the
-n option identifies the processes
accessing a port. This
is especially useful in combination with nmap. root# nmap localhost.localdomain
PORT STATE SERVICE
25/tcp open smtp
root# fuser -un tcp 25
25/tcp: 2095(root)
root# ps ax | grep 2095 | grep -v grep
2095 ? Ss 0:00 sendmail: accepting connections
|
- cron
Administrative program scheduler, performing such
duties as cleaning up and deleting system log
files and updating the slocate
database. This is the superuser version of at (although each user may have their
own crontab file which can be changed
with the crontab command). It runs
as a daemon and executes
scheduled entries from /etc/crontab.  | Some flavors of Linux run
crond, Matthew Dillon's version of
cron. |
Process Control and Booting - init
The init command is the parent of all processes. Called
in the final step of a bootup, init
determines the runlevel of the system from
/etc/inittab. Invoked by its alias
telinit, and by root only. - telinit
Symlinked to init, this is a means of changing the system runlevel,
usually done for system maintenance or emergency filesystem
repairs. Invoked only by root. This command can be dangerous - be
certain you understand it well before using! - runlevel
Shows the current and last runlevel, that is, whether the system
is halted (runlevel 0), in single-user mode
(1), in multi-user mode (2
or 3), in X Windows (5), or
rebooting (6). This command accesses the
/var/run/utmp file. - halt, shutdown, reboot
Command set to shut the system down, usually just prior to a power down. - service
Starts or stops a system service.
The startup scripts in /etc/init.d
and /etc/rc.d use this
command to start services at bootup. root# /sbin/service iptables stop
Flushing firewall rules: [ OK ]
Setting chains to policy ACCEPT: filter [ OK ]
Unloading iptables modules: [ OK ]
|
Network - ifconfig
Network interface configuration
and tuning utility. bash$ ifconfig -a
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:10 errors:0 dropped:0 overruns:0 frame:0
TX packets:10 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:700 (700.0 b) TX bytes:700 (700.0 b) |
The ifconfig command is most often used
at bootup to set up the interfaces, or to shut them down
when rebooting. # Code snippets from /etc/rc.d/init.d/network
# ...
# Check that networking is up.
[ ${NETWORKING} = "no" ] && exit 0
[ -x /sbin/ifconfig ] || exit 0
# ...
for i in $interfaces ; do
if ifconfig $i 2>/dev/null | grep -q "UP" >/dev/null 2>&1 ; then
action "Shutting down interface $i: " ./ifdown $i boot
fi
# The GNU-specific "-q" option to "grep" means "quiet", i.e., producing no output.
# Redirecting output to /dev/null is therefore not strictly necessary.
# ...
echo "Currently active devices:"
echo `/sbin/ifconfig | grep ^[a-z] | awk '{print $1}'`
# ^^^^^ should be quoted to prevent globbing.
# The following also work.
# echo $(/sbin/ifconfig | awk '/^[a-z]/ { print $1 })'
# echo $(/sbin/ifconfig | sed -e 's/ .*//')
# Thanks, S.C., for additional comments. |
See also Example 29-6. - iwconfig
This is the command set for configuring a wireless network.
It is the wireless equivalent of ifconfig,
above. - route
Show info about or make changes to the kernel routing table. bash$ route
Destination Gateway Genmask Flags MSS Window irtt Iface
pm3-67.bozosisp * 255.255.255.255 UH 40 0 0 ppp0
127.0.0.0 * 255.0.0.0 U 40 0 0 lo
default pm3-67.bozosisp 0.0.0.0 UG 40 0 0 ppp0
|
- chkconfig
Check network configuration. This command lists and
manages the network services started at bootup in the
/etc/rc?.d
directory. Originally a port from IRIX to Red Hat Linux,
chkconfig may not be part of the core
installation of some Linux flavors. bash$ chkconfig --list
atd 0:off 1:off 2:off 3:on 4:on 5:on 6:off
rwhod 0:off 1:off 2:off 3:off 4:off 5:off 6:off
...
|
- tcpdump
Network packet "sniffer". This is a tool for
analyzing and troubleshooting traffic on a network by dumping
packet headers that match specified criteria. Dump ip packet traffic between hosts
bozoville and
caduceus:
bash$ tcpdump ip host bozoville and caduceus
|
Of course, the output of tcpdump can be
parsed, using certain of the previously discussed text processing
utilities.
Filesystem - mount
Mount a filesystem, usually on an external device,
such as a floppy or CDROM. The file
/etc/fstab provides a handy listing
of available filesystems, partitions, and devices,
including options, that may be automatically or manually
mounted. The file /etc/mtab shows
the currently mounted filesystems and partitions
(including the virtual ones, such as /proc). mount -a mounts all filesystems and
partitions listed in /etc/fstab,
except those with a noauto
option. At bootup, a startup script in
/etc/rc.d
(rc.sysinit or something similar)
invokes this to get everything mounted. mount -t iso9660 /dev/cdrom /mnt/cdrom
# Mounts CDROM
mount /mnt/cdrom
# Shortcut, if /mnt/cdrom listed in /etc/fstab |
This versatile command can even mount an ordinary file
on a block device, and the file will act as if it were a
filesystem. Mount accomplishes that by
associating the file with a loopback
device. One application of this is to mount and examine
an ISO9660 image before burning it onto a CDR.
Example 13-7. Checking a CD image # As root...
mkdir /mnt/cdtest # Prepare a mount point, if not already there.
mount -r -t iso9660 -o loop cd-image.iso /mnt/cdtest # Mount the image.
# "-o loop" option equivalent to "losetup /dev/loop0"
cd /mnt/cdtest # Now, check the image.
ls -alR # List the files in the directory tree there.
# And so forth. |
- umount
Unmount a currently mounted filesystem. Before physically removing a
previously mounted floppy or CDROM disk, the device must be
umounted, else filesystem corruption may result.
umount /mnt/cdrom
# You may now press the eject button and safely remove the disk. |
 | The automount utility, if
properly installed, can mount and unmount floppies or
CDROM disks as they are accessed or removed. On laptops
with swappable floppy and CDROM drives, this can cause
problems, though. |
- sync
Forces an immediate write of all updated data from
buffers to hard drive (synchronize drive
with buffers). While not strictly necessary, a
sync assures the sys admin or
user that the data just changed will survive a sudden
power failure. In the olden days, a sync;
sync (twice, just to make absolutely sure) was a
useful precautionary measure before a system reboot. At times, you may wish to force an immediate buffer
flush, as when securely deleting a file (see Example 12-55) or when the lights begin to
flicker. - losetup
Sets up and configures loopback devices. Example 13-8. Creating a filesystem in a file SIZE=1000000 # 1 meg
head -c $SIZE < /dev/zero > file # Set up file of designated size.
losetup /dev/loop0 file # Set it up as loopback device.
mke2fs /dev/loop0 # Create filesystem.
mount -o loop /dev/loop0 /mnt # Mount it.
# Thanks, S.C. |
- mkswap
Creates a swap partition or file. The swap area must
subsequently be enabled with
swapon. - swapon, swapoff
Enable / disable swap partitition or file.
These commands usually take effect at bootup and
shutdown. - mke2fs
Create a Linux ext2 filesystem. This command must
be invoked as root. Example 13-9. Adding a new hard drive #!/bin/bash
# Adding a second hard drive to system.
# Software configuration. Assumes hardware already mounted.
# From an article by the author of this document.
# In issue #38 of "Linux Gazette", http://www.linuxgazette.com.
ROOT_UID=0 # This script must be run as root.
E_NOTROOT=67 # Non-root exit error.
if [ "$UID" -ne "$ROOT_UID" ]
then
echo "Must be root to run this script."
exit $E_NOTROOT
fi
# Use with extreme caution!
# If something goes wrong, you may wipe out your current filesystem.
NEWDISK=/dev/hdb # Assumes /dev/hdb vacant. Check!
MOUNTPOINT=/mnt/newdisk # Or choose another mount point.
fdisk $NEWDISK
mke2fs -cv $NEWDISK1 # Check for bad blocks & verbose output.
# Note: /dev/hdb1, *not* /dev/hdb!
mkdir $MOUNTPOINT
chmod 777 $MOUNTPOINT # Makes new drive accessible to all users.
# Now, test...
# mount -t ext2 /dev/hdb1 /mnt/newdisk
# Try creating a directory.
# If it works, umount it, and proceed.
# Final step:
# Add the following line to /etc/fstab.
# /dev/hdb1 /mnt/newdisk ext2 defaults 1 1
exit 0 |
See also Example 13-8 and Example 28-3. - tune2fs
Tune ext2 filesystem. May be used to change filesystem
parameters, such as maximum mount count. This must be
invoked as root.  | This is an extremely dangerous command. Use it at
your own risk, as you may inadvertently destroy your filesystem.
|
- dumpe2fs
Dump (list to stdout) very verbose
filesystem info. This must be invoked as root. root# dumpe2fs /dev/hda7 | grep 'ount count'
dumpe2fs 1.19, 13-Jul-2000 for EXT2 FS 0.5b, 95/08/09
Mount count: 6
Maximum mount count: 20 |
- hdparm
List or change hard disk parameters. This command must be
invoked as root, and it may be dangerous if misused. - fdisk
Create or change a partition table on a storage device,
usually a hard drive. This command must be invoked as
root.  | Use this command with extreme caution. If something
goes wrong, you may destroy an existing
filesystem. |
- fsck, e2fsck, debugfs
Filesystem check, repair, and debug command set. fsck: a front end for checking a UNIX
filesystem (may invoke other utilities). The actual
filesystem type generally defaults to ext2. e2fsck: ext2 filesystem checker. debugfs: ext2 filesystem debugger.
One of the uses of this versatile, but dangerous command
is to (attempt to) recover deleted files. For advanced users
only!  | All of these should be invoked as root, and they
can damage or destroy a filesystem if misused. |
- badblocks
Checks for bad blocks (physical media flaws) on a
storage device. This command finds use when formatting
a newly installed hard drive or testing the integrity
of backup media.
As an example, badblocks /dev/fd0
tests a floppy disk. The badblocks command
may be invoked destructively (overwrite all data) or in
non-destructive read-only mode. If root user owns the
device to be tested, as is generally the case, then root
must invoke this command. - lsusb, usbmodules
The lsusb command lists all USB
(Universal Serial Bus) buses and the devices hooked up to
them. The usbmodules command outputs
information about the driver modules for connected USB
devices. root# lsusb
Bus 001 Device 001: ID 0000:0000
Device Descriptor:
bLength 18
bDescriptorType 1
bcdUSB 1.00
bDeviceClass 9 Hub
bDeviceSubClass 0
bDeviceProtocol 0
bMaxPacketSize0 8
idVendor 0x0000
idProduct 0x0000
. . .
|
- mkbootdisk
Creates a boot floppy which can be used to bring up the
system if, for example, the MBR (master boot record) becomes
corrupted. The mkbootdisk command is actually
a Bash script, written by Erik Troan, in the /sbin directory. - chroot
CHange ROOT directory. Normally commands are fetched
from $PATH, relative to
/, the default root
directory. This changes the root directory to a different
one (and also changes the working directory to there).
This is useful for security purposes, for instance when
the system administrator wishes to restrict certain users,
such as those telnetting
in, to a secured portion of the filesystem (this
is sometimes referred to as confining a guest user
to a "chroot jail"). Note that after a
chroot, the execution path for system
binaries is no longer valid. A chroot /opt would cause
references to /usr/bin
to be translated to /opt/usr/bin. Likewise,
chroot /aaa/bbb /bin/ls would
redirect future instances of ls
to /aaa/bbb as the base directory,
rather than / as is
normally the case. An alias XX 'chroot /aaa/bbb
ls' in a user's ~/.bashrc
effectively restricts which portion of the filesystem
she may run command "XX" on. The chroot command is also handy
when running from an emergency boot floppy
(chroot to /dev/fd0),
or as an option to lilo when recovering
from a system crash. Other uses include installation from a
different filesystem (an rpm
option) or running a readonly filesystem from a CD ROM.
Invoke only as root, and use with care.  | It might be necessary to copy certain system
files to a chrooted directory,
since the normal $PATH can no longer
be relied upon. |
- lockfile
This utility is part of the procmail
package (www.procmail.org).
It creates a lock file, a semaphore file that
controls access to a file, device, or resource. The lock file
serves as a flag that this particular file, device, or resource is
in use by a particular process ("busy"), and
this permits only restricted access (or no access) to other
processes. Lock files are used in such applications as protecting
system mail folders from simultaneously being changed
by multiple users, indicating that a modem port
is being accessed, and showing that an instance of
Netscape is using its cache.
Scripts may check for the existence of a lock file created
by a certain process to check if that process is running.
Note that if a script attempts to create a lock file that
already exists, the script will likely hang. Normally, applications create and check for lock files
in the /var/lock
directory. A script can test for the presence of a lock file by
something like the following.
appname=xyzip
# Application "xyzip" created lock file "/var/lock/xyzip.lock".
if [ -e "/var/lock/$appname.lock" ]
then
... |
- mknod
Creates block or character device files (may be
necessary when installing new hardware on the system). The
MAKEDEV utility has virtually
all of the functionality of mknod,
and is easier to use. - MAKEDEV
Utility for creating device files. It must be run as root,
and in the /dev
directory.
This is a sort of advanced version of
mknod.- tmpwatch
Automatically deletes files which have not been accessed
within a specified period of time. Usually invoked by
cron to remove stale log
files.
Backup - dump, restore
The dump command is an elaborate
filesystem backup utility, generally used on larger
installations and networks.
It reads raw disk partitions and writes a backup file
in a binary format. Files to be backed up may be saved
to a variety of storage media, including disks and tape
drives. The restore command restores
backups made with dump. - fdformat
Perform a low-level format on a floppy disk.
System Resources - ulimit
Sets an upper limit on use
of system resources. Usually invoked with the
-f option, which sets a limit on file size
(ulimit -f 1000 limits files to 1 meg
maximum). The -t option limits the coredump
size (ulimit -c 0 eliminates coredumps).
Normally, the value of ulimit
would be set in /etc/profile
and/or ~/.bash_profile (see Appendix G).  | Judicious use of ulimit can
protect a system against the dreaded fork
bomb. #!/bin/bash
# This script is for illustrative purposes only.
# Run it at your own peril -- it *will* freeze your system.
while true # Endless loop.
do
$0 & # This script invokes itself . . .
#+ forks an infinite number of times . . .
#+ until the system freezes up because all resources exhausted.
done # This is the notorious "sorcerer's appentice" scenario.
exit 0 # Will not exit here, because this script will never terminate. |
A ulimit -Hu XX (where
XX is the user process limit) in
/etc/profile would abort
this script when it exceeds the preset limit.
|
- quota
Display user or group disk quotas. - setquota
Set user or group disk quotas from the command line. - umask
User file creation permissions
mask. Limit the default file
attributes for a particular user. All files created
by that user take on the attributes specified by
umask. The (octal) value passed to
umask defines the file permissions
disabled. For example, umask
022 ensures that new files will have at most
755 permissions (777 NAND 022).
Of course, the user may later change the
attributes of particular files with chmod. The usual practice
is to set the value of umask
in /etc/profile and/or
~/.bash_profile (see Appendix G). Example 13-10. Using umask to hide an output file
from prying eyes #!/bin/bash
# rot13a.sh: Same as "rot13.sh" script, but writes output to "secure" file.
# Usage: ./rot13a.sh filename
# or ./rot13a.sh <filename
# or ./rot13a.sh and supply keyboard input (stdin)
umask 177 # File creation mask.
# Files created by this script
#+ will have 600 permissions.
OUTFILE=decrypted.txt # Results output to file "decrypted.txt"
#+ which can only be read/written
# by invoker of script (or root).
cat "$@" | tr 'a-zA-Z' 'n-za-mN-ZA-M' > $OUTFILE
# ^^ Input from stdin or a file. ^^^^^^^^^^ Output redirected to file.
exit 0 |
- rdev
Get info about or make changes to root device, swap space, or video
mode. The functionality of rdev has generally been taken over by
lilo, but rdev remains
useful for setting up a ram disk. This is a dangerous command, if misused.
Modules - lsmod
List installed kernel modules. bash$ lsmod
Module Size Used by
autofs 9456 2 (autoclean)
opl3 11376 0
serial_cs 5456 0 (unused)
sb 34752 0
uart401 6384 0 [sb]
sound 58368 0 [opl3 sb uart401]
soundlow 464 0 [sound]
soundcore 2800 6 [sb sound]
ds 6448 2 [serial_cs]
i82365 22928 2
pcmcia_core 45984 0 [serial_cs ds i82365]
|
 | Doing a cat /proc/modules gives the
same information. |
- insmod
Force installation of a kernel module (use
modprobe instead, when possible). Must
be invoked as root. - rmmod
Force unloading of a kernel module. Must be invoked
as root. - modprobe
Module loader that is normally invoked automatically
in a startup script. Must be invoked as root. - depmod
Creates module dependency file, usually invoked from
startup script. - modinfo
Output information about a loadable module. bash$ modinfo hid
filename: /lib/modules/2.4.20-6/kernel/drivers/usb/hid.o
description: "USB HID support drivers"
author: "Andreas Gal, Vojtech Pavlik <vojtech@suse.cz>"
license: "GPL"
|
Miscellaneous - env
Runs a program or script with certain environmental variables
set or changed (without changing the overall system
environment). The [varname=xxx]
permits changing the environmental variable
varname for the duration of the
script. With no options specified, this command lists all
the environmental variable settings.  | In Bash and other Bourne shell derivatives, it is
possible to set variables in a single command's environment.
var1=value1 var2=value2 commandXXX
# $var1 and $var2 set in the environment of 'commandXXX' only. |
|
 | The first line of a script (the
"sha-bang" line) may use env
when the path to the shell or interpreter is unknown.
#! /usr/bin/env perl
print "This Perl script will run,\n";
print "even when I don't know where to find Perl.\n";
# Good for portable cross-platform scripts,
# where the Perl binaries may not be in the expected place.
# Thanks, S.C. |
|
- ldd
Show shared lib dependencies for an executable file. bash$ ldd /bin/ls
libc.so.6 => /lib/libc.so.6 (0x4000c000)
/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x80000000) |
- watch
Run a command repeatedly, at specified time intervals. The default is two-second intervals, but this may be changed
with the -n option. watch -n 5 tail /var/log/messages
# Shows tail end of system log, /var/log/messages, every five seconds. |
- strip
Remove the debugging symbolic references from an executable
binary. This decreases its size, but makes debugging it
impossible. This command often occurs in a Makefile,
but rarely in a shell script. - nm
List symbols in an unstripped compiled binary. - rdist
Remote distribution client: synchronizes, clones,
or backs up a file system on a remote server.
13.1. Analyzing a System ScriptUsing our knowledge of administrative commands, let us examine a system
script. One of the shortest and simplest to understand scripts is
killall, used to suspend running processes at system shutdown. Example 13-11. killall, from /etc/rc.d/init.d #!/bin/sh
# --> Comments added by the author of this document marked by "# -->".
# --> This is part of the 'rc' script package
# --> by Miquel van Smoorenburg, <miquels@drinkel.nl.mugnet.org>.
# --> This particular script seems to be Red Hat / FC specific
# --> (may not be present in other distributions).
# Bring down all unneeded services that are still running
#+ (there shouldn't be any, so this is just a sanity check)
for i in /var/lock/subsys/*; do
# --> Standard for/in loop, but since "do" is on same line,
# --> it is necessary to add ";".
# Check if the script is there.
[ ! -f $i ] && continue
# --> This is a clever use of an "and list", equivalent to:
# --> if [ ! -f "$i" ]; then continue
# Get the subsystem name.
subsys=${i#/var/lock/subsys/}
# --> Match variable name, which, in this case, is the file name.
# --> This is the exact equivalent of subsys=`basename $i`.
# --> It gets it from the lock file name
# -->+ (if there is a lock file,
# -->+ that's proof the process has been running).
# --> See the "lockfile" entry, above.
# Bring the subsystem down.
if [ -f /etc/rc.d/init.d/$subsys.init ]; then
/etc/rc.d/init.d/$subsys.init stop
else
/etc/rc.d/init.d/$subsys stop
# --> Suspend running jobs and daemons.
# --> Note that "stop" is a positional parameter,
# -->+ not a shell builtin.
fi
done |
That wasn't so bad. Aside from a little fancy footwork with variable
matching, there is no new material there.
Chapter 14. Command Substitution Command
substitution reassigns the output of a command
or even multiple commands; it literally plugs the command
output into another context.
The classic form of command
substitution uses backquotes
(`...`). Commands within backquotes (backticks) generate
command line text.
script_name=`basename $0`
echo "The name of this script is $script_name." |
rm `cat filename` # "filename" contains a list of files to delete.
#
# S. C. points out that "arg list too long" error might result.
# Better is xargs rm -- < filename
# ( -- covers those cases where "filename" begins with a "-" )
textfile_listing=`ls *.txt`
# Variable contains names of all *.txt files in current working directory.
echo $textfile_listing
textfile_listing2=$(ls *.txt) # The alternative form of command substitution.
echo $textfile_listing2
# Same result.
# A possible problem with putting a list of files into a single string
# is that a newline may creep in.
#
# A safer way to assign a list of files to a parameter is with an array.
# shopt -s nullglob # If no match, filename expands to nothing.
# textfile_listing=( *.txt )
#
# Thanks, S.C. |
 | Command substitution invokes a subshell. |
 | Command substitution may result in word splitting.
COMMAND `echo a b` # 2 args: a and b
COMMAND "`echo a b`" # 1 arg: "a b"
COMMAND `echo` # no arg
COMMAND "`echo`" # one empty arg
# Thanks, S.C. |
Even when there is no word splitting, command
substitution can remove trailing newlines.
# cd "`pwd`" # This should always work.
# However...
mkdir 'dir with trailing newline
'
cd 'dir with trailing newline
'
cd "`pwd`" # Error message:
# bash: cd: /tmp/file with trailing newline: No such file or directory
cd "$PWD" # Works fine.
old_tty_setting=$(stty -g) # Save old terminal setting.
echo "Hit a key "
stty -icanon -echo # Disable "canonical" mode for terminal.
# Also, disable *local* echo.
key=$(dd bs=1 count=1 2> /dev/null) # Using 'dd' to get a keypress.
stty "$old_tty_setting" # Restore old setting.
echo "You hit ${#key} key." # ${#variable} = number of characters in $variable
#
# Hit any key except RETURN, and the output is "You hit 1 key."
# Hit RETURN, and it's "You hit 0 key."
# The newline gets eaten in the command substitution.
Thanks, S.C. |
|
 | Using echo to output an
unquoted variable set with command
substitution removes trailing newlines characters from
the output of the reassigned command(s). This can cause
unpleasant surprises.
dir_listing=`ls -l`
echo $dir_listing # unquoted
# Expecting a nicely ordered directory listing.
# However, what you get is:
# total 3 -rw-rw-r-- 1 bozo bozo 30 May 13 17:15 1.txt -rw-rw-r-- 1 bozo
# bozo 51 May 15 20:57 t2.sh -rwxr-xr-x 1 bozo bozo 217 Mar 5 21:13 wi.sh
# The newlines disappeared.
echo "$dir_listing" # quoted
# -rw-rw-r-- 1 bozo 30 May 13 17:15 1.txt
# -rw-rw-r-- 1 bozo 51 May 15 20:57 t2.sh
# -rwxr-xr-x 1 bozo 217 Mar 5 21:13 wi.sh |
|
Command substitution even permits setting a variable to the
contents of a file, using either redirection or the cat command. variable1=`<file1` # Set "variable1" to contents of "file1".
variable2=`cat file2` # Set "variable2" to contents of "file2".
# This, however, forks a new process,
#+ so the line of code executes slower than the above version.
# Note:
# The variables may contain embedded whitespace,
#+ or even (horrors), control characters. |
# Excerpts from system file, /etc/rc.d/rc.sysinit
#+ (on a Red Hat Linux installation)
if [ -f /fsckoptions ]; then
fsckoptions=`cat /fsckoptions`
...
fi
#
#
if [ -e "/proc/ide/${disk[$device]}/media" ] ; then
hdmedia=`cat /proc/ide/${disk[$device]}/media`
...
fi
#
#
if [ ! -n "`uname -r | grep -- "-"`" ]; then
ktag="`cat /proc/version`"
...
fi
#
#
if [ $usb = "1" ]; then
sleep 5
mouseoutput=`cat /proc/bus/usb/devices 2>/dev/null|grep -E "^I.*Cls=03.*Prot=02"`
kbdoutput=`cat /proc/bus/usb/devices 2>/dev/null|grep -E "^I.*Cls=03.*Prot=01"`
...
fi |
 | Do not set a variable to the contents of a
long text file unless you have a very good
reason for doing so. Do not set a variable to the contents of a
binary file, even as a joke. Example 14-1. Stupid script tricks #!/bin/bash
# stupid-script-tricks.sh: Don't try this at home, folks.
# From "Stupid Script Tricks," Volume I.
dangerous_variable=`cat /boot/vmlinuz` # The compressed Linux kernel itself.
echo "string-length of \$dangerous_variable = ${#dangerous_variable}"
# string-length of $dangerous_variable = 794151
# (Does not give same count as 'wc -c /boot/vmlinuz'.)
# echo "$dangerous_variable"
# Don't try this! It would hang the script.
# The document author is aware of no useful applications for
#+ setting a variable to the contents of a binary file.
exit 0 |
Notice that a buffer overrun
does not occur. This is one instance where an interpreted
language, such as Bash, provides more protection from
programmer mistakes than a compiled language. |
Command substitution permits setting a variable to the
output of a loop. The
key to this is grabbing the output of an echo command within the
loop. Example 14-2. Generating a variable from a loop #!/bin/bash
# csubloop.sh: Setting a variable to the output of a loop.
variable1=`for i in 1 2 3 4 5
do
echo -n "$i" # The 'echo' command is critical
done` #+ to command substitution here.
echo "variable1 = $variable1" # variable1 = 12345
i=0
variable2=`while [ "$i" -lt 10 ]
do
echo -n "$i" # Again, the necessary 'echo'.
let "i += 1" # Increment.
done`
echo "variable2 = $variable2" # variable2 = 0123456789
# Demonstrates that it's possible to embed a loop
#+ within a variable declaration.
exit 0 |
 | The $(COMMAND) form has
superseded backticks for command substitution. output=$(sed -n /"$1"/p $file) # From "grp.sh" example.
# Setting a variable to the contents of a text file.
File_contents1=$(cat $file1)
File_contents2=$(<$file2) # Bash permits this also. |
The $(...) form of command substitution
treats a double backslash in a different way than
`...`.
bash$ echo `echo \\`
bash$ echo $(echo \\)
\
|
The $(...) form of command
substitution permits nesting.
word_count=$( wc -w $(ls -l | awk '{print $9}') ) |
Or, for something a bit more elaborate . . . Example 14-3. Finding anagrams #!/bin/bash
# agram2.sh
# Example of nested command substitution.
# Uses "anagram" utility
#+ that is part of the author's "yawl" word list package.
# http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz
# http://personal.riverusers.com/~thegrendel/yawl-0.3.2.tar.gz
E_NOARGS=66
E_BADARG=67
MINLEN=7
if [ -z "$1" ]
then
echo "Usage $0 LETTERSET"
exit $E_NOARGS # Script needs a command-line argument.
elif [ ${#1} -lt $MINLEN ]
then
echo "Argument must have at least $MINLEN letters."
exit $E_BADARG
fi
FILTER='.......' # Must have at least 7 letters.
# 1234567
Anagrams=( $(echo $(anagram $1 | grep $FILTER) ) )
# | | nested command sub. | |
# ( array assignment )
echo
echo "${#Anagrams[*]} 7+ letter anagrams found"
echo
echo ${Anagrams[0]} # First anagram.
echo ${Anagrams[1]} # Second anagram.
# Etc.
# echo "${Anagrams[*]}" # To list all the anagrams in a single line . . .
# Look ahead to the "Arrays" chapter for enlightenment on
#+ what's going on here.
# See also the agram.sh script for an example of anagram finding.
exit $? |
|
Examples of command substitution in shell scripts:
Example 10-7 Example 10-26 Example 9-28 Example 12-3 Example 12-19 Example 12-15 Example 12-49 Example 10-13 Example 10-10 Example 12-29 Example 16-8 Example A-17 Example 27-2 Example 12-42 Example 12-43 Example 12-44
Chapter 15. Arithmetic ExpansionArithmetic expansion provides a
powerful tool for performing (integer) arithmetic operations
in scripts. Translating a string into a numerical
expression is relatively straightforward using
backticks, double parentheses, or let. Variations - Arithmetic expansion with backticks (often used in
conjunction with expr)
z=`expr $z + 3` # The 'expr' command performs the expansion. |
- Arithmetic expansion with double parentheses, and using let
The use of backticks in arithmetic
expansion has been superseded by double parentheses --
((...)) and
$((...)) -- and also by the very
convenient let construction. z=$(($z+3))
z=$((z+3)) # Also correct.
# Within double parentheses,
#+ parameter dereferencing
#+ is optional.
# $((EXPRESSION)) is arithmetic expansion. # Not to be confused with
#+ command substitution.
# You may also use operations within double parentheses without assignment.
n=0
echo "n = $n" # n = 0
(( n += 1 )) # Increment.
# (( $n += 1 )) is incorrect!
echo "n = $n" # n = 1
let z=z+3
let "z += 3" # Quotes permit the use of spaces in variable assignment.
# The 'let' operator actually performs arithmetic evaluation,
#+ rather than expansion. |
Examples of arithmetic expansion in scripts:
Example 12-9 Example 10-14 Example 26-1 Example 26-11 Example A-17
Chapter 16. I/O Redirection
There are always three default "files"
open, stdin (the keyboard),
stdout (the screen), and
stderr (error messages output to the
screen). These, and any other open files, can be redirected.
Redirection simply means capturing output from a file, command,
program, script, or even code block within a script (see Example 3-1 and Example 3-2) and sending it as
input to another file, command, program, or script. Each open file gets assigned a file descriptor.
The file descriptors for stdin,
stdout, and stderr are
0, 1, and 2, respectively. For opening additional files, there
remain descriptors 3 to 9. It is sometimes useful to assign one of
these additional file descriptors to stdin,
stdout, or stderr
as a temporary duplicate link.
This simplifies restoration to normal after complex redirection
and reshuffling (see Example 16-1).
COMMAND_OUTPUT >
# Redirect stdout to a file.
# Creates the file if not present, otherwise overwrites it.
ls -lR > dir-tree.list
# Creates a file containing a listing of the directory tree.
: > filename
# The > truncates file "filename" to zero length.
# If file not present, creates zero-length file (same effect as 'touch').
# The : serves as a dummy placeholder, producing no output.
> filename
# The > truncates file "filename" to zero length.
# If file not present, creates zero-length file (same effect as 'touch').
# (Same result as ": >", above, but this does not work with some shells.)
COMMAND_OUTPUT >>
# Redirect stdout to a file.
# Creates the file if not present, otherwise appends to it.
# Single-line redirection commands (affect only the line they are on):
# --------------------------------------------------------------------
1>filename
# Redirect stdout to file "filename".
1>>filename
# Redirect and append stdout to file "filename".
2>filename
# Redirect stderr to file "filename".
2>>filename
# Redirect and append stderr to file "filename".
&>filename
# Redirect both stdout and stderr to file "filename".
#==============================================================================
# Redirecting stdout, one line at a time.
LOGFILE=script.log
echo "This statement is sent to the log file, \"$LOGFILE\"." 1>$LOGFILE
echo "This statement is appended to \"$LOGFILE\"." 1>>$LOGFILE
echo "This statement is also appended to \"$LOGFILE\"." 1>>$LOGFILE
echo "This statement is echoed to stdout, and will not appear in \"$LOGFILE\"."
# These redirection commands automatically "reset" after each line.
# Redirecting stderr, one line at a time.
ERRORFILE=script.errors
bad_command1 2>$ERRORFILE # Error message sent to $ERRORFILE.
bad_command2 2>>$ERRORFILE # Error message appended to $ERRORFILE.
bad_command3 # Error message echoed to stderr,
#+ and does not appear in $ERRORFILE.
# These redirection commands also automatically "reset" after each line.
#==============================================================================
2>&1
# Redirects stderr to stdout.
# Error messages get sent to same place as standard output.
i>&j
# Redirects file descriptor i to j.
# All output of file pointed to by i gets sent to file pointed to by j.
>&j
# Redirects, by default, file descriptor 1 (stdout) to j.
# All stdout gets sent to file pointed to by j.
0< FILENAME
< FILENAME
# Accept input from a file.
# Companion command to ">", and often used in combination with it.
#
# grep search-word <filename
[j]<>filename
# Open file "filename" for reading and writing, and assign file descriptor "j" to it.
# If "filename" does not exist, create it.
# If file descriptor "j" is not specified, default to fd 0, stdin.
#
# An application of this is writing at a specified place in a file.
echo 1234567890 > File # Write string to "File".
exec 3<> File # Open "File" and assign fd 3 to it.
read -n 4 <&3 # Read only 4 characters.
echo -n . >&3 # Write a decimal point there.
exec 3>&- # Close fd 3.
cat File # ==> 1234.67890
# Random access, by golly.
|
# Pipe.
# General purpose process and command chaining tool.
# Similar to ">", but more general in effect.
# Useful for chaining commands, scripts, files, and programs together.
cat *.txt | sort | uniq > result-file
# Sorts the output of all the .txt files and deletes duplicate lines,
# finally saves results to "result-file". |
Multiple instances of input and output redirection
and/or pipes can be combined in a single command
line.
command < input-file > output-file
command1 | command2 | command3 > output-file |
See Example 12-28 and Example A-15. Multiple output streams may be redirected to one file.
ls -yz >> command.log 2>&1
# Capture result of illegal options "yz" in file "command.log."
# Because stderr is redirected to the file,
#+ any error messages will also be there.
# Note, however, that the following does *not* give the same result.
ls -yz 2>&1 >> command.log
# Outputs an error message and does not write to file.
# If redirecting both stdout and stderr,
#+ the order of the commands makes a difference. |
Closing File Descriptors - n<&-
Close input file descriptor
n. - 0<&-, <&-
Close stdin. - n>&-
Close output file descriptor n. - 1>&-, >&-
Close stdout.
Child processes inherit open file descriptors. This is why pipes
work. To prevent an fd from being inherited, close it.
# Redirecting only stderr to a pipe.
exec 3>&1 # Save current "value" of stdout.
ls -l 2>&1 >&3 3>&- | grep bad 3>&- # Close fd 3 for 'grep' (but not 'ls').
# ^^^^ ^^^^
exec 3>&- # Now close it for the remainder of the script.
# Thanks, S.C. |
For a more detailed introduction to I/O redirection see
Appendix E.
16.1. Using exec
An exec <filename command redirects
stdin to a file. From that point on, all
stdin comes from that file, rather than
its normal source (usually keyboard input). This provides a
method of reading a file line by line and possibly parsing
each line of input using sed
and/or awk. Example 16-1. Redirecting stdin using
exec #!/bin/bash
# Redirecting stdin using 'exec'.
exec 6<&0 # Link file descriptor #6 with stdin.
# Saves stdin.
exec < data-file # stdin replaced by file "data-file"
read a1 # Reads first line of file "data-file".
read a2 # Reads second line of file "data-file."
echo
echo "Following lines read from file."
echo "-------------------------------"
echo $a1
echo $a2
echo; echo; echo
exec 0<&6 6<&-
# Now restore stdin from fd #6, where it had been saved,
#+ and close fd #6 ( 6<&- ) to free it for other processes to use.
#
# <&6 6<&- also works.
echo -n "Enter data "
read b1 # Now "read" functions as expected, reading from normal stdin.
echo "Input read from stdin."
echo "----------------------"
echo "b1 = $b1"
echo
exit 0 |
Similarly, an exec >filename
command redirects stdout to a designated
file. This sends all command output that would normally go
to stdout to that file. Example 16-2. Redirecting stdout using
exec #!/bin/bash
# reassign-stdout.sh
LOGFILE=logfile.txt
exec 6>&1 # Link file descriptor #6 with stdout.
# Saves stdout.
exec > $LOGFILE # stdout replaced with file "logfile.txt".
# ----------------------------------------------------------- #
# All output from commands in this block sent to file $LOGFILE.
echo -n "Logfile: "
date
echo "-------------------------------------"
echo
echo "Output of \"ls -al\" command"
echo
ls -al
echo; echo
echo "Output of \"df\" command"
echo
df
# ----------------------------------------------------------- #
exec 1>&6 6>&- # Restore stdout and close file descriptor #6.
echo
echo "== stdout now restored to default == "
echo
ls -al
echo
exit 0 |
Example 16-3. Redirecting both stdin and
stdout in the same script with
exec #!/bin/bash
# upperconv.sh
# Converts a specified input file to uppercase.
E_FILE_ACCESS=70
E_WRONG_ARGS=71
if [ ! -r "$1" ] # Is specified input file readable?
then
echo "Can't read from input file!"
echo "Usage: $0 input-file output-file"
exit $E_FILE_ACCESS
fi # Will exit with same error
#+ even if input file ($1) not specified (why?).
if [ -z "$2" ]
then
echo "Need to specify output file."
echo "Usage: $0 input-file output-file"
exit $E_WRONG_ARGS
fi
exec 4<&0
exec < $1 # Will read from input file.
exec 7>&1
exec > $2 # Will write to output file.
# Assumes output file writable (add check?).
# -----------------------------------------------
cat - | tr a-z A-Z # Uppercase conversion.
# ^^^^^ # Reads from stdin.
# ^^^^^^^^^^ # Writes to stdout.
# However, both stdin and stdout were redirected.
# -----------------------------------------------
exec 1>&7 7>&- # Restore stout.
exec 0<&4 4<&- # Restore stdin.
# After restoration, the following line prints to stdout as expected.
echo "File \"$1\" written to \"$2\" as uppercase conversion."
exit 0 |
I/O redirection is a clever way of avoiding the dreaded inaccessible variables within a subshell
problem. Example 16-4. Avoiding a subshell #!/bin/bash
# avoid-subshell.sh
# Suggested by Matthew Walker.
Lines=0
echo
cat myfile.txt | while read line;
do {
echo $line
(( Lines++ )); # Incremented values of this variable
#+ inaccessible outside loop.
# Subshell problem.
}
done
echo "Number of lines read = $Lines" # 0
# Wrong!
echo "------------------------"
exec 3<> myfile.txt
while read line <&3
do {
echo "$line"
(( Lines++ )); # Incremented values of this variable
#+ accessible outside loop.
# No subshell, no problem.
}
done
exec 3>&-
echo "Number of lines read = $Lines" # 8
echo
exit 0
# Lines below not seen by script.
$ cat myfile.txt
Line 1.
Line 2.
Line 3.
Line 4.
Line 5.
Line 6.
Line 7.
Line 8. |
16.2. Redirecting Code BlocksBlocks of code, such as while, until, and for loops, even if/then test blocks can also incorporate
redirection of stdin. Even a function may
use this form of redirection (see Example 23-11).
The < operator at the end of the code block
accomplishes this. Example 16-5. Redirected while loop #!/bin/bash
# redir2.sh
if [ -z "$1" ]
then
Filename=names.data # Default, if no filename specified.
else
Filename=$1
fi
#+ Filename=${1:-names.data}
# can replace the above test (parameter substitution).
count=0
echo
while [ "$name" != Smith ] # Why is variable $name in quotes?
do
read name # Reads from $Filename, rather than stdin.
echo $name
let "count += 1"
done <"$Filename" # Redirects stdin to file $Filename.
# ^^^^^^^^^^^^
echo; echo "$count names read"; echo
exit 0
# Note that in some older shell scripting languages,
#+ the redirected loop would run as a subshell.
# Therefore, $count would return 0, the initialized value outside the loop.
# Bash and ksh avoid starting a subshell *whenever possible*,
#+ so that this script, for example, runs correctly.
# (Thanks to Heiner Steven for pointing this out.)
# However . . .
# Bash *can* sometimes start a subshell in a *redirected* "while" loop.
abc=hi
echo -e "1\n2\n3" | while read l
do abc="$l"
echo $abc
done
echo $abc
# (Thanks, Bruno de Oliveira Schneider, for demonstrating this
#+ with the above snippet of code.) |
Example 16-6. Alternate form of redirected while loop #!/bin/bash
# This is an alternate form of the preceding script.
# Suggested by Heiner Steven
#+ as a workaround in those situations when a redirect loop
#+ runs as a subshell, and therefore variables inside the loop
# +do not keep their values upon loop termination.
if [ -z "$1" ]
then
Filename=names.data # Default, if no filename specified.
else
Filename=$1
fi
exec 3<&0 # Save stdin to file descriptor 3.
exec 0<"$Filename" # Redirect standard input.
count=0
echo
while [ "$name" != Smith ]
do
read name # Reads from redirected stdin ($Filename).
echo $name
let "count += 1"
done # Loop reads from file $Filename
#+ because of line 20.
# The original version of this script terminated the "while" loop with
#+ done <"$Filename"
# Exercise:
# Why is this unnecessary?
exec 0<&3 # Restore old stdin.
exec 3<&- # Close temporary fd 3.
echo; echo "$count names read"; echo
exit 0 |
Example 16-7. Redirected until loop #!/bin/bash
# Same as previous example, but with "until" loop.
if [ -z "$1" ]
then
Filename=names.data # Default, if no filename specified.
else
Filename=$1
fi
# while [ "$name" != Smith ]
until [ "$name" = Smith ] # Change != to =.
do
read name # Reads from $Filename, rather than stdin.
echo $name
done <"$Filename" # Redirects stdin to file $Filename.
# ^^^^^^^^^^^^
# Same results as with "while" loop in previous example.
exit 0 |
Example 16-8. Redirected for loop #!/bin/bash
if [ -z "$1" ]
then
Filename=names.data # Default, if no filename specified.
else
Filename=$1
fi
line_count=`wc $Filename | awk '{ print $1 }'`
# Number of lines in target file.
#
# Very contrived and kludgy, nevertheless shows that
#+ it's possible to redirect stdin within a "for" loop...
#+ if you're clever enough.
#
# More concise is line_count=$(wc -l < "$Filename")
for name in `seq $line_count` # Recall that "seq" prints sequence of numbers.
# while [ "$name" != Smith ] -- more complicated than a "while" loop --
do
read name # Reads from $Filename, rather than stdin.
echo $name
if [ "$name" = Smith ] # Need all this extra baggage here.
then
break
fi
done <"$Filename" # Redirects stdin to file $Filename.
# ^^^^^^^^^^^^
exit 0 |
We can modify the previous example to also redirect the output of
the loop. Example 16-9. Redirected for loop (both
stdin and stdout
redirected) #!/bin/bash
if [ -z "$1" ]
then
Filename=names.data # Default, if no filename specified.
else
Filename=$1
fi
Savefile=$Filename.new # Filename to save results in.
FinalName=Jonah # Name to terminate "read" on.
line_count=`wc $Filename | awk '{ print $1 }'` # Number of lines in target file.
for name in `seq $line_count`
do
read name
echo "$name"
if [ "$name" = "$FinalName" ]
then
break
fi
done < "$Filename" > "$Savefile" # Redirects stdin to file $Filename,
# ^^^^^^^^^^^^^^^^^^^^^^^^^^^ and saves it to backup file.
exit 0 |
Example 16-10. Redirected if/then test #!/bin/bash
if [ -z "$1" ]
then
Filename=names.data # Default, if no filename specified.
else
Filename=$1
fi
TRUE=1
if [ "$TRUE" ] # if true and if : also work.
then
read name
echo $name
fi <"$Filename"
# ^^^^^^^^^^^^
# Reads only first line of file.
# An "if/then" test has no way of iterating unless embedded in a loop.
exit 0 |
Example 16-11. Data file "names.data" for above examples Aristotle
Belisarius
Capablanca
Euler
Goethe
Hamurabi
Jonah
Laplace
Maroczy
Purcell
Schmidt
Semmelweiss
Smith
Turing
Venn
Wilson
Znosko-Borowski
# This is a data file for
#+ "redir2.sh", "redir3.sh", "redir4.sh", "redir4a.sh", "redir5.sh". |
Redirecting the stdout of a code
block has the effect of saving its output to a file. See Example 3-2. Here documents
are a special case of redirected code blocks.
16.3. ApplicationsClever use of I/O redirection permits parsing and stitching
together snippets of command output (see Example 11-7). This permits
generating report and log files. Example 16-12. Logging events #!/bin/bash
# logevents.sh, by Stephane Chazelas.
# Event logging to a file.
# Must be run as root (for write access in /var/log).
ROOT_UID=0 # Only users with $UID 0 have root privileges.
E_NOTROOT=67 # Non-root exit error.
if [ "$UID" -ne "$ROOT_UID" ]
then
echo "Must be root to run this script."
exit $E_NOTROOT
fi
FD_DEBUG1=3
FD_DEBUG2=4
FD_DEBUG3=5
# Uncomment one of the two lines below to activate script.
# LOG_EVENTS=1
# LOG_VARS=1
log() # Writes time and date to log file.
{
echo "$(date) $*" >&7 # This *appends* the date to the file.
# See below.
}
case $LOG_LEVEL in
1) exec 3>&2 4> /dev/null 5> /dev/null;;
2) exec 3>&2 4>&2 5> /dev/null;;
3) exec 3>&2 4>&2 5>&2;;
*) exec 3> /dev/null 4> /dev/null 5> /dev/null;;
esac
FD_LOGVARS=6
if [[ $LOG_VARS ]]
then exec 6>> /var/log/vars.log
else exec 6> /dev/null # Bury output.
fi
FD_LOGEVENTS=7
if [[ $LOG_EVENTS ]]
then
# then exec 7 >(exec gawk '{print strftime(), $0}' >> /var/log/event.log)
# Above line will not work in Bash, version 2.04.
exec 7>> /var/log/event.log # Append to "event.log".
log # Write time and date.
else exec 7> /dev/null # Bury output.
fi
echo "DEBUG3: beginning" >&${FD_DEBUG3}
ls -l >&5 2>&4 # command1 >&5 2>&4
echo "Done" # command2
echo "sending mail" >&${FD_LOGEVENTS} # Writes "sending mail" to fd #7.
exit 0 |
Chapter 17. Here Documents | Here and now, boys. | | Aldous Huxley, "Island" |
A here document is a special-purpose
code block. It uses a form of I/O
redirection to feed a command list to
an interactive program or a command, such as ftp, cat,
or the ex text editor. COMMAND <<InputComesFromHERE
...
InputComesFromHERE |
A limit string delineates (frames)
the command list. The special symbol << designates
the limit string. This has the effect of redirecting the output
of a file into the stdin of the program
or command. It is similar to interactive-program <
command-file, where command-file
contains
command #1
command #2
... |
The here document alternative looks
like this: #!/bin/bash
interactive-program <<LimitString
command #1
command #2
...
LimitString |
Choose a limit string sufficiently
unusual that it will not occur anywhere in the command list and
confuse matters. Note that here documents may sometimes
be used to good effect with non-interactive utilities and commands,
such as, for example, wall. Example 17-1. broadcast: Sends message to everyone logged in #!/bin/bash
wall <<zzz23EndOfMessagezzz23
E-mail your noontime orders for pizza to the system administrator.
(Add an extra dollar for anchovy or mushroom topping.)
# Additional message text goes here.
# Note: 'wall' prints comment lines.
zzz23EndOfMessagezzz23
# Could have been done more efficiently by
# wall <message-file
# However, embedding the message template in a script
#+ is a quick-and-dirty one-off solution.
exit 0 |
Even such unlikely candidates as vi lend
themselves to here documents. Example 17-2. dummyfile: Creates a 2-line dummy file #!/bin/bash
# Non-interactive use of 'vi' to edit a file.
# Emulates 'sed'.
E_BADARGS=65
if [ -z "$1" ]
then
echo "Usage: `basename $0` filename"
exit $E_BADARGS
fi
TARGETFILE=$1
# Insert 2 lines in file, then save.
#--------Begin here document-----------#
vi $TARGETFILE <<x23LimitStringx23
i
This is line 1 of the example file.
This is line 2 of the example file.
^[
ZZ
x23LimitStringx23
#----------End here document-----------#
# Note that ^[ above is a literal escape
#+ typed by Control-V <Esc>.
# Bram Moolenaar points out that this may not work with 'vim',
#+ because of possible problems with terminal interaction.
exit 0 |
The above script could just as effectively have been implemented with
ex, rather than
vi. Here documents
containing a list of ex commands are common
enough to form their own category, known as ex
scripts.
#!/bin/bash
# Replace all instances of "Smith" with "Jones"
#+ in files with a ".txt" filename suffix.
ORIGINAL=Smith
REPLACEMENT=Jones
for word in $(fgrep -l $ORIGINAL *.txt)
do
# -------------------------------------
ex $word <<EOF
:%s/$ORIGINAL/$REPLACEMENT/g
:wq
EOF
# :%s is the "ex" substitution command.
# :wq is write-and-quit.
# -------------------------------------
done |
Analogous to "ex scripts" are cat
scripts. Example 17-3. Multi-line message using cat #!/bin/bash
# 'echo' is fine for printing single line messages,
#+ but somewhat problematic for for message blocks.
# A 'cat' here document overcomes this limitation.
cat <<End-of-message
-------------------------------------
This is line 1 of the message.
This is line 2 of the message.
This is line 3 of the message.
This is line 4 of the message.
This is the last line of the message.
-------------------------------------
End-of-message
# Replacing line 7, above, with
#+ cat > $Newfile <<End-of-message
#+ ^^^^^^^^^^
#+ writes the output to the file $Newfile, rather than to stdout.
exit 0
#--------------------------------------------
# Code below disabled, due to "exit 0" above.
# S.C. points out that the following also works.
echo "-------------------------------------
This is line 1 of the message.
This is line 2 of the message.
This is line 3 of the message.
This is line 4 of the message.
This is the last line of the message.
-------------------------------------"
# However, text may not include double quotes unless they are escaped. |
The - option to mark a here document limit string
(<<-LimitString) suppresses leading
tabs (but not spaces) in the output. This may be useful in making
a script more readable. Example 17-4. Multi-line message, with tabs suppressed #!/bin/bash
# Same as previous example, but...
# The - option to a here document <<-
#+ suppresses leading tabs in the body of the document,
#+ but *not* spaces.
cat <<-ENDOFMESSAGE
This is line 1 of the message.
This is line 2 of the message.
This is line 3 of the message.
This is line 4 of the message.
This is the last line of the message.
ENDOFMESSAGE
# The output of the script will be flush left.
# Leading tab in each line will not show.
# Above 5 lines of "message" prefaced by a tab, not spaces.
# Spaces not affected by <<- .
# Note that this option has no effect on *embedded* tabs.
exit 0 |
A here document supports parameter and
command substitution. It is therefore possible to pass different
parameters to the body of the here document, changing its output
accordingly. Example 17-5. Here document with parameter substitution #!/bin/bash
# Another 'cat' here document, using parameter substitution.
# Try it with no command line parameters, ./scriptname
# Try it with one command line parameter, ./scriptname Mortimer
# Try it with one two-word quoted command line parameter,
# ./scriptname "Mortimer Jones"
CMDLINEPARAM=1 # Expect at least command line parameter.
if [ $# -ge $CMDLINEPARAM ]
then
NAME=$1 # If more than one command line param,
#+ then just take the first.
else
NAME="John Doe" # Default, if no command line parameter.
fi
RESPONDENT="the author of this fine script"
cat <<Endofmessage
Hello, there, $NAME.
Greetings to you, $NAME, from $RESPONDENT.
# This comment shows up in the output (why?).
Endofmessage
# Note that the blank lines show up in the output.
# So does the "comment".
exit 0 |
This is a useful script containing a here document with
parameter substitution. Example 17-6. Upload a file pair to "Sunsite" incoming
directory #!/bin/bash
# upload.sh
# Upload file pair (Filename.lsm, Filename.tar.gz)
#+ to incoming directory at Sunsite/UNC (ibiblio.org).
# Filename.tar.gz is the tarball itself.
# Filename.lsm is the descriptor file.
# Sunsite requires "lsm" file, otherwise will bounce contributions.
E_ARGERROR=65
if [ -z "$1" ]
then
echo "Usage: `basename $0` Filename-to-upload"
exit $E_ARGERROR
fi
Filename=`basename $1` # Strips pathname out of file name.
Server="ibiblio.org"
Directory="/incoming/Linux"
# These need not be hard-coded into script,
#+ but may instead be changed to command line argument.
Password="your.e-mail.address" # Change above to suit.
ftp -n $Server <<End-Of-Session
# -n option disables auto-logon
user anonymous "$Password"
binary
bell # Ring 'bell' after each file transfer.
cd $Directory
put "$Filename.lsm"
put "$Filename.tar.gz"
bye
End-Of-Session
exit 0 |
Quoting or escaping the "limit string" at the
head of a here document disables parameter substitution within its
body. Example 17-7. Parameter substitution turned off #!/bin/bash
# A 'cat' here document, but with parameter substitution disabled.
NAME="John Doe"
RESPONDENT="the author of this fine script"
cat <<'Endofmessage'
Hello, there, $NAME.
Greetings to you, $NAME, from $RESPONDENT.
Endofmessage
# No parameter substitution when the "limit string" is quoted or escaped.
# Either of the following at the head of the here document would have the same effect.
# cat <<"Endofmessage"
# cat <<\Endofmessage
exit 0 |
Disabling parameter substitution permits outputting literal text.
Generating scripts or even program code is one use for this. Example 17-8. A script that generates another script #!/bin/bash
# generate-script.sh
# Based on an idea by Albert Reiner.
OUTFILE=generated.sh # Name of the file to generate.
# -----------------------------------------------------------
# 'Here document containing the body of the generated script.
(
cat <<'EOF'
#!/bin/bash
echo "This is a generated shell script."
# Note that since we are inside a subshell,
#+ we can't access variables in the "outside" script.
echo "Generated file will be named: $OUTFILE"
# Above line will not work as normally expected
#+ because parameter expansion has been disabled.
# Instead, the result is literal output.
a=7
b=3
let "c = $a * $b"
echo "c = $c"
exit 0
EOF
) > $OUTFILE
# -----------------------------------------------------------
# Quoting the 'limit string' prevents variable expansion
#+ within the body of the above 'here document.'
# This permits outputting literal strings in the output file.
if [ -f "$OUTFILE" ]
then
chmod 755 $OUTFILE
# Make the generated file executable.
else
echo "Problem in creating file: \"$OUTFILE\""
fi
# This method can also be used for generating
#+ C programs, Perl programs, Python programs, Makefiles,
#+ and the like.
exit 0 |
It is possible to set a variable from the output of a here document.
variable=$(cat <<SETVAR
This variable
runs over multiple lines.
SETVAR)
echo "$variable" |
A here document can supply input to a function in the same
script. Example 17-9. Here documents and functions #!/bin/bash
# here-function.sh
GetPersonalData ()
{
read firstname
read lastname
read address
read city
read state
read zipcode
} # This certainly looks like an interactive function, but...
# Supply input to the above function.
GetPersonalData <<RECORD001
Bozo
Bozeman
2726 Nondescript Dr.
Baltimore
MD
21226
RECORD001
echo
echo "$firstname $lastname"
echo "$address"
echo "$city, $state $zipcode"
echo
exit 0 |
It is possible to use : as a dummy command
accepting output from a here document. This, in effect, creates an
"anonymous" here document. Example 17-10. "Anonymous" Here Document #!/bin/bash
: <<TESTVARIABLES
${HOSTNAME?}${USER?}${MAIL?} # Print error message if one of the variables not set.
TESTVARIABLES
exit 0 |
 | A variation of the above technique permits "commenting
out" blocks of code. |
Example 17-11. Commenting out a block of code #!/bin/bash
# commentblock.sh
: <<COMMENTBLOCK
echo "This line will not echo."
This is a comment line missing the "#" prefix.
This is another comment line missing the "#" prefix.
&*@!!++=
The above line will cause no error message,
because the Bash interpreter will ignore it.
COMMENTBLOCK
echo "Exit value of above \"COMMENTBLOCK\" is $?." # 0
# No error shown.
# The above technique also comes in useful for commenting out
#+ a block of working code for debugging purposes.
# This saves having to put a "#" at the beginning of each line,
#+ then having to go back and delete each "#" later.
: <<DEBUGXXX
for file in *
do
cat "$file"
done
DEBUGXXX
exit 0 |
 | Yet another twist of this nifty trick makes
"self-documenting" scripts possible. |
Example 17-12. A self-documenting script #!/bin/bash
# self-document.sh: self-documenting script
# Modification of "colm.sh".
DOC_REQUEST=70
if [ "$1" = "-h" -o "$1" = "--help" ] # Request help.
then
echo; echo "Usage: $0 [directory-name]"; echo
sed --silent -e '/DOCUMENTATIONXX$/,/^DOCUMENTATIONXX$/p' "$0" |
sed -e '/DOCUMENTATIONXX$/d'; exit $DOC_REQUEST; fi
: <<DOCUMENTATIONXX
List the statistics of a specified directory in tabular format.
---------------------------------------------------------------
The command line parameter gives the directory to be listed.
If no directory specified or directory specified cannot be read,
then list the current working directory.
DOCUMENTATIONXX
if [ -z "$1" -o ! -r "$1" ]
then
directory=.
else
directory="$1"
fi
echo "Listing of "$directory":"; echo
(printf "PERMISSIONS LINKS OWNER GROUP SIZE MONTH DAY HH:MM PROG-NAME\n" \
; ls -l "$directory" | sed 1d) | column -t
exit 0 |
Using a cat script is an
alternate way of accomplishing this. DOC_REQUEST=70
if [ "$1" = "-h" -o "$1" = "--help" ] # Request help.
then # Use a "cat script" . . .
cat <<DOCUMENTATIONXX
List the statistics of a specified directory in tabular format.
---------------------------------------------------------------
The command line parameter gives the directory to be listed.
If no directory specified or directory specified cannot be read,
then list the current working directory.
DOCUMENTATIONXX
exit $DOC_REQUEST
fi |
See also Example A-27 for one more excellent example
of a self-documenting script.  | Here documents create temporary files, but these
files are deleted after opening and are not accessible to
any other process. bash$ bash -c 'lsof -a -p $$ -d0' << EOF
> EOF
lsof 1213 bozo 0r REG 3,5 0 30386 /tmp/t1213-0-sh (deleted)
|
|
 | Some utilities will not work inside a
here document. |
 | The closing limit string,
on the final line of a here document, must start in the
first character position. There can
be no leading whitespace. Trailing
whitespace after the limit string likewise causes unexpected
behavior. The whitespace prevents the limit string from being
recognized. #!/bin/bash
echo "----------------------------------------------------------------------"
cat <<LimitString
echo "This is line 1 of the message inside the here document."
echo "This is line 2 of the message inside the here document."
echo "This is the final line of the message inside the here document."
LimitString
#^^^^Indented limit string. Error! This script will not behave as expected.
echo "----------------------------------------------------------------------"
# These comments are outside the 'here document',
#+ and should not echo.
echo "Outside the here document."
exit 0
echo "This line had better not echo." # Follows an 'exit' command. |
|
For those tasks too complex for a "here
document", consider using the expect
scripting language, which is specifically tailored for feeding
input into interactive programs.
17.1. Here Strings
A here string can be considered as
a stripped-down form of here document. It
consists of nothing more than COMMAND
<<<$WORD, where $WORD
is expanded and fed to the stdin of
COMMAND. Example 17-13. Prepending a line to a file #!/bin/bash
# prepend.sh: Add text at beginning of file.
#
# Example contributed by Kenny Stauffer,
#+ and slightly modified by document author.
E_NOSUCHFILE=65
read -p "File: " file # -p arg to 'read' displays prompt.
if [ ! -e "$file" ]
then # Bail out if no such file.
echo "File $file not found."
exit $E_NOSUCHFILE
fi
read -p "Title: " title
cat - $file <<<$title > $file.new
echo "Modified file is $file.new"
exit 0
# from 'man bash':
# Here Strings
# A variant of here documents, the format is:
#
# <<<word
#
# The word is expanded and supplied to the command on its standard input. |
Exercise: Find other uses for here
strings.
Chapter 18. Recess Time
This bizarre little intermission gives the reader a chance to
relax and maybe laugh a bit.
Fellow Linux user, greetings! You are reading something which
will bring you luck and good fortune. Just e-mail a copy of
this document to 10 of your friends. Before making the copies,
send a 100-line Bash script to the first person on the list
at the bottom of this letter. Then delete their name and add
yours to the bottom of the list.
Don't break the chain! Make the copies within 48 hours.
Wilfred P. of Brooklyn failed to send out his ten copies and
woke the next morning to find his job description changed
to "COBOL programmer." Howard L. of Newport News sent
out his ten copies and within a month had enough hardware
to build a 100-node Beowulf cluster dedicated to playing
Tuxracer. Amelia V. of Chicago laughed
at this letter and broke the chain. Shortly thereafter, a fire
broke out in her terminal and she now spends her days writing
documentation for MS Windows.
Don't break the chain! Send out your ten copies today!
Courtesy 'NIX "fortune cookies", with some
alterations and many apologies
Part 4. Advanced TopicsAt this point, we are ready to delve into certain of the
difficult and unusual aspects of scripting. Along the way, we
will attempt to "push the envelope" in various
ways and examine boundary conditions
(what happens when we move into uncharted territory?).
Chapter 19. Regular Expressions
To fully utilize the power of shell scripting, you need to
master Regular Expressions. Certain commands and utilities
commonly used in scripts, such as
grep, expr, sed
and awk interpret and use
REs.
19.1. A Brief Introduction to Regular ExpressionsAn expression is a string of characters. Those characters
having an interpretation above and beyond their literal
meaning are called metacharacters. A
quote symbol, for example, may denote speech by a person,
ditto, or a meta-meaning for the symbols
that follow. Regular Expressions are sets of characters and/or
metacharacters that match (or specify) patterns. A Regular Expression contains one or more of the
following: A character set. These are the
characters retaining their literal meaning. The
simplest type of Regular Expression consists
only of a character set, with no
metacharacters. An anchor. These designate
(anchor) the position in the line of
text that the RE is to match. For example, ^,
and $ are anchors. Modifiers. These expand or narrow
(modify) the range of text the RE is
to match. Modifiers include the asterisk, brackets, and
the backslash.
The main uses for Regular Expressions
(REs) are text searches and string
manipulation. An RE matches a single
character or a set of characters -- a string or a part of
a string. The asterisk -- * -- matches any number of
repeats of the character string or RE preceding it,
including zero. "1133*" matches 11 +
one or more 3's + possibly other characters:
113, 1133,
111312, and so forth. The dot -- . -- matches
any one character, except a newline.
"13." matches 13 + at
least one of any character (including a
space): 1133,
11333, but not
13 (additional character
missing). The caret -- ^ -- matches the beginning of
a line, but sometimes, depending on context, negates the
meaning of a set of characters in an RE.
The dollar sign -- $ -- at the end of an
RE matches the end of a line. "^$" matches blank lines.
Brackets -- [...] -- enclose a set of characters
to match in a single RE. "[xyz]" matches the characters
x, y,
or z. "[c-n]" matches any of the
characters in the range c
to n. "[B-Pk-y]" matches any of the
characters in the ranges B
to P
and k to
y. "[a-z0-9]" matches any lowercase letter or any
digit. "[^b-d]" matches all characters
except those in
the range b to
d. This is an instance of
^ negating or inverting the meaning
of the following RE (taking on a role similar to
! in a different context). Combined sequences of bracketed characters match
common word patterns. "[Yy][Ee][Ss]" matches
yes, Yes,
YES, yEs,
and so forth.
"[0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9][0-9][0-9]"
matches any Social Security number. The backslash -- \ -- escapes a special character, which
means that character gets interpreted literally. A "\$" reverts back to its
literal meaning of "$", rather than its
RE meaning of end-of-line. Likewise a "\\"
has the literal meaning of "\".
Escaped "angle
brackets" -- \<...\> -- mark word
boundaries. The angle brackets must be escaped, since otherwise
they have only their literal character meaning. "\<the\>" matches the word
"the", but not the words "them",
"there", "other", etc. bash$ cat textfile
This is line 1, of which there is only one instance.
This is the only instance of line 2.
This is line 3, another line.
This is line 4.
bash$ grep 'the' textfile
This is line 1, of which there is only one instance.
This is the only instance of line 2.
This is line 3, another line.
bash$ grep '\<the\>' textfile
This is the only instance of line 2.
|
The question mark -- ? -- matches zero or
one of the previous RE. It is generally used for matching
single characters.
The plus -- + -- matches one or more of the
previous RE. It serves a role similar to the *, but
does not match zero occurrences. # GNU versions of sed and awk can use "+",
# but it needs to be escaped.
echo a111b | sed -ne '/a1\+b/p'
echo a111b | grep 'a1\+b'
echo a111b | gawk '/a1+b/'
# All of above are equivalent.
# Thanks, S.C. |
Escaped "curly
brackets" -- \{ \} -- indicate the number
of occurrences of a preceding RE to match. It is necessary to escape the curly brackets since
they have only their literal character meaning
otherwise. This usage is technically not part of the basic
RE set. "[0-9]\{5\}" matches exactly five digits
(characters in the range of 0 to 9).  | Curly brackets are not available as an RE in the
"classic" (non-POSIX compliant) version
of awk. However,
gawk has the
--re-interval option that permits them
(without being escaped). bash$ echo 2222 | gawk --re-interval '/2{3}/'
2222
|
Perl and some
egrep versions do not require escaping
the curly brackets. |
Parentheses -- ( ) -- enclose groups of
REs. They are useful with the following
"|" operator and in substring extraction using expr. The -- | -- "or" RE operator
matches any of a set of alternate characters. bash$ egrep 're(a|e)d' misc.txt
People who read seem to be better informed than those who do not.
The clarinet produces sound by the vibration of its reed.
|
 | Some versions of sed,
ed, and ex support
escaped versions of the extended Regular Expressions
described above, as do the GNU utilities. |
This is an alternate method of specifying a range of
characters to match. [:alnum:] matches alphabetic or
numeric characters. This is equivalent to
A-Za-z0-9. [:alpha:] matches alphabetic
characters. This is equivalent to
A-Za-z. [:blank:] matches a space or a
tab. [:cntrl:] matches control
characters. [:digit:] matches (decimal)
digits. This is equivalent to
0-9. [:graph:] (graphic printable
characters). Matches characters in the range of ASCII 33 -
126. This is the same as [:print:],
below, but excluding the space character. [:lower:] matches lowercase
alphabetic characters. This is equivalent to
a-z. [:print:] (printable
characters). Matches characters in the range of ASCII 32 -
126. This is the same as [:graph:],
above, but adding the space character. [:space:] matches whitespace
characters (space and horizontal tab). [:upper:] matches uppercase
alphabetic characters. This is equivalent to
A-Z. [:xdigit:] matches hexadecimal
digits. This is equivalent to
0-9A-Fa-f.  | POSIX character classes generally require quoting
or double brackets
([[ ]]). bash$ grep [[:digit:]] test.file
abc=723
|
These character classes may even be used with globbing, to a limited
extent. bash$ ls -l ?[[:digit:]][[:digit:]]?
-rw-rw-r-- 1 bozo bozo 0 Aug 21 14:47 a33b
|
To see POSIX character classes used in scripts, refer to
Example 12-18 and Example 12-19. |
Sed, awk, and Perl, used as filters in scripts, take
REs as arguments when "sifting" or transforming files or I/O
streams. See Example A-12 and Example A-17
for illustrations of this. The standard reference on this complex topic is Friedl's
Mastering Regular
Expressions. Sed &
Awk, by Dougherty and Robbins also gives a very
lucid treatment of REs. See the Bibliography for
more information on these books.
19.2. GlobbingBash itself cannot recognize Regular Expressions. Inside
scripts, it is commands and utilities -- such as
sed and awk -- that interpret RE's. Bash does carry out filename
expansion
-- a process known as globbing -- but
this does not use the standard RE set.
Instead, globbing recognizes and expands wildcards. Globbing
interprets the standard wildcard characters, *
and ?, character lists in square brackets, and
certain other special characters (such as ^ for
negating the sense of a match). There are important limitations
on wildcard characters in globbing, however. Strings containing
* will not match filenames that
start with a dot, as, for example, .bashrc.
Likewise, the ? has a different
meaning in globbing than as part of an RE. bash$ ls -l
total 2
-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 a.1
-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 b.1
-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 c.1
-rw-rw-r-- 1 bozo bozo 466 Aug 6 17:48 t2.sh
-rw-rw-r-- 1 bozo bozo 758 Jul 30 09:02 test1.txt
bash$ ls -l t?.sh
-rw-rw-r-- 1 bozo bozo 466 Aug 6 17:48 t2.sh
bash$ ls -l [ab]*
-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 a.1
-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 b.1
bash$ ls -l [a-c]*
-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 a.1
-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 b.1
-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 c.1
bash$ ls -l [^ab]*
-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 c.1
-rw-rw-r-- 1 bozo bozo 466 Aug 6 17:48 t2.sh
-rw-rw-r-- 1 bozo bozo 758 Jul 30 09:02 test1.txt
bash$ ls -l {b*,c*,*est*}
-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 b.1
-rw-rw-r-- 1 bozo bozo 0 Aug 6 18:42 c.1
-rw-rw-r-- 1 bozo bozo 758 Jul 30 09:02 test1.txt
|
Bash performs filename expansion on unquoted command-line
arguments. The echo command
demonstrates this. bash$ echo *
a.1 b.1 c.1 t2.sh test1.txt
bash$ echo t*
t2.sh test1.txt
|
 | It is possible to modify the way Bash interprets
special characters in globbing. A set -f
command disables globbing, and the
nocaseglob and nullglob
options to shopt change
globbing behavior. |
See also Example 10-4.
Chapter 20. Subshells
Running a shell script launches another instance of the
command processor. Just as your commands are interpreted at the
command line prompt, similarly does a script batch process a list
of commands in a file. Each shell script running is, in effect,
a subprocess of the parent shell,
the one that gives you the prompt at the console or in an
xterm window. A shell script can also launch subprocesses. These
subshells let the script do
parallel processing, in effect executing multiple subtasks
simultaneously. Command List in
Parentheses - ( command1; command2; command3; ... )
A command list embedded between
parentheses runs as a
subshell.
 | Variables in a subshell are
not visible outside the block of code
in the subshell. They are not accessible to the parent process, to the shell
that launched the subshell. These are, in effect, local variables. |
Example 20-1. Variable scope in a subshell #!/bin/bash
# subshell.sh
echo
echo "Subshell level OUTSIDE subshell = $BASH_SUBSHELL"
# Bash, version 3, adds the new $BASH_SUBSHELL variable.
echo
outer_variable=Outer
(
echo "Subshell level INSIDE subshell = $BASH_SUBSHELL"
inner_variable=Inner
echo "From subshell, \"inner_variable\" = $inner_variable"
echo "From subshell, \"outer\" = $outer_variable"
)
echo
echo "Subshell level OUTSIDE subshell = $BASH_SUBSHELL"
echo
if [ -z "$inner_variable" ]
then
echo "inner_variable undefined in main body of shell"
else
echo "inner_variable defined in main body of shell"
fi
echo "From main body of shell, \"inner_variable\" = $inner_variable"
# $inner_variable will show as uninitialized
#+ because variables defined in a subshell are "local variables".
# Is there any remedy for this?
echo
exit 0 |
See also Example 31-2. + Directory changes made in a subshell do not carry over to the
parent shell. Example 20-2. List User Profiles #!/bin/bash
# allprofs.sh: print all user profiles
# This script written by Heiner Steven, and modified by the document author.
FILE=.bashrc # File containing user profile,
#+ was ".profile" in original script.
for home in `awk -F: '{print $6}' /etc/passwd`
do
[ -d "$home" ] || continue # If no home directory, go to next.
[ -r "$home" ] || continue # If not readable, go to next.
(cd $home; [ -e $FILE ] && less $FILE)
done
# When script terminates, there is no need to 'cd' back to original directory,
#+ because 'cd $home' takes place in a subshell.
exit 0 |
A subshell may be used to set up a "dedicated
environment" for a command group.
COMMAND1
COMMAND2
COMMAND3
(
IFS=:
PATH=/bin
unset TERMINFO
set -C
shift 5
COMMAND4
COMMAND5
exit 3 # Only exits the subshell.
)
# The parent shell has not been affected, and the environment is preserved.
COMMAND6
COMMAND7 |
One application of this is testing whether a variable is defined.
if (set -u; : $variable) 2> /dev/null
then
echo "Variable is set."
fi # Variable has been set in current script,
#+ or is an an internal Bash variable,
#+ or is present in environment (has been exported).
# Could also be written [[ ${variable-x} != x || ${variable-y} != y ]]
# or [[ ${variable-x} != x$variable ]]
# or [[ ${variable+x} = x ]]
# or [[ ${variable-x} != x ]] |
Another application is checking for a lock file:
if (set -C; : > lock_file) 2> /dev/null
then
: # lock_file didn't exist: no user running the script
else
echo "Another user is already running that script."
exit 65
fi
# Code snippet by Stéphane Chazelas,
#+ with modifications by Paulo Marcel Coelho Aragao. |
Processes may execute in parallel within different
subshells. This permits breaking a complex task into subcomponents
processed concurrently. Example 20-3. Running parallel processes in subshells (cat list1 list2 list3 | sort | uniq > list123) &
(cat list4 list5 list6 | sort | uniq > list456) &
# Merges and sorts both sets of lists simultaneously.
# Running in background ensures parallel execution.
#
# Same effect as
# cat list1 list2 list3 | sort | uniq > list123 &
# cat list4 list5 list6 | sort | uniq > list456 &
wait # Don't execute the next command until subshells finish.
diff list123 list456 |
Redirecting I/O to a subshell uses the "|" pipe
operator, as in ls -al | (command).  | A command block between curly
braces does not launch
a subshell. { command1; command2; command3; ... } |
Chapter 21. Restricted ShellsDisabled commands in restricted
shells Changing the values of the
$PATH,
$SHELL,
$BASH_ENV,
or $ENV environmental variables. Reading or changing the $SHELLOPTS,
shell environmental options. Output redirection. Invoking commands containing one or more
/'s. Invoking exec to substitute a different
process for the shell. Various other commands that would enable monkeying
with or attempting to subvert the script for an unintended
purpose. Getting out of restricted mode within the script.
Example 21-1. Running a script in restricted mode #!/bin/bash
# Starting the script with "#!/bin/bash -r"
#+ runs entire script in restricted mode.
echo
echo "Changing directory."
cd /usr/local
echo "Now in `pwd`"
echo "Coming back home."
cd
echo "Now in `pwd`"
echo
# Everything up to here in normal, unrestricted mode.
set -r
# set --restricted has same effect.
echo "==> Now in restricted mode. <=="
echo
echo
echo "Attempting directory change in restricted mode."
cd ..
echo "Still in `pwd`"
echo
echo
echo "\$SHELL = $SHELL"
echo "Attempting to change shell in restricted mode."
SHELL="/bin/ash"
echo
echo "\$SHELL= $SHELL"
echo
echo
echo "Attempting to redirect output in restricted mode."
ls -l /usr/bin > bin.files
ls -l bin.files # Try to list attempted file creation effort.
echo
exit 0 |
Chapter 22. Process SubstitutionProcess
substitution is the counterpart to command substitution. Command
substitution sets a variable to the result of a command, as in
dir_contents=`ls -al` or xref=$(
grep word datafile). Process substitution feeds the
output of a process to another process (in other words, it sends
the results of a command to another command). Command substitution template - command within parentheses
>(command) <(command) These initiate process substitution. This uses
/dev/fd/<n> files to send the
results of the process within parentheses to another process.
 | There is no space between the
the "<" or ">" and the parentheses.
Space there would give an error message. |
bash$ echo >(true)
/dev/fd/63
bash$ echo <(true)
/dev/fd/63
|
Bash creates a pipe with two file
descriptors, --fIn and
fOut--. The stdin
of true connects
to fOut (dup2(fOut, 0)),
then Bash passes a /dev/fd/fIn
argument to echo. On systems lacking
/dev/fd/<n> files, Bash may use
temporary files. (Thanks, S.C.) Process substitution can compare the output of two
different commands, or even the output of different options
to the same command. bash$ comm <(ls -l) <(ls -al)
total 12
-rw-rw-r-- 1 bozo bozo 78 Mar 10 12:58 File0
-rw-rw-r-- 1 bozo bozo 42 Mar 10 12:58 File2
-rw-rw-r-- 1 bozo bozo 103 Mar 10 12:58 t2.sh
total 20
drwxrwxrwx 2 bozo bozo 4096 Mar 10 18:10 .
drwx------ 72 bozo bozo 4096 Mar 10 17:58 ..
-rw-rw-r-- 1 bozo bozo 78 Mar 10 12:58 File0
-rw-rw-r-- 1 bozo bozo 42 Mar 10 12:58 File2
-rw-rw-r-- 1 bozo bozo 103 Mar 10 12:58 t2.sh |
Using process substitution to compare the contents
of two directories (to see which filenames are in one,
but not the other):
diff <(ls $first_directory) <(ls $second_directory) |
Some other usages and uses of process substitution: cat <(ls -l)
# Same as ls -l | cat
sort -k 9 <(ls -l /bin) <(ls -l /usr/bin) <(ls -l /usr/X11R6/bin)
# Lists all the files in the 3 main 'bin' directories, and sorts by filename.
# Note that three (count 'em) distinct commands are fed to 'sort'.
diff <(command1) <(command2) # Gives difference in command output.
tar cf >(bzip2 -c > file.tar.bz2) $directory_name
# Calls "tar cf /dev/fd/?? $directory_name", and "bzip2 -c > file.tar.bz2".
#
# Because of the /dev/fd/<n> system feature,
# the pipe between both commands does not need to be named.
#
# This can be emulated.
#
bzip2 -c < pipe > file.tar.bz2&
tar cf pipe $directory_name
rm pipe
# or
exec 3>&1
tar cf /dev/fd/4 $directory_name 4>&1 >&3 3>&- | bzip2 -c > file.tar.bz2 3>&-
exec 3>&-
# Thanks, St´phane Chazelas |
A reader sent in the following interesting example of process
substitution. # Script fragment taken from SuSE distribution:
while read des what mask iface; do
# Some commands ...
done < <(route -n)
# To test it, let's make it do something.
while read des what mask iface; do
echo $des $what $mask $iface
done < <(route -n)
# Output:
# Kernel IP routing table
# Destination Gateway Genmask Flags Metric Ref Use Iface
# 127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo
# As Stéphane Chazelas points out, an easier-to-understand equivalent is:
route -n |
while read des what mask iface; do # Variables set from output of pipe.
echo $des $what $mask $iface
done # This yields the same output as above.
# However, as Ulrich Gayer points out . . .
#+ this simplified equivalent uses a subshell for the while loop,
#+ and therefore the variables disappear when the pipe terminates.
# However, Filip Moritz comments that there is a subtle difference
#+ between the above two examples, as the following shows.
(
route -n | while read x; do ((y++)); done
echo $y # $y is still unset
while read x; do ((y++)); done < <(route -n)
echo $y # $y has the number of lines of output of route -n
)
More generally spoken
(
: | x=x
# seems to start a subshell like
: | ( x=x )
# while
x=x < <(:)
# does not
)
# This is useful, when parsing csv and the like.
# That is, in effect, what the original SuSE code fragment does. |
Chapter 23. Functions
Like "real" programming languages,
Bash has functions, though in a somewhat limited implementation.
A function is a subroutine, a code
block that implements a set of operations, a "black
box" that performs a specified task. Wherever there is
repetitive code, when a task repeats with only slight variations,
then consider using a function. function function_name { command... }
or
function_name () { command... }
This second form will cheer the hearts of C programmers
(and is more portable). As in C, the function's opening bracket may optionally appear
on the second line. function_name () { command... }
Functions are called, triggered, simply by
invoking their names. Example 23-1. Simple functions #!/bin/bash
JUST_A_SECOND=1
funky ()
{ # This is about as simple as functions get.
echo "This is a funky function."
echo "Now exiting funky function."
} # Function declaration must precede call.
fun ()
{ # A somewhat more complex function.
i=0
REPEATS=30
echo
echo "And now the fun really begins."
echo
sleep $JUST_A_SECOND # Hey, wait a second!
while [ $i -lt $REPEATS ]
do
echo "----------FUNCTIONS---------->"
echo "<------------ARE-------------"
echo "<------------FUN------------>"
echo
let "i+=1"
done
}
# Now, call the functions.
funky
fun
exit 0 |
The function definition must precede the first call to
it. There is no method of "declaring" the function,
as, for example, in C.
f1
# Will give an error message, since function "f1" not yet defined.
declare -f f1 # This doesn't help either.
f1 # Still an error message.
# However...
f1 ()
{
echo "Calling function \"f2\" from within function \"f1\"."
f2
}
f2 ()
{
echo "Function \"f2\"."
}
f1 # Function "f2" is not actually called until this point,
#+ although it is referenced before its definition.
# This is permissible.
# Thanks, S.C. |
It is even possible to nest a function within another function,
although this is not very useful.
f1 ()
{
f2 () # nested
{
echo "Function \"f2\", inside \"f1\"."
}
}
f2 # Gives an error message.
# Even a preceding "declare -f f2" wouldn't help.
echo
f1 # Does nothing, since calling "f1" does not automatically call "f2".
f2 # Now, it's all right to call "f2",
#+ since its definition has been made visible by calling "f1".
# Thanks, S.C. |
Function declarations can appear in unlikely places, even where a
command would otherwise go.
ls -l | foo() { echo "foo"; } # Permissible, but useless.
if [ "$USER" = bozo ]
then
bozo_greet () # Function definition embedded in an if/then construct.
{
echo "Hello, Bozo."
}
fi
bozo_greet # Works only for Bozo, and other users get an error.
# Something like this might be useful in some contexts.
NO_EXIT=1 # Will enable function definition below.
[[ $NO_EXIT -eq 1 ]] && exit() { true; } # Function definition in an "and-list".
# If $NO_EXIT is 1, declares "exit ()".
# This disables the "exit" builtin by aliasing it to "true".
exit # Invokes "exit ()" function, not "exit" builtin.
# Thanks, S.C. |
23.1. Complex Functions and Function ComplexitiesFunctions may process arguments passed to them and return
an exit status to the script
for further processing. function_name $arg1 $arg2 |
The function refers to the passed arguments by position (as if they were
positional parameters),
that is, $1, $2, and
so forth. Example 23-2. Function Taking Parameters #!/bin/bash
# Functions and parameters
DEFAULT=default # Default param value.
func2 () {
if [ -z "$1" ] # Is parameter #1 zero length?
then
echo "-Parameter #1 is zero length.-" # Or no parameter passed.
else
echo "-Param #1 is \"$1\".-"
fi
variable=${1-$DEFAULT} # What does
echo "variable = $variable" #+ parameter substitution show?
# ---------------------------
# It distinguishes between
#+ no param and a null param.
if [ "$2" ]
then
echo "-Parameter #2 is \"$2\".-"
fi
return 0
}
echo
echo "Nothing passed."
func2 # Called with no params
echo
echo "Zero-length parameter passed."
func2 "" # Called with zero-length param
echo
echo "Null parameter passed."
func2 "$uninitialized_param" # Called with uninitialized param
echo
echo "One parameter passed."
func2 first # Called with one param
echo
echo "Two parameters passed."
func2 first second # Called with two params
echo
echo "\"\" \"second\" passed."
func2 "" second # Called with zero-length first parameter
echo # and ASCII string as a second one.
exit 0 |
But, what about command-line arguments passed to the script?
Does a function see them? Well, let's clear up the confusion. Example 23-3. Functions and command-line args passed to the script #!/bin/bash
# func-cmdlinearg.sh
# Call this script with a command-line argument,
#+ something like $0 arg1.
func ()
{
echo "$1"
}
echo "First call to function: no arg passed."
echo "See if command-line arg is seen."
func
# No! Command-line arg not seen.
echo "============================================================"
echo
echo "Second call to function: command-line arg passed explicitly."
func $1
# Now it's seen!
exit 0 |
In contrast to certain other programming languages,
shell scripts normally pass only value parameters to
functions. Variable names (which are actually pointers), if
passed as parameters to functions, will be treated as string
literals. Functions interpret their arguments
literally. Indirect variable
references (see Example 34-2) provide a clumsy
sort of mechanism for passing variable pointers to
functions. Example 23-4. Passing an indirect reference to a function #!/bin/bash
# ind-func.sh: Passing an indirect reference to a function.
echo_var ()
{
echo "$1"
}
message=Hello
Hello=Goodbye
echo_var "$message" # Hello
# Now, let's pass an indirect reference to the function.
echo_var "${!message}" # Goodbye
echo "-------------"
# What happens if we change the contents of "hello" variable?
Hello="Hello, again!"
echo_var "$message" # Hello
echo_var "${!message}" # Hello, again!
exit 0 |
The next logical question is whether parameters can be
dereferenced after being passed to a
function. Example 23-5. Dereferencing a parameter passed to a function #!/bin/bash
# dereference.sh
# Dereferencing parameter passed to a function.
# Script by Bruce W. Clare.
dereference ()
{
y=\$"$1" # Name of variable.
echo $y # $Junk
x=`eval "expr \"$y\" "`
echo $1=$x
eval "$1=\"Some Different Text \"" # Assign new value.
}
Junk="Some Text"
echo $Junk "before" # Some Text before
dereference Junk
echo $Junk "after" # Some Different Text after
exit 0 |
Example 23-6. Again, dereferencing a parameter passed to a function #!/bin/bash
# ref-params.sh: Dereferencing a parameter passed to a function.
# (Complex Example)
ITERATIONS=3 # How many times to get input.
icount=1
my_read () {
# Called with my_read varname,
#+ outputs the previous value between brackets as the default value,
#+ then asks for a new value.
local local_var
echo -n "Enter a value "
eval 'echo -n "[$'$1'] "' # Previous value.
# eval echo -n "[\$$1] " # Easier to understand,
#+ but loses trailing space in user prompt.
read local_var
[ -n "$local_var" ] && eval $1=\$local_var
# "And-list": if "local_var" then set "$1" to its value.
}
echo
while [ "$icount" -le "$ITERATIONS" ]
do
my_read var
echo "Entry #$icount = $var"
let "icount += 1"
echo
done
# Thanks to Stephane Chazelas for providing this instructive example.
exit 0 |
Exit and Return - exit status
Functions return a value, called an exit
status. The exit status may be explicitly
specified by a return statement,
otherwise it is the exit status of the last command
in the function (0 if
successful, and a non-zero error code if not). This
exit status
may be used in the script by referencing it as
$?. This mechanism
effectively permits script functions to have a "return
value" similar to C functions. - return
Terminates a function. A return command
optionally takes an integer
argument, which is returned to the calling script as
the "exit status" of the function, and
this exit status is assigned to the variable $?. Example 23-7. Maximum of two numbers #!/bin/bash
# max.sh: Maximum of two integers.
E_PARAM_ERR=-198 # If less than 2 params passed to function.
EQUAL=-199 # Return value if both params equal.
max2 () # Returns larger of two numbers.
{ # Note: numbers compared must be less than 257.
if [ -z "$2" ]
then
return $E_PARAM_ERR
fi
if [ "$1" -eq "$2" ]
then
return $EQUAL
else
if [ "$1" -gt "$2" ]
then
return $1
else
return $2
fi
fi
}
max2 33 34
return_val=$?
if [ "$return_val" -eq $E_PARAM_ERR ]
then
echo "Need to pass two parameters to the function."
elif [ "$return_val" -eq $EQUAL ]
then
echo "The two numbers are equal."
else
echo "The larger of the two numbers is $return_val."
fi
exit 0
# Exercise (easy):
# ---------------
# Convert this to an interactive script,
#+ that is, have the script ask for input (two numbers). |
 | For a function to return a string or array, use a
dedicated variable.
count_lines_in_etc_passwd()
{
[[ -r /etc/passwd ]] && REPLY=$(echo $(wc -l < /etc/passwd))
# If /etc/passwd is readable, set REPLY to line count.
# Returns both a parameter value and status information.
# The 'echo' seems unnecessary, but . . .
#+ it removes excess whitespace from the output.
}
if count_lines_in_etc_passwd
then
echo "There are $REPLY lines in /etc/passwd."
else
echo "Cannot count lines in /etc/passwd."
fi
# Thanks, S.C. |
|
Example 23-8. Converting numbers to Roman numerals #!/bin/bash
# Arabic number to Roman numeral conversion
# Range: 0 - 200
# It's crude, but it works.
# Extending the range and otherwise improving the script is left as an exercise.
# Usage: roman number-to-convert
LIMIT=200
E_ARG_ERR=65
E_OUT_OF_RANGE=66
if [ -z "$1" ]
then
echo "Usage: `basename $0` number-to-convert"
exit $E_ARG_ERR
fi
num=$1
if [ "$num" -gt $LIMIT ]
then
echo "Out of range!"
exit $E_OUT_OF_RANGE
fi
to_roman () # Must declare function before first call to it.
{
number=$1
factor=$2
rchar=$3
let "remainder = number - factor"
while [ "$remainder" -ge 0 ]
do
echo -n $rchar
let "number -= factor"
let "remainder = number - factor"
done
return $number
# Exercise:
# --------
# Explain how this function works.
# Hint: division by successive subtraction.
}
to_roman $num 100 C
num=$?
to_roman $num 90 LXXXX
num=$?
to_roman $num 50 L
num=$?
to_roman $num 40 XL
num=$?
to_roman $num 10 X
num=$?
to_roman $num 9 IX
num=$?
to_roman $num 5 V
num=$?
to_roman $num 4 IV
num=$?
to_roman $num 1 I
echo
exit 0 |
See also Example 10-28.  | The largest positive integer a function can return is
255. The return command is closely tied
to the concept of exit
status, which accounts for this particular
limitation. Fortunately, there are various workarounds for those situations
requiring a large integer return value from a
function. Example 23-9. Testing large return values in a function #!/bin/bash
# return-test.sh
# The largest positive value a function can return is 255.
return_test () # Returns whatever passed to it.
{
return $1
}
return_test 27 # o.k.
echo $? # Returns 27.
return_test 255 # Still o.k.
echo $? # Returns 255.
return_test 257 # Error!
echo $? # Returns 1 (return code for miscellaneous error).
# ======================================================
return_test -151896 # Do large negative numbers work?
echo $? # Will this return -151896?
# No! It returns 168.
# Version of Bash before 2.05b permitted
#+ large negative integer return values.
# Newer versions of Bash plug this loophole.
# This may break older scripts.
# Caution!
# ======================================================
exit 0 |
A workaround for obtaining large integer "return
values" is to simply assign the "return
value" to a global variable.
Return_Val= # Global variable to hold oversize return value of function.
alt_return_test ()
{
fvar=$1
Return_Val=$fvar
return # Returns 0 (success).
}
alt_return_test 1
echo $? # 0
echo "return value = $Return_Val" # 1
alt_return_test 256
echo "return value = $Return_Val" # 256
alt_return_test 257
echo "return value = $Return_Val" # 257
alt_return_test 25701
echo "return value = $Return_Val" #25701 |
A more elegant method is to have the function
echo its "return
value to stdout," and
then capture it by command
substitution. See the discussion
of this in Section 33.7. Example 23-10. Comparing two large integers #!/bin/bash
# max2.sh: Maximum of two LARGE integers.
# This is the previous "max.sh" example,
#+ modified to permit comparing large integers.
EQUAL=0 # Return value if both params equal.
E_PARAM_ERR=-99999 # Not enough params passed to function.
max2 () # "Returns" larger of two numbers.
{
if [ -z "$2" ]
then
echo $E_PARAM_ERR
return
fi
if [ "$1" -eq "$2" ]
then
echo $EQUAL
return
else
if [ "$1" -gt "$2" ]
then
retval=$1
else
retval=$2
fi
fi
echo $retval # Echoes (to stdout), rather than returning value.
}
return_val=$(max2 33001 33997)
# ^^^^ Function name
# ^^^^^ ^^^^^ Params passed
# This is actually a form of command substitution:
#+ treating a function as if it were a command,
#+ and assigning the stdout of the function to the variable "return_val."
# ========================= OUTPUT ========================
if [ "$return_val" -eq "$E_PARAM_ERR" ]
then
echo "Error in parameters passed to comparison function!"
elif [ "$return_val" -eq "$EQUAL" ]
then
echo "The two numbers are equal."
else
echo "The larger of the two numbers is $return_val."
fi
# =========================================================
exit 0
# Exercises:
# ---------
# 1) Find a more elegant way of testing
#+ the parameters passed to the function.
# 2) Simplify the if/then structure at "OUTPUT."
# 3) Rewrite the script to take input from command-line parameters. |
Here is another example of capturing a function
"return value." Understanding it requires some
knowledge of awk.
month_length () # Takes month number as an argument.
{ # Returns number of days in month.
monthD="31 28 31 30 31 30 31 31 30 31 30 31" # Declare as local?
echo "$monthD" | awk '{ print $'"${1}"' }' # Tricky.
# ^^^^^^^^^
# Parameter passed to function ($1 -- month number), then to awk.
# Awk sees this as "print $1 . . . print $12" (depending on month number)
# Template for passing a parameter to embedded awk script:
# $'"${script_parameter}"'
# Needs error checking for correct parameter range (1-12)
#+ and for February in leap year.
}
# ----------------------------------------------
# Usage example:
month=4 # April, for example (4th month).
days_in=$(month_length $month)
echo $days_in # 30
# ---------------------------------------------- |
See also Example A-7. Exercise: Using what we have
just learned, extend the previous Roman numerals example to accept
arbitrarily large input. |
Redirection - Redirecting the stdin of a
function
A function is essentially a code block, which means its
stdin can be redirected (as in Example 3-1). Example 23-11. Real name from username #!/bin/bash
# realname.sh
#
# From username, gets "real name" from /etc/passwd.
ARGCOUNT=1 # Expect one arg.
E_WRONGARGS=65
file=/etc/passwd
pattern=$1
if [ $# -ne "$ARGCOUNT" ]
then
echo "Usage: `basename $0` USERNAME"
exit $E_WRONGARGS
fi
file_excerpt () # Scan file for pattern, then print relevant portion of line.
{
while read line # "while" does not necessarily need "[ condition ]"
do
echo "$line" | grep $1 | awk -F":" '{ print $5 }' # Have awk use ":" delimiter.
done
} <$file # Redirect into function's stdin.
file_excerpt $pattern
# Yes, this entire script could be reduced to
# grep PATTERN /etc/passwd | awk -F":" '{ print $5 }'
# or
# awk -F: '/PATTERN/ {print $5}'
# or
# awk -F: '($1 == "username") { print $5 }' # real name from username
# However, it might not be as instructive.
exit 0 |
There is an alternate, and perhaps less confusing
method of redirecting a function's
stdin. This involves redirecting the
stdin to an embedded bracketed code
block within the function.
# Instead of:
Function ()
{
...
} < file
# Try this:
Function ()
{
{
...
} < file
}
# Similarly,
Function () # This works.
{
{
echo $*
} | tr a b
}
Function () # This doesn't work.
{
echo $*
} | tr a b # A nested code block is mandatory here.
# Thanks, S.C. |
23.2. Local VariablesWhat makes a variable "local"? - local variables
A variable declared as local
is one that is visible only within the block of code in which it
appears. It has local "scope". In
a function, a local variable has
meaning only within that function block. Example 23-12. Local variable visibility #!/bin/bash
# Global and local variables inside a function.
func ()
{
local loc_var=23 # Declared as local variable.
echo # Uses the 'local' builtin.
echo "\"loc_var\" in function = $loc_var"
global_var=999 # Not declared as local.
# Defaults to global.
echo "\"global_var\" in function = $global_var"
}
func
# Now, to see if local variable "loc_var" exists outside function.
echo
echo "\"loc_var\" outside function = $loc_var"
# $loc_var outside function =
# No, $loc_var not visible globally.
echo "\"global_var\" outside function = $global_var"
# $global_var outside function = 999
# $global_var is visible globally.
echo
exit 0
# In contrast to C, a Bash variable declared inside a function
#+ is local *only* if declared as such. |
 | Before a function is called, all
variables declared within the function are invisible outside
the body of the function, not just those explicitly declared
as local.
#!/bin/bash
func ()
{
global_var=37 # Visible only within the function block
#+ before the function has been called.
} # END OF FUNCTION
echo "global_var = $global_var" # global_var =
# Function "func" has not yet been called,
#+ so $global_var is not visible here.
func
echo "global_var = $global_var" # global_var = 37
# Has been set by function call. |
|
23.2.1. Local variables help make recursion possible.Local variables permit recursion,
but this practice generally involves much computational
overhead and is definitely not
recommended in a shell script.
Example 23-13. Recursion, using a local variable #!/bin/bash
# factorial
# ---------
# Does bash permit recursion?
# Well, yes, but...
# It's so slow that you gotta have rocks in your head to try it.
MAX_ARG=5
E_WRONG_ARGS=65
E_RANGE_ERR=66
if [ -z "$1" ]
then
echo "Usage: `basename $0` number"
exit $E_WRONG_ARGS
fi
if [ "$1" -gt $MAX_ARG ]
then
echo "Out of range (5 is maximum)."
# Let's get real now.
# If you want greater range than this,
#+ rewrite it in a Real Programming Language.
exit $E_RANGE_ERR
fi
fact ()
{
local number=$1
# Variable "number" must be declared as local,
#+ otherwise this doesn't work.
if [ "$number" -eq 0 ]
then
factorial=1 # Factorial of 0 = 1.
else
let "decrnum = number - 1"
fact $decrnum # Recursive function call (the function calls itself).
let "factorial = $number * $?"
fi
return $factorial
}
fact $1
echo "Factorial of $1 is $?."
exit 0 |
See also Example A-16 for an example of
recursion in a script. Be aware that recursion is
resource-intensive and executes slowly, and is therefore
generally not appropriate to use in a script.
23.3. Recursion Without Local VariablesA function may recursively call itself even without use of
local variables. Example 23-14. The Towers of Hanoi #! /bin/bash
#
# The Towers Of Hanoi
# Bash script
# Copyright (C) 2000 Amit Singh. All Rights Reserved.
# http://hanoi.kernelthread.com
#
# Last tested under bash version 2.05b.0(13)-release
#
# Used in "Advanced Bash Scripting Guide"
#+ with permission of script author.
# Slightly modified and commented by ABS author.
#=================================================================#
# The Tower of Hanoi is a mathematical puzzle attributed to
#+ Edouard Lucas, a nineteenth-century French mathematician.
#
# There are three vertical posts set in a base.
# The first post has a set of annular rings stacked on it.
# These rings are flat disks with a hole drilled out of the center,
#+ so they can slip over the posts.
# The rings have different diameters, and they stack in descending
#+ order, according to size.
# The smallest ring is on top, and the largest on the bottom.
#
# The task is to transfer the stack of rings
#+ to one of the other posts.
# You can move only one ring at a time to another post.
# You are permitted to move rings back to the original post.
# You may place a smaller ring atop a larger one,
#+ but *not* vice versa.
# Again, it is forbidden to place a larger ring atop a smaller one.
#
# For a small number of rings, only a few moves are required.
#+ For each additional ring,
#+ the required number of moves approximately doubles,
#+ and the "strategy" becomes increasingly complicated.
#
# For more information, see http://hanoi.kernelthread.com.
#
#
# ... ... ...
# | | | | | |
# _|_|_ | | | |
# |_____| | | | |
# |_______| | | | |
# |_________| | | | |
# |___________| | | | |
# | | | | | |
# .--------------------------------------------------------------.
# |**************************************************************|
# #1 #2 #3
#
#=================================================================#
E_NOPARAM=66 # No parameter passed to script.
E_BADPARAM=67 # Illegal number of disks passed to script.
Moves= # Global variable holding number of moves.
# Modifications to original script.
dohanoi() { # Recursive function.
case $1 in
0)
;;
*)
dohanoi "$(($1-1))" $2 $4 $3
echo move $2 "-->" $3
let "Moves += 1" # Modification to original script.
dohanoi "$(($1-1))" $4 $3 $2
;;
esac
}
case $# in
1)
case $(($1>0)) in # Must have at least one disk.
1)
dohanoi $1 1 3 2
echo "Total moves = $Moves"
exit 0;
;;
*)
echo "$0: illegal value for number of disks";
exit $E_BADPARAM;
;;
esac
;;
*)
echo "usage: $0 N"
echo " Where \"N\" is the number of disks."
exit $E_NOPARAM;
;;
esac
# Exercises:
# ---------
# 1) Would commands beyond this point ever be executed?
# Why not? (Easy)
# 2) Explain the workings of the workings of the "dohanoi" function.
# (Difficult) |
Chapter 24. Aliases
A Bash alias is essentially nothing more than
a keyboard shortcut, an abbreviation, a means of avoiding
typing a long command sequence. If, for example, we include
alias lm="ls -l | more" in the ~/.bashrc file,
then each lm typed at the command
line will automatically be replaced by a ls -l |
more. This can save a great deal of typing at the
command line and avoid having to remember complex combinations of
commands and options. Setting alias rm="rm -i"
(interactive mode delete) may save a good deal of grief, since
it can prevent inadvertently losing important files. In a script, aliases have very limited usefulness. It would be
quite nice if aliases could assume some of the functionality of
the C preprocessor, such as macro expansion, but unfortunately
Bash does not expand arguments within the alias body.
Moreover, a script fails to expand an alias itself
within "compound constructs", such as if/then statements, loops, and
functions. An added limitation is that an alias will not expand
recursively. Almost invariably, whatever we would like an alias
to do could be accomplished much more effectively with a function. Example 24-1. Aliases within a script #!/bin/bash
# alias.sh
shopt -s expand_aliases
# Must set this option, else script will not expand aliases.
# First, some fun.
alias Jesse_James='echo "\"Alias Jesse James\" was a 1959 comedy starring Bob Hope."'
Jesse_James
echo; echo; echo;
alias ll="ls -l"
# May use either single (') or double (") quotes to define an alias.
echo "Trying aliased \"ll\":"
ll /usr/X11R6/bin/mk* #* Alias works.
echo
directory=/usr/X11R6/bin/
prefix=mk* # See if wild card causes problems.
echo "Variables \"directory\" + \"prefix\" = $directory$prefix"
echo
alias lll="ls -l $directory$prefix"
echo "Trying aliased \"lll\":"
lll # Long listing of all files in /usr/X11R6/bin stating with mk.
# An alias can handle concatenated variables -- including wild card -- o.k.
TRUE=1
echo
if [ TRUE ]
then
alias rr="ls -l"
echo "Trying aliased \"rr\" within if/then statement:"
rr /usr/X11R6/bin/mk* #* Error message results!
# Aliases not expanded within compound statements.
echo "However, previously expanded alias still recognized:"
ll /usr/X11R6/bin/mk*
fi
echo
count=0
while [ $count -lt 3 ]
do
alias rrr="ls -l"
echo "Trying aliased \"rrr\" within \"while\" loop:"
rrr /usr/X11R6/bin/mk* #* Alias will not expand here either.
# alias.sh: line 57: rrr: command not found
let count+=1
done
echo; echo
alias xyz='cat $0' # Script lists itself.
# Note strong quotes.
xyz
# This seems to work,
#+ although the Bash documentation suggests that it shouldn't.
#
# However, as Steve Jacobson points out,
#+ the "$0" parameter expands immediately upon declaration of the alias.
exit 0 |
The unalias command removes a previously
set alias. Example 24-2. unalias: Setting and unsetting an alias #!/bin/bash
# unalias.sh
shopt -s expand_aliases # Enables alias expansion.
alias llm='ls -al | more'
llm
echo
unalias llm # Unset alias.
llm
# Error message results, since 'llm' no longer recognized.
exit 0 |
bash$ ./unalias.sh
total 6
drwxrwxr-x 2 bozo bozo 3072 Feb 6 14:04 .
drwxr-xr-x 40 bozo bozo 2048 Feb 6 14:04 ..
-rwxr-xr-x 1 bozo bozo 199 Feb 6 14:04 unalias.sh
./unalias.sh: llm: command not found |
Chapter 25. List Constructs
The "and list" and "or list"
constructs provide a means of processing a number of commands
consecutively. These can effectively replace complex
nested if/then or even
case statements. Chaining together commands - and list
command-1 && command-2 && command-3 && ... command-n |
Each command executes in turn provided that
the previous command has given a return value of
true (zero). At the first
false (non-zero) return, the
command chain terminates (the first command returning
false is the last one to
execute).Example 25-1. Using an "and list" to test for command-line arguments #!/bin/bash
# "and list"
if [ ! -z "$1" ] && echo "Argument #1 = $1" && [ ! -z "$2" ] && echo "Argument #2 = $2"
then
echo "At least 2 arguments passed to script."
# All the chained commands return true.
else
echo "Less than 2 arguments passed to script."
# At least one of the chained commands returns false.
fi
# Note that "if [ ! -z $1 ]" works, but its supposed equivalent,
# if [ -n $1 ] does not.
# However, quoting fixes this.
# if [ -n "$1" ] works.
# Careful!
# It is always best to QUOTE tested variables.
# This accomplishes the same thing, using "pure" if/then statements.
if [ ! -z "$1" ]
then
echo "Argument #1 = $1"
fi
if [ ! -z "$2" ]
then
echo "Argument #2 = $2"
echo "At least 2 arguments passed to script."
else
echo "Less than 2 arguments passed to script."
fi
# It's longer and less elegant than using an "and list".
exit 0 |
Example 25-2. Another command-line arg test using an "and list" #!/bin/bash
ARGS=1 # Number of arguments expected.
E_BADARGS=65 # Exit value if incorrect number of args passed.
test $# -ne $ARGS && echo "Usage: `basename $0` $ARGS argument(s)" && exit $E_BADARGS
# If condition 1 tests true (wrong number of args passed to script),
#+ then the rest of the line executes, and script terminates.
# Line below executes only if the above test fails.
echo "Correct number of arguments passed to this script."
exit 0
# To check exit value, do a "echo $?" after script termination. |
Of course, an and list can also
set variables to a default value.
arg1=$@ # Set $arg1 to command line arguments, if any.
[ -z "$arg1" ] && arg1=DEFAULT
# Set to DEFAULT if not specified on command line. |
- or list
command-1 || command-2 || command-3 || ... command-n |
Each command executes in turn for as long as the previous
command returns false. At
the first true return, the
command chain terminates (the first command returning
true is the last one to
execute). This is obviously the inverse of the "and
list".Example 25-3. Using "or lists" in combination with an "and list" #!/bin/bash
# delete.sh, not-so-cunning file deletion utility.
# Usage: delete filename
E_BADARGS=65
if [ -z "$1" ]
then
echo "Usage: `basename $0` filename"
exit $E_BADARGS # No arg? Bail out.
else
file=$1 # Set filename.
fi
[ ! -f "$file" ] && echo "File \"$file\" not found. \
Cowardly refusing to delete a nonexistent file."
# AND LIST, to give error message if file not present.
# Note echo message continued on to a second line with an escape.
[ ! -f "$file" ] || (rm -f $file; echo "File \"$file\" deleted.")
# OR LIST, to delete file if present.
# Note logic inversion above.
# AND LIST executes on true, OR LIST on false.
exit 0 |
 | If the first command in an "or list"
returns true, it
will execute. |
# ==> The following snippets from the /etc/rc.d/init.d/single script by Miquel van Smoorenburg
#+==> illustrate use of "and" and "or" lists.
# ==> "Arrowed" comments added by document author.
[ -x /usr/bin/clear ] && /usr/bin/clear
# ==> If /usr/bin/clear exists, then invoke it.
# ==> Checking for the existence of a command before calling it
#+==> avoids error messages and other awkward consequences.
# ==> . . .
# If they want to run something in single user mode, might as well run it...
for i in /etc/rc1.d/S[0-9][0-9]* ; do
# Check if the script is there.
[ -x "$i" ] || continue
# ==> If corresponding file in $PWD *not* found,
#+==> then "continue" by jumping to the top of the loop.
# Reject backup files and files generated by rpm.
case "$1" in
*.rpmsave|*.rpmorig|*.rpmnew|*~|*.orig)
continue;;
esac
[ "$i" = "/etc/rc1.d/S00single" ] && continue
# ==> Set script name, but don't execute it yet.
$i start
done
# ==> . . . |
 | The exit
status of an and list or an
or list is the exit status of the last
command executed. |
Clever combinations of "and" and "or"
lists are possible, but the logic may easily become convoluted and
require extensive debugging.
false && true || echo false # false
# Same result as
( false && true ) || echo false # false
# But *not*
false && ( true || echo false ) # (nothing echoed)
# Note left-to-right grouping and evaluation of statements,
#+ since the logic operators "&&" and "||" have equal precedence.
# It's best to avoid such complexities, unless you know what you're doing.
# Thanks, S.C. |
See Example A-7 and Example 7-4 for illustrations of using an and
/ or list to test variables.
Chapter 26. Arrays
Newer versions of Bash support one-dimensional arrays.
Array elements may be initialized with the
variable[xx] notation. Alternatively,
a script may introduce the entire array by an explicit
declare -a variable statement. To
dereference (find the contents of) an array element, use
curly bracket notation, that is,
${variable[xx]}. Example 26-1. Simple array usage #!/bin/bash
area[11]=23
area[13]=37
area[51]=UFOs
# Array members need not be consecutive or contiguous.
# Some members of the array can be left uninitialized.
# Gaps in the array are okay.
# In fact, arrays with sparse data ("sparse arrays")
#+ are useful in spreadsheet-processing software.
echo -n "area[11] = "
echo ${area[11]} # {curly brackets} needed.
echo -n "area[13] = "
echo ${area[13]}
echo "Contents of area[51] are ${area[51]}."
# Contents of uninitialized array variable print blank (null variable).
echo -n "area[43] = "
echo ${area[43]}
echo "(area[43] unassigned)"
echo
# Sum of two array variables assigned to third
area[5]=`expr ${area[11]} + ${area[13]}`
echo "area[5] = area[11] + area[13]"
echo -n "area[5] = "
echo ${area[5]}
area[6]=`expr ${area[11]} + ${area[51]}`
echo "area[6] = area[11] + area[51]"
echo -n "area[6] = "
echo ${area[6]}
# This fails because adding an integer to a string is not permitted.
echo; echo; echo
# -----------------------------------------------------------------
# Another array, "area2".
# Another way of assigning array variables...
# array_name=( XXX YYY ZZZ ... )
area2=( zero one two three four )
echo -n "area2[0] = "
echo ${area2[0]}
# Aha, zero-based indexing (first element of array is [0], not [1]).
echo -n "area2[1] = "
echo ${area2[1]} # [1] is second element of array.
# -----------------------------------------------------------------
echo; echo; echo
# -----------------------------------------------
# Yet another array, "area3".
# Yet another way of assigning array variables...
# array_name=([xx]=XXX [yy]=YYY ...)
area3=([17]=seventeen [24]=twenty-four)
echo -n "area3[17] = "
echo ${area3[17]}
echo -n "area3[24] = "
echo ${area3[24]}
# -----------------------------------------------
exit 0 |
 | Bash permits array operations on variables, even if
the variables are not explicitly declared as arrays.
string=abcABC123ABCabc
echo ${string[@]} # abcABC123ABCabc
echo ${string[*]} # abcABC123ABCabc
echo ${string[0]} # abcABC123ABCabc
echo ${string[1]} # No output!
# Why?
echo ${#string[@]} # 1
# One element in the array.
# The string itself.
# Thank you, Michael Zick, for pointing this out. |
Once again this demonstrates that Bash
variables are untyped.
|
Example 26-2. Formatting a poem #!/bin/bash
# poem.sh: Pretty-prints one of the document author's favorite poems.
# Lines of the poem (single stanza).
Line[1]="I do not know which to prefer,"
Line[2]="The beauty of inflections"
Line[3]="Or the beauty of innuendoes,"
Line[4]="The blackbird whistling"
Line[5]="Or just after."
# Attribution.
Attrib[1]=" Wallace Stevens"
Attrib[2]="\"Thirteen Ways of Looking at a Blackbird\""
# This poem is in the Public Domain (copyright expired).
echo
for index in 1 2 3 4 5 # Five lines.
do
printf " %s\n" "${Line[index]}"
done
for index in 1 2 # Two attribution lines.
do
printf " %s\n" "${Attrib[index]}"
done
echo
exit 0
# Exercise:
# --------
# Modify this script to pretty-print a poem from a text data file. |
Array variables have a syntax all their own, and even
standard Bash commands and operators have special options adapted
for array use. Example 26-3. Various array operations #!/bin/bash
# array-ops.sh: More fun with arrays.
array=( zero one two three four five )
# Element 0 1 2 3 4 5
echo ${array[0]} # zero
echo ${array:0} # zero
# Parameter expansion of first element,
#+ starting at position # 0 (1st character).
echo ${array:1} # ero
# Parameter expansion of first element,
#+ starting at position # 1 (2nd character).
echo "--------------"
echo ${#array[0]} # 4
# Length of first element of array.
echo ${#array} # 4
# Length of first element of array.
# (Alternate notation)
echo ${#array[1]} # 3
# Length of second element of array.
# Arrays in Bash have zero-based indexing.
echo ${#array[*]} # 6
# Number of elements in array.
echo ${#array[@]} # 6
# Number of elements in array.
echo "--------------"
array2=( [0]="first element" [1]="second element" [3]="fourth element" )
echo ${array2[0]} # first element
echo ${array2[1]} # second element
echo ${array2[2]} #
# Skipped in initialization, and therefore null.
echo ${array2[3]} # fourth element
exit 0 |
Many of the standard string
operations work on arrays. Example 26-4. String operations on arrays #!/bin/bash
# array-strops.sh: String operations on arrays.
# Script by Michael Zick.
# Used with permission.
# In general, any string operation in the ${name ... } notation
#+ can be applied to all string elements in an array
#+ with the ${name[@] ... } or ${name[*] ...} notation.
arrayZ=( one two three four five five )
echo
# Trailing Substring Extraction
echo ${arrayZ[@]:0} # one two three four five five
# All elements.
echo ${arrayZ[@]:1} # two three four five five
# All elements following element[0].
echo ${arrayZ[@]:1:2} # two three
# Only the two elements after element[0].
echo "-----------------------"
# Substring Removal
# Removes shortest match from front of string(s),
#+ where the substring is a regular expression.
echo ${arrayZ[@]#f*r} # one two three five five
# Applied to all elements of the array.
# Matches "four" and removes it.
# Longest match from front of string(s)
echo ${arrayZ[@]##t*e} # one two four five five
# Applied to all elements of the array.
# Matches "three" and removes it.
# Shortest match from back of string(s)
echo ${arrayZ[@]%h*e} # one two t four five five
# Applied to all elements of the array.
# Matches "hree" and removes it.
# Longest match from back of string(s)
echo ${arrayZ[@]%%t*e} # one two four five five
# Applied to all elements of the array.
# Matches "three" and removes it.
echo "-----------------------"
# Substring Replacement
# Replace first occurance of substring with replacement
echo ${arrayZ[@]/fiv/XYZ} # one two three four XYZe XYZe
# Applied to all elements of the array.
# Replace all occurances of substring
echo ${arrayZ[@]//iv/YY} # one two three four fYYe fYYe
# Applied to all elements of the array.
# Delete all occurances of substring
# Not specifing a replacement means 'delete'
echo ${arrayZ[@]//fi/} # one two three four ve ve
# Applied to all elements of the array.
# Replace front-end occurances of substring
echo ${arrayZ[@]/#fi/XY} # one two three four XYve XYve
# Applied to all elements of the array.
# Replace back-end occurances of substring
echo ${arrayZ[@]/%ve/ZZ} # one two three four fiZZ fiZZ
# Applied to all elements of the array.
echo ${arrayZ[@]/%o/XX} # one twXX three four five five
# Why?
echo "-----------------------"
# Before reaching for awk (or anything else) --
# Recall:
# $( ... ) is command substitution.
# Functions run as a sub-process.
# Functions write their output to stdout.
# Assignment reads the function's stdout.
# The name[@] notation specifies a "for-each" operation.
newstr() {
echo -n "!!!"
}
echo ${arrayZ[@]/%e/$(newstr)}
# on!!! two thre!!! four fiv!!! fiv!!!
# Q.E.D: The replacement action is an 'assignment.'
# Accessing the "For-Each"
echo ${arrayZ[@]//*/$(newstr optional_arguments)}
# Now, if Bash would just pass the matched string as $0
#+ to the function being called . . .
echo
exit 0 |
Command substitution can
construct the individual elements of an array. Example 26-5. Loading the contents of a script into an array #!/bin/bash
# script-array.sh: Loads this script into an array.
# Inspired by an e-mail from Chris Martin (thanks!).
script_contents=( $(cat "$0") ) # Stores contents of this script ($0)
#+ in an array.
for element in $(seq 0 $((${#script_contents[@]} - 1)))
do # ${#script_contents[@]}
#+ gives number of elements in the array.
#
# Question:
# Why is seq 0 necessary?
# Try changing it to seq 1.
echo -n "${script_contents[$element]}"
# List each field of this script on a single line.
echo -n " -- " # Use " -- " as a field separator.
done
echo
exit 0
# Exercise:
# --------
# Modify this script so it lists itself
#+ in its original format,
#+ complete with whitespace, line breaks, etc. |
In an array context, some Bash builtins have a slightly altered
meaning. For example, unset
deletes array elements, or even an entire array. Example 26-6. Some special properties of arrays #!/bin/bash
declare -a colors
# All subsequent commands in this script will treat
#+ the variable "colors" as an array.
echo "Enter your favorite colors (separated from each other by a space)."
read -a colors # Enter at least 3 colors to demonstrate features below.
# Special option to 'read' command,
#+ allowing assignment of elements in an array.
echo
element_count=${#colors[@]}
# Special syntax to extract number of elements in array.
# element_count=${#colors[*]} works also.
#
# The "@" variable allows word splitting within quotes
#+ (extracts variables separated by whitespace).
#
# This corresponds to the behavior of "$@" and "$*"
#+ in positional parameters.
index=0
while [ "$index" -lt "$element_count" ]
do # List all the elements in the array.
echo ${colors[$index]}
let "index = $index + 1"
done
# Each array element listed on a separate line.
# If this is not desired, use echo -n "${colors[$index]} "
#
# Doing it with a "for" loop instead:
# for i in "${colors[@]}"
# do
# echo "$i"
# done
# (Thanks, S.C.)
echo
# Again, list all the elements in the array, but using a more elegant method.
echo ${colors[@]} # echo ${colors[*]} also works.
echo
# The "unset" command deletes elements of an array, or entire array.
unset colors[1] # Remove 2nd element of array.
# Same effect as colors[1]=
echo ${colors[@]} # List array again, missing 2nd element.
unset colors # Delete entire array.
# unset colors[*] and
#+ unset colors[@] also work.
echo; echo -n "Colors gone."
echo ${colors[@]} # List array again, now empty.
exit 0 |
As seen in the previous example, either
${array_name[@]} or
${array_name[*]} refers to
all the elements of the array.
Similarly, to get a count of the number of elements in an
array, use either ${#array_name[@]}
or ${#array_name[*]}.
${#array_name} is the length (number of
characters) of ${array_name[0]}, the first
element of the array. Example 26-7. Of empty arrays and empty elements #!/bin/bash
# empty-array.sh
# Thanks to Stephane Chazelas for the original example,
#+ and to Michael Zick for extending it.
# An empty array is not the same as an array with empty elements.
array0=( first second third )
array1=( '' ) # "array1" consists of one empty element.
array2=( ) # No elements . . . "array2" is empty.
echo
ListArray()
{
echo
echo "Elements in array0: ${array0[@]}"
echo "Elements in array1: ${array1[@]}"
echo "Elements in array2: ${array2[@]}"
echo
echo "Length of first element in array0 = ${#array0}"
echo "Length of first element in array1 = ${#array1}"
echo "Length of first element in array2 = ${#array2}"
echo
echo "Number of elements in array0 = ${#array0[*]}" # 3
echo "Number of elements in array1 = ${#array1[*]}" # 1 (Surprise!)
echo "Number of elements in array2 = ${#array2[*]}" # 0
}
# ===================================================================
ListArray
# Try extending those arrays.
# Adding an element to an array.
array0=( "${array0[@]}" "new1" )
array1=( "${array1[@]}" "new1" )
array2=( "${array2[@]}" "new1" )
ListArray
# or
array0[${#array0[*]}]="new2"
array1[${#array1[*]}]="new2"
array2[${#array2[*]}]="new2"
ListArray
# When extended as above; arrays are 'stacks'
# The above is the 'push'
# The stack 'height' is:
height=${#array2[@]}
echo
echo "Stack height for array2 = $height"
# The 'pop' is:
unset array2[${#array2[@]}-1] # Arrays are zero-based,
height=${#array2[@]} #+ which means first element has index 0.
echo
echo "POP"
echo "New stack height for array2 = $height"
ListArray
# List only 2nd and 3rd elements of array0.
from=1 # Zero-based numbering.
to=2 #
array3=( ${array0[@]:1:2} )
echo
echo "Elements in array3: ${array3[@]}"
# Works like a string (array of characters).
# Try some other "string" forms.
# Replacement:
array4=( ${array0[@]/second/2nd} )
echo
echo "Elements in array4: ${array4[@]}"
# Replace all matching wildcarded string.
array5=( ${array0[@]//new?/old} )
echo
echo "Elements in array5: ${array5[@]}"
# Just when you are getting the feel for this . . .
array6=( ${array0[@]#*new} )
echo # This one might surprise you.
echo "Elements in array6: ${array6[@]}"
array7=( ${array0[@]#new1} )
echo # After array6 this should not be a surprise.
echo "Elements in array7: ${array7[@]}"
# Which looks a lot like . . .
array8=( ${array0[@]/new1/} )
echo
echo "Elements in array8: ${array8[@]}"
# So what can one say about this?
# The string operations are performed on
#+ each of the elements in var[@] in succession.
# Therefore : Bash supports string vector operations
#+ if the result is a zero length string,
#+ that element disappears in the resulting assignment.
# Question, are those strings hard or soft quotes?
zap='new*'
array9=( ${array0[@]/$zap/} )
echo
echo "Elements in array9: ${array9[@]}"
# Just when you thought you where still in Kansas . . .
array10=( ${array0[@]#$zap} )
echo
echo "Elements in array10: ${array10[@]}"
# Compare array7 with array10.
# Compare array8 with array9.
# Answer: must be soft quotes.
exit 0 |
The relationship of ${array_name[@]}
and ${array_name[*]} is analogous to that
between $@ and $*. This powerful
array notation has a number of uses. # Copying an array.
array2=( "${array1[@]}" )
# or
array2="${array1[@]}"
# Adding an element to an array.
array=( "${array[@]}" "new element" )
# or
array[${#array[*]}]="new element"
# Thanks, S.C. |
 | The array=( element1 element2 ... elementN )
initialization operation, with the help of command substitution, makes it
possible to load the contents of a text file into an array.
#!/bin/bash
filename=sample_file
# cat sample_file
#
# 1 a b c
# 2 d e fg
declare -a array1
array1=( `cat "$filename"`) # Loads contents
# List file to stdout #+ of $filename into array1.
#
# array1=( `cat "$filename" | tr '\n' ' '`)
# change linefeeds in file to spaces.
# Not necessary because Bash does word splitting,
#+ changing linefeeds to spaces.
echo ${array1[@]} # List the array.
# 1 a b c 2 d e fg
#
# Each whitespace-separated "word" in the file
#+ has been assigned to an element of the array.
element_count=${#array1[*]}
echo $element_count # 8 |
|
Clever scripting makes it possible to add array operations. Example 26-8. Initializing arrays #! /bin/bash
# array-assign.bash
# Array operations are Bash specific,
#+ hence the ".bash" in the script name.
# Copyright (c) Michael S. Zick, 2003, All rights reserved.
# License: Unrestricted reuse in any form, for any purpose.
# Version: $ID$
#
# Clarification and additional comments by William Park.
# Based on an example provided by Stephane Chazelas
#+ which appeared in the book: Advanced Bash Scripting Guide.
# Output format of the 'times' command:
# User CPU <space> System CPU
# User CPU of dead children <space> System CPU of dead children
# Bash has two versions of assigning all elements of an array
#+ to a new array variable.
# Both drop 'null reference' elements
#+ in Bash versions 2.04, 2.05a and 2.05b.
# An additional array assignment that maintains the relationship of
#+ [subscript]=value for arrays may be added to newer versions.
# Constructs a large array using an internal command,
#+ but anything creating an array of several thousand elements
#+ will do just fine.
declare -a bigOne=( /dev/* )
echo
echo 'Conditions: Unquoted, default IFS, All-Elements-Of'
echo "Number of elements in array is ${#bigOne[@]}"
# set -vx
echo
echo '- - testing: =( ${array[@]} ) - -'
times
declare -a bigTwo=( ${bigOne[@]} )
# ^ ^
times
echo
echo '- - testing: =${array[@]} - -'
times
declare -a bigThree=${bigOne[@]}
# No parentheses this time.
times
# Comparing the numbers shows that the second form, pointed out
#+ by Stephane Chazelas, is from three to four times faster.
#
# William Park explains:
#+ The bigTwo array assigned as single string, whereas
#+ bigThree assigned element by element.
# So, in essence, you have:
# bigTwo=( [0]="... ... ..." )
# bigThree=( [0]="..." [1]="..." [2]="..." ... )
# I will continue to use the first form in my example descriptions
#+ because I think it is a better illustration of what is happening.
# The reusable portions of my examples will actual contain
#+ the second form where appropriate because of the speedup.
# MSZ: Sorry about that earlier oversight folks.
# Note:
# ----
# The "declare -a" statements in lines 31 and 43
#+ are not strictly necessary, since it is implicit
#+ in the Array=( ... ) assignment form.
# However, eliminating these declarations slows down
#+ the execution of the following sections of the script.
# Try it, and see what happens.
exit 0 |
 | Adding a superfluous declare -a
statement to an array declaration may speed up execution of
subsequent operations on the array. |
Example 26-9. Copying and concatenating arrays #! /bin/bash
# CopyArray.sh
#
# This script written by Michael Zick.
# Used here with permission.
# How-To "Pass by Name & Return by Name"
#+ or "Building your own assignment statement".
CpArray_Mac() {
# Assignment Command Statement Builder
echo -n 'eval '
echo -n "$2" # Destination name
echo -n '=( ${'
echo -n "$1" # Source name
echo -n '[@]} )'
# That could all be a single command.
# Matter of style only.
}
declare -f CopyArray # Function "Pointer"
CopyArray=CpArray_Mac # Statement Builder
Hype()
{
# Hype the array named $1.
# (Splice it together with array containing "Really Rocks".)
# Return in array named $2.
local -a TMP
local -a hype=( Really Rocks )
$($CopyArray $1 TMP)
TMP=( ${TMP[@]} ${hype[@]} )
$($CopyArray TMP $2)
}
declare -a before=( Advanced Bash Scripting )
declare -a after
echo "Array Before = ${before[@]}"
Hype before after
echo "Array After = ${after[@]}"
# Too much hype?
echo "What ${after[@]:3:2}?"
declare -a modest=( ${after[@]:2:1} ${after[@]:3:2} )
# ---- substring extraction ----
echo "Array Modest = ${modest[@]}"
# What happened to 'before' ?
echo "Array Before = ${before[@]}"
exit 0 |
Example 26-10. More on concatenating arrays #! /bin/bash
# array-append.bash
# Copyright (c) Michael S. Zick, 2003, All rights reserved.
# License: Unrestricted reuse in any form, for any purpose.
# Version: $ID$
#
# Slightly modified in formatting by M.C.
# Array operations are Bash-specific.
# Legacy UNIX /bin/sh lacks equivalents.
# Pipe the output of this script to 'more'
#+ so it doesn't scroll off the terminal.
# Subscript packed.
declare -a array1=( zero1 one1 two1 )
# Subscript sparse ([1] is not defined).
declare -a array2=( [0]=zero2 [2]=two2 [3]=three2 )
echo
echo '- Confirm that the array is really subscript sparse. -'
echo "Number of elements: 4" # Hard-coded for illustration.
for (( i = 0 ; i < 4 ; i++ ))
do
echo "Element [$i]: ${array2[$i]}"
done
# See also the more general code example in basics-reviewed.bash.
declare -a dest
# Combine (append) two arrays into a third array.
echo
echo 'Conditions: Unquoted, default IFS, All-Elements-Of operator'
echo '- Undefined elements not present, subscripts not maintained. -'
# # The undefined elements do not exist; they are not being dropped.
dest=( ${array1[@]} ${array2[@]} )
# dest=${array1[@]}${array2[@]} # Strange results, possibly a bug.
# Now, list the result.
echo
echo '- - Testing Array Append - -'
cnt=${#dest[@]}
echo "Number of elements: $cnt"
for (( i = 0 ; i < cnt ; i++ ))
do
echo "Element [$i]: ${dest[$i]}"
done
# Assign an array to a single array element (twice).
dest[0]=${array1[@]}
dest[1]=${array2[@]}
# List the result.
echo
echo '- - Testing modified array - -'
cnt=${#dest[@]}
echo "Number of elements: $cnt"
for (( i = 0 ; i < cnt ; i++ ))
do
echo "Element [$i]: ${dest[$i]}"
done
# Examine the modified second element.
echo
echo '- - Reassign and list second element - -'
declare -a subArray=${dest[1]}
cnt=${#subArray[@]}
echo "Number of elements: $cnt"
for (( i = 0 ; i < cnt ; i++ ))
do
echo "Element [$i]: ${subArray[$i]}"
done
# The assignment of an entire array to a single element
#+ of another array using the '=${ ... }' array assignment
#+ has converted the array being assigned into a string,
#+ with the elements separated by a space (the first character of IFS).
# If the original elements didn't contain whitespace . . .
# If the original array isn't subscript sparse . . .
# Then we could get the original array structure back again.
# Restore from the modified second element.
echo
echo '- - Listing restored element - -'
declare -a subArray=( ${dest[1]} )
cnt=${#subArray[@]}
echo "Number of elements: $cnt"
for (( i = 0 ; i < cnt ; i++ ))
do
echo "Element [$i]: ${subArray[$i]}"
done
echo '- - Do not depend on this behavior. - -'
echo '- - This behavior is subject to change - -'
echo '- - in versions of Bash newer than version 2.05b - -'
# MSZ: Sorry about any earlier confusion folks.
exit 0 |
-- Arrays permit deploying old familiar algorithms as shell scripts.
Whether this is necessarily a good idea is left to the reader to
decide. Example 26-11. An old friend:
The Bubble Sort #!/bin/bash
# bubble.sh: Bubble sort, of sorts.
# Recall the algorithm for a bubble sort. In this particular version...
# With each successive pass through the array to be sorted,
#+ compare two adjacent elements, and swap them if out of order.
# At the end of the first pass, the "heaviest" element has sunk to bottom.
# At the end of the second pass, the next "heaviest" one has sunk next to bottom.
# And so forth.
# This means that each successive pass needs to traverse less of the array.
# You will therefore notice a speeding up in the printing of the later passes.
exchange()
{
# Swaps two members of the array.
local temp=${Countries[$1]} # Temporary storage
#+ for element getting swapped out.
Countries[$1]=${Countries[$2]}
Countries[$2]=$temp
return
}
declare -a Countries # Declare array,
#+ optional here since it's initialized below.
# Is it permissable to split an array variable over multiple lines
#+ using an escape (\)?
# Yes.
Countries=(Netherlands Ukraine Zaire Turkey Russia Yemen Syria \
Brazil Argentina Nicaragua Japan Mexico Venezuela Greece England \
Israel Peru Canada Oman Denmark Wales France Kenya \
Xanadu Qatar Liechtenstein Hungary)
# "Xanadu" is the mythical place where, according to Coleridge,
#+ Kubla Khan did a pleasure dome decree.
clear # Clear the screen to start with.
echo "0: ${Countries[*]}" # List entire array at pass 0.
number_of_elements=${#Countries[@]}
let "comparisons = $number_of_elements - 1"
count=1 # Pass number.
while [ "$comparisons" -gt 0 ] # Beginning of outer loop
do
index=0 # Reset index to start of array after each pass.
while [ "$index" -lt "$comparisons" ] # Beginning of inner loop
do
if [ ${Countries[$index]} \> ${Countries[`expr $index + 1`]} ]
# If out of order...
# Recalling that \> is ASCII comparison operator
#+ within single brackets.
# if [[ ${Countries[$index]} > ${Countries[`expr $index + 1`]} ]]
#+ also works.
then
exchange $index `expr $index + 1` # Swap.
fi
let "index += 1"
done # End of inner loop
# ----------------------------------------------------------------------
# Paulo Marcel Coelho Aragao suggests for-loops as a simpler altenative.
#
# for (( last = $number_of_elements - 1 ; last > 1 ; last-- ))
# do
# for (( i = 0 ; i < last ; i++ ))
# do
# [[ "${Countries[$i]}" > "${Countries[$((i+1))]}" ]] \
# && exchange $i $((i+1))
# done
# done
# ----------------------------------------------------------------------
let "comparisons -= 1" # Since "heaviest" element bubbles to bottom,
#+ we need do one less comparison each pass.
echo
echo "$count: ${Countries[@]}" # Print resultant array at end of each pass.
echo
let "count += 1" # Increment pass count.
done # End of outer loop
# All done.
exit 0 |
-- Is it possible to nest arrays within arrays? #!/bin/bash
# "Nested" array.
# Michael Zick provided this example,
#+ with corrections and clarifications by William Park.
AnArray=( $(ls --inode --ignore-backups --almost-all \
--directory --full-time --color=none --time=status \
--sort=time -l ${PWD} ) ) # Commands and options.
# Spaces are significant . . . and don't quote anything in the above.
SubArray=( ${AnArray[@]:11:1} ${AnArray[@]:6:5} )
# This array has six elements:
#+ SubArray=( [0]=${AnArray[11]} [1]=${AnArray[6]} [2]=${AnArray[7]}
# [3]=${AnArray[8]} [4]=${AnArray[9]} [5]=${AnArray[10]} )
#
# Arrays in Bash are (circularly) linked lists
#+ of type string (char *).
# So, this isn't actually a nested array,
#+ but it's functionally similar.
echo "Current directory and date of last status change:"
echo "${SubArray[@]}"
exit 0 |
-- Embedded arrays in combination with indirect references create some fascinating
possibilities Example 26-12. Embedded arrays and indirect references #!/bin/bash
# embedded-arrays.sh
# Embedded arrays and indirect references.
# This script by Dennis Leeuw.
# Used with permission.
# Modified by document author.
ARRAY1=(
VAR1_1=value11
VAR1_2=value12
VAR1_3=value13
)
ARRAY2=(
VARIABLE="test"
STRING="VAR1=value1 VAR2=value2 VAR3=value3"
ARRAY21=${ARRAY1[*]}
) # Embed ARRAY1 within this second array.
function print () {
OLD_IFS="$IFS"
IFS=$'\n' # To print each array element
#+ on a separate line.
TEST1="ARRAY2[*]"
local ${!TEST1} # See what happens if you delete this line.
# Indirect reference.
# This makes the components of $TEST1
#+ accessible to this function.
# Let's see what we've got so far.
echo
echo "\$TEST1 = $TEST1" # Just the name of the variable.
echo; echo
echo "{\$TEST1} = ${!TEST1}" # Contents of the variable.
# That's what an indirect
#+ reference does.
echo
echo "-------------------------------------------"; echo
echo
# Print variable
echo "Variable VARIABLE: $VARIABLE"
# Print a string element
IFS="$OLD_IFS"
TEST2="STRING[*]"
local ${!TEST2} # Indirect reference (as above).
echo "String element VAR2: $VAR2 from STRING"
# Print an array element
TEST2="ARRAY21[*]"
local ${!TEST2} # Indirect reference (as above).
echo "Array element VAR1_1: $VAR1_1 from ARRAY21"
}
print
echo
exit 0
# As the author of the script notes,
#+ "you can easily expand it to create named-hashes in bash."
# (Difficult) exercise for the reader: implement this. |
-- Arrays enable implementing a shell script version of the Sieve of
Eratosthenes. Of course, a resource-intensive application of this
nature should really be written in a compiled language, such as C. It
runs excruciatingly slowly as a script. Example 26-13. Complex array application:
Sieve of Eratosthenes #!/bin/bash
# sieve.sh (ex68.sh)
# Sieve of Eratosthenes
# Ancient algorithm for finding prime numbers.
# This runs a couple of orders of magnitude slower
#+ than the equivalent program written in C.
LOWER_LIMIT=1 # Starting with 1.
UPPER_LIMIT=1000 # Up to 1000.
# (You may set this higher . . . if you have time on your hands.)
PRIME=1
NON_PRIME=0
let SPLIT=UPPER_LIMIT/2
# Optimization:
# Need to test numbers only halfway to upper limit (why?).
declare -a Primes
# Primes[] is an array.
initialize ()
{
# Initialize the array.
i=$LOWER_LIMIT
until [ "$i" -gt "$UPPER_LIMIT" ]
do
Primes[i]=$PRIME
let "i += 1"
done
# Assume all array members guilty (prime)
#+ until proven innocent.
}
print_primes ()
{
# Print out the members of the Primes[] array tagged as prime.
i=$LOWER_LIMIT
until [ "$i" -gt "$UPPER_LIMIT" ]
do
if [ "${Primes[i]}" -eq "$PRIME" ]
then
printf "%8d" $i
# 8 spaces per number gives nice, even columns.
fi
let "i += 1"
done
}
sift () # Sift out the non-primes.
{
let i=$LOWER_LIMIT+1
# We know 1 is prime, so let's start with 2.
until [ "$i" -gt "$UPPER_LIMIT" ]
do
if [ "${Primes[i]}" -eq "$PRIME" ]
# Don't bother sieving numbers already sieved (tagged as non-prime).
then
t=$i
while [ "$t" -le "$UPPER_LIMIT" ]
do
let "t += $i "
Primes[t]=$NON_PRIME
# Tag as non-prime all multiples.
done
fi
let "i += 1"
done
}
# ==============================================
# main ()
# Invoke the functions sequentially.
initialize
sift
print_primes
# This is what they call structured programming.
# ==============================================
echo
exit 0
# -------------------------------------------------------- #
# Code below line will not execute, because of 'exit.'
# This improved version of the Sieve, by Stephane Chazelas,
#+ executes somewhat faster.
# Must invoke with command-line argument (limit of primes).
UPPER_LIMIT=$1 # From command line.
let SPLIT=UPPER_LIMIT/2 # Halfway to max number.
Primes=( '' $(seq $UPPER_LIMIT) )
i=1
until (( ( i += 1 ) > SPLIT )) # Need check only halfway.
do
if [[ -n $Primes[i] ]]
then
t=$i
until (( ( t += i ) > UPPER_LIMIT ))
do
Primes[t]=
done
fi
done
echo ${Primes[*]}
exit 0 |
Compare this array-based prime number generator with an
alternative that does not use arrays, Example A-16. -- Arrays lend themselves, to some extent, to emulating data
structures for which Bash has no native support. Example 26-14. Emulating a push-down stack #!/bin/bash
# stack.sh: push-down stack simulation
# Similar to the CPU stack, a push-down stack stores data items
#+ sequentially, but releases them in reverse order, last-in first-out.
BP=100 # Base Pointer of stack array.
# Begin at element 100.
SP=$BP # Stack Pointer.
# Initialize it to "base" (bottom) of stack.
Data= # Contents of stack location.
# Must use global variable,
#+ because of limitation on function return range.
declare -a stack
push() # Push item on stack.
{
if [ -z "$1" ] # Nothing to push?
then
return
fi
let "SP -= 1" # Bump stack pointer.
stack[$SP]=$1
return
}
pop() # Pop item off stack.
{
Data= # Empty out data item.
if [ "$SP" -eq "$BP" ] # Stack empty?
then
return
fi # This also keeps SP from getting past 100,
#+ i.e., prevents a runaway stack.
Data=${stack[$SP]}
let "SP += 1" # Bump stack pointer.
return
}
status_report() # Find out what's happening.
{
echo "-------------------------------------"
echo "REPORT"
echo "Stack Pointer = $SP"
echo "Just popped \""$Data"\" off the stack."
echo "-------------------------------------"
echo
}
# =======================================================
# Now, for some fun.
echo
# See if you can pop anything off empty stack.
pop
status_report
echo
push garbage
pop
status_report # Garbage in, garbage out.
value1=23; push $value1
value2=skidoo; push $value2
value3=FINAL; push $value3
pop # FINAL
status_report
pop # skidoo
status_report
pop # 23
status_report # Last-in, first-out!
# Notice how the stack pointer decrements with each push,
#+ and increments with each pop.
echo
exit 0
# =======================================================
# Exercises:
# ---------
# 1) Modify the "push()" function to permit pushing
# + multiple element on the stack with a single function call.
# 2) Modify the "pop()" function to permit popping
# + multiple element from the stack with a single function call.
# 3) Add error checking to the critical functions.
# That is, return an error code, depending on
# + successful or unsuccessful completion of the operation,
# + and take appropriate action.
# 4) Using this script as a starting point,
# + write a stack-based 4-function calculator. |
-- Fancy manipulation of array "subscripts" may require
intermediate variables. For projects involving this, again consider
using a more powerful programming language, such as Perl or C. Example 26-15. Complex array application:
Exploring a weird mathematical series #!/bin/bash
# Douglas Hofstadter's notorious "Q-series":
# Q(1) = Q(2) = 1
# Q(n) = Q(n - Q(n-1)) + Q(n - Q(n-2)), for n>2
# This is a "chaotic" integer series with strange and unpredictable behavior.
# The first 20 terms of the series are:
# 1 1 2 3 3 4 5 5 6 6 6 8 8 8 10 9 10 11 11 12
# See Hofstadter's book, "Goedel, Escher, Bach: An Eternal Golden Braid",
#+ p. 137, ff.
LIMIT=100 # Number of terms to calculate.
LINEWIDTH=20 # Number of terms printed per line.
Q[1]=1 # First two terms of series are 1.
Q[2]=1
echo
echo "Q-series [$LIMIT terms]:"
echo -n "${Q[1]} " # Output first two terms.
echo -n "${Q[2]} "
for ((n=3; n <= $LIMIT; n++)) # C-like loop conditions.
do # Q[n] = Q[n - Q[n-1]] + Q[n - Q[n-2]] for n>2
# Need to break the expression into intermediate terms,
#+ since Bash doesn't handle complex array arithmetic very well.
let "n1 = $n - 1" # n-1
let "n2 = $n - 2" # n-2
t0=`expr $n - ${Q[n1]}` # n - Q[n-1]
t1=`expr $n - ${Q[n2]}` # n - Q[n-2]
T0=${Q[t0]} # Q[n - Q[n-1]]
T1=${Q[t1]} # Q[n - Q[n-2]]
Q[n]=`expr $T0 + $T1` # Q[n - Q[n-1]] + Q[n - Q[n-2]]
echo -n "${Q[n]} "
if [ `expr $n % $LINEWIDTH` -eq 0 ] # Format output.
then # ^ Modula operator
echo # Break lines into neat chunks.
fi
done
echo
exit 0
# This is an iterative implementation of the Q-series.
# The more intuitive recursive implementation is left as an exercise.
# Warning: calculating this series recursively takes a VERY long time. |
-- Bash supports only one-dimensional arrays, though a little
trickery permits simulating multi-dimensional ones. Example 26-16. Simulating a two-dimensional array, then tilting it #!/bin/bash
# twodim.sh: Simulating a two-dimensional array.
# A one-dimensional array consists of a single row.
# A two-dimensional array stores rows sequentially.
Rows=5
Columns=5
# 5 X 5 Array.
declare -a alpha # char alpha [Rows] [Columns];
# Unnecessary declaration. Why?
load_alpha ()
{
local rc=0
local index
for i in A B C D E F G H I J K L M N O P Q R S T U V W X Y
do # Use different symbols if you like.
local row=`expr $rc / $Columns`
local column=`expr $rc % $Rows`
let "index = $row * $Rows + $column"
alpha[$index]=$i
# alpha[$row][$column]
let "rc += 1"
done
# Simpler would be
#+ declare -a alpha=( A B C D E F G H I J K L M N O P Q R S T U V W X Y )
#+ but this somehow lacks the "flavor" of a two-dimensional array.
}
print_alpha ()
{
local row=0
local index
echo
while [ "$row" -lt "$Rows" ] # Print out in "row major" order:
do #+ columns vary,
#+ while row (outer loop) remains the same.
local column=0
echo -n " " # Lines up "square" array with rotated one.
while [ "$column" -lt "$Columns" ]
do
let "index = $row * $Rows + $column"
echo -n "${alpha[index]} " # alpha[$row][$column]
let "column += 1"
done
let "row += 1"
echo
done
# The simpler equivalent is
# echo ${alpha[*]} | xargs -n $Columns
echo
}
filter () # Filter out negative array indices.
{
echo -n " " # Provides the tilt.
# Explain how.
if [[ "$1" -ge 0 && "$1" -lt "$Rows" && "$2" -ge 0 && "$2" -lt "$Columns" ]]
then
let "index = $1 * $Rows + $2"
# Now, print it rotated.
echo -n " ${alpha[index]}"
# alpha[$row][$column]
fi
}
rotate () # Rotate the array 45 degrees --
{ #+ "balance" it on its lower lefthand corner.
local row
local column
for (( row = Rows; row > -Rows; row-- ))
do # Step through the array backwards. Why?
for (( column = 0; column < Columns; column++ ))
do
if [ "$row" -ge 0 ]
then
let "t1 = $column - $row"
let "t2 = $column"
else
let "t1 = $column"
let "t2 = $column + $row"
fi
filter $t1 $t2 # Filter out negative array indices.
# What happens if you don't do this?
done
echo; echo
done
# Array rotation inspired by examples (pp. 143-146) in
#+ "Advanced C Programming on the IBM PC," by Herbert Mayer
#+ (see bibliography).
# This just goes to show that much of what can be done in C
#+ can also be done in shell scripting.
}
#--------------- Now, let the show begin. ------------#
load_alpha # Load the array.
print_alpha # Print it out.
rotate # Rotate it 45 degrees counterclockwise.
#-----------------------------------------------------#
exit 0
# This is a rather contrived, not to mention inelegant simulation.
# Exercises:
# ---------
# 1) Rewrite the array loading and printing functions
# in a more intuitive and less kludgy fashion.
#
# 2) Figure out how the array rotation functions work.
# Hint: think about the implications of backwards-indexing an array.
#
# 3) Rewrite this script to handle a non-square array,
# such as a 6 X 4 one.
# Try to minimize "distortion" when the array is rotated. |
A two-dimensional array is essentially equivalent to a
one-dimensional one, but with additional addressing modes
for referencing and manipulating the individual elements
by row and column
position. For an even more elaborate example of simulating a
two-dimensional array, see Example A-10. -- For yet another interesting script using arrays, see:
Chapter 27. /dev and /proc
A Linux or UNIX machine typically has the
/dev and
/proc special-purpose
directories.
27.1. /devThe /dev directory
contains entries for the physical devices
that may or may not be present in the hardware.
The hard drive partitions containing the mounted filesystem(s)
have entries in /dev,
as a simple df shows.
bash$ df
Filesystem 1k-blocks Used Available Use%
Mounted on
/dev/hda6 495876 222748 247527 48% /
/dev/hda1 50755 3887 44248 9% /boot
/dev/hda8 367013 13262 334803 4% /home
/dev/hda5 1714416 1123624 503704 70% /usr
|
Among other things, the /dev directory also
contains loopback devices, such as
/dev/loop0. A loopback device is a gimmick
that allows an ordinary file to be accessed as if it were a
block device.
This enables mounting an entire filesystem within a
single large file. See Example 13-8 and Example 13-7. A few of the pseudo-devices in /dev
have other specialized uses, such as /dev/null, /dev/zero, /dev/urandom,
/dev/sda1, /dev/udp,
and /dev/tcp. For instance: To mount a USB flash drive,
append the following line to /etc/fstab.
/dev/sda1 /mnt/flashdrive auto noauto,user,noatime 0 0 |
(See also Example A-23.)
When executing a command on a
/dev/tcp/$host/$port pseudo-device file, Bash
opens a TCP connection to the associated socket.
Getting the time from nist.gov: bash$ cat </dev/tcp/time.nist.gov/13
53082 04-03-18 04:26:54 68 0 0 502.3 UTC(NIST) *
|
[Mark contributed the above example.] Downloading a URL: bash$ exec 5<>/dev/tcp/www.net.cn/80
bash$ echo -e "GET / HTTP/1.0\n" >&5
bash$ cat <&5
|
[Thanks, Mark and Mihai Maties.] Example 27-1. Using /dev/tcp for troubleshooting #!/bin/bash
# dev-tcp.sh: /dev/tcp redirection to check Internet connection.
# Script by Troy Engel.
# Used with permission.
TCP_HOST=www.dns-diy.com # A known spam-friendly ISP.
TCP_PORT=80 # Port 80 is http.
# Try to connect. (Somewhat similar to a 'ping' . . .)
echo "HEAD / HTTP/1.0" >/dev/tcp/${TCP_HOST}/${TCP_PORT}
MYEXIT=$?
: <<EXPLANATION
If bash was compiled with --enable-net-redirections, it has the capability of
using a special character device for both TCP and UDP redirections. These
redirections are used identically as STDIN/STDOUT/STDERR. The device entries
are 30,36 for /dev/tcp:
mknod /dev/tcp c 30 36
>From the bash reference:
/dev/tcp/host/port
If host is a valid hostname or Internet address, and port is an integer
port number or service name, Bash attempts to open a TCP connection to the
corresponding socket.
EXPLANATION
if [ "X$MYEXIT" = "X0" ]; then
echo "Connection successful. Exit code: $MYEXIT"
else
echo "Connection unsuccessful. Exit code: $MYEXIT"
fi
exit $MYEXIT |
27.2. /procThe /proc directory
is actually a pseudo-filesystem. The files in /proc mirror currently running
system and kernel processes and contain
information and statistics about them. bash$ cat /proc/devices
Character devices:
1 mem
2 pty
3 ttyp
4 ttyS
5 cua
7 vcs
10 misc
14 sound
29 fb
36 netlink
128 ptm
136 pts
162 raw
254 pcmcia
Block devices:
1 ramdisk
2 fd
3 ide0
9 md
bash$ cat /proc/interrupts
CPU0
0: 84505 XT-PIC timer
1: 3375 XT-PIC keyboard
2: 0 XT-PIC cascade
5: 1 XT-PIC soundblaster
8: 1 XT-PIC rtc
12: 4231 XT-PIC PS/2 Mouse
14: 109373 XT-PIC ide0
NMI: 0
ERR: 0
bash$ cat /proc/partitions
major minor #blocks name rio rmerge rsect ruse wio wmerge wsect wuse running use aveq
3 0 3007872 hda 4472 22260 114520 94240 3551 18703 50384 549710 0 111550 644030
3 1 52416 hda1 27 395 844 960 4 2 14 180 0 800 1140
3 2 1 hda2 0 0 0 0 0 0 0 0 0 0 0
3 4 165280 hda4 10 0 20 210 0 0 0 0 0 210 210
...
bash$ cat /proc/loadavg
0.13 0.42 0.27 2/44 1119
bash$ cat /proc/apm
1.16 1.2 0x03 0x01 0xff 0x80 -1% -1 ?
|
Shell scripts may extract data from certain of the files in
/proc.
FS=iso # ISO filesystem support in kernel?
grep $FS /proc/filesystems # iso9660 |
kernel_version=$( awk '{ print $3 }' /proc/version ) |
CPU=$( awk '/model name/ {print $4}' < /proc/cpuinfo )
if [ $CPU = Pentium ]
then
run_some_commands
...
else
run_different_commands
...
fi |
devfile="/proc/bus/usb/devices"
USB1="Spd=12"
USB2="Spd=480"
bus_speed=$(grep Spd $devfile | awk '{print $9}')
if [ "$bus_speed" = "$USB1" ]
then
echo "USB 1.1 port found."
# Do something appropriate for USB 1.1.
fi |
The /proc directory
contains subdirectories with unusual numerical
names. Every one of these names maps to the process ID of a currently running
process. Within each of these subdirectories, there are
a number of files that hold useful information about the
corresponding process. The stat and
status files keep running statistics
on the process, the cmdline file holds
the command-line arguments the process was invoked with, and
the exe file is a symbolic link to the
complete path name of the invoking process. There are a few
more such files, but these seem to be the most interesting
from a scripting standpoint. Example 27-2. Finding the process associated with a PID #!/bin/bash
# pid-identifier.sh: Gives complete path name to process associated with pid.
ARGNO=1 # Number of arguments the script expects.
E_WRONGARGS=65
E_BADPID=66
E_NOSUCHPROCESS=67
E_NOPERMISSION=68
PROCFILE=exe
if [ $# -ne $ARGNO ]
then
echo "Usage: `basename $0` PID-number" >&2 # Error message >stderr.
exit $E_WRONGARGS
fi
pidno=$( ps ax | grep $1 | awk '{ print $1 }' | grep $1 )
# Checks for pid in "ps" listing, field #1.
# Then makes sure it is the actual process, not the process invoked by this script.
# The last "grep $1" filters out this possibility.
#
# pidno=$( ps ax | awk '{ print $1 }' | grep $1 )
# also works, as Teemu Huovila, points out.
if [ -z "$pidno" ] # If, after all the filtering, the result is a zero-length string,
then # no running process corresponds to the pid given.
echo "No such process running."
exit $E_NOSUCHPROCESS
fi
# Alternatively:
# if ! ps $1 > /dev/null 2>&1
# then # no running process corresponds to the pid given.
# echo "No such process running."
# exit $E_NOSUCHPROCESS
# fi
# To simplify the entire process, use "pidof".
if [ ! -r "/proc/$1/$PROCFILE" ] # Check for read permission.
then
echo "Process $1 running, but..."
echo "Can't get read permission on /proc/$1/$PROCFILE."
exit $E_NOPERMISSION # Ordinary user can't access some files in /proc.
fi
# The last two tests may be replaced by:
# if ! kill -0 $1 > /dev/null 2>&1 # '0' is not a signal, but
# this will test whether it is possible
# to send a signal to the process.
# then echo "PID doesn't exist or you're not its owner" >&2
# exit $E_BADPID
# fi
exe_file=$( ls -l /proc/$1 | grep "exe" | awk '{ print $11 }' )
# Or exe_file=$( ls -l /proc/$1/exe | awk '{print $11}' )
#
# /proc/pid-number/exe is a symbolic link
# to the complete path name of the invoking process.
if [ -e "$exe_file" ] # If /proc/pid-number/exe exists...
then # the corresponding process exists.
echo "Process #$1 invoked by $exe_file."
else
echo "No such process running."
fi
# This elaborate script can *almost* be replaced by
# ps ax | grep $1 | awk '{ print $5 }'
# However, this will not work...
# because the fifth field of 'ps' is argv[0] of the process,
# not the executable file path.
#
# However, either of the following would work.
# find /proc/$1/exe -printf '%l\n'
# lsof -aFn -p $1 -d txt | sed -ne 's/^n//p'
# Additional commentary by Stephane Chazelas.
exit 0 |
Example 27-3. On-line connect status #!/bin/bash
PROCNAME=pppd # ppp daemon
PROCFILENAME=status # Where to look.
NOTCONNECTED=65
INTERVAL=2 # Update every 2 seconds.
pidno=$( ps ax | grep -v "ps ax" | grep -v grep | grep $PROCNAME | awk '{ print $1 }' )
# Finding the process number of 'pppd', the 'ppp daemon'.
# Have to filter out the process lines generated by the search itself.
#
# However, as Oleg Philon points out,
#+ this could have been considerably simplified by using "pidof".
# pidno=$( pidof $PROCNAME )
#
# Moral of the story:
#+ When a command sequence gets too complex, look for a shortcut.
if [ -z "$pidno" ] # If no pid, then process is not running.
then
echo "Not connected."
exit $NOTCONNECTED
else
echo "Connected."; echo
fi
while [ true ] # Endless loop, script can be improved here.
do
if [ ! -e "/proc/$pidno/$PROCFILENAME" ]
# While process running, then "status" file exists.
then
echo "Disconnected."
exit $NOTCONNECTED
fi
netstat -s | grep "packets received" # Get some connect statistics.
netstat -s | grep "packets delivered"
sleep $INTERVAL
echo; echo
done
exit 0
# As it stands, this script must be terminated with a Control-C.
# Exercises:
# ---------
# Improve the script so it exits on a "q" keystroke.
# Make the script more user-friendly in other ways. |
 | In general, it is dangerous to
write to the files in /proc, as this can corrupt the
filesystem or crash the machine. |
Chapter 28. Of Zeros and Nulls
/dev/zero
and /dev/null - Uses of /dev/null
Think of /dev/null as a "black
hole." It is the nearest equivalent to a
write-only file. Everything written to it disappears
forever. Attempts to read or output from it result in
nothing. Nevertheless, /dev/null
can be quite useful from both the command line and in
scripts. Suppressing stdout.
cat $filename >/dev/null
# Contents of the file will not list to stdout. |
Suppressing stderr
(from Example 12-3).
rm $badname 2>/dev/null
# So error messages [stderr] deep-sixed. |
Suppressing output from both
stdout and stderr.
cat $filename 2>/dev/null >/dev/null
# If "$filename" does not exist, there will be no error message output.
# If "$filename" does exist, the contents of the file will not list to stdout.
# Therefore, no output at all will result from the above line of code.
#
# This can be useful in situations where the return code from a command
#+ needs to be tested, but no output is desired.
#
# cat $filename &>/dev/null
# also works, as Baris Cicek points out. |
Deleting contents of a file, but preserving the file itself, with
all attendant permissions (from Example 2-1 and Example 2-3):
cat /dev/null > /var/log/messages
# : > /var/log/messages has same effect, but does not spawn a new process.
cat /dev/null > /var/log/wtmp |
Automatically emptying the contents of a logfile
(especially good
|