Advanced Bash-Scripting HOWTO

Advanced Bash-Scripting HOWTOAdvanced Bash-Scripting HOWTOA guide to shell scripting, using BashMendel Cooperthegrendel@theriver.comv0.2, 30 October 2000This is a major update on version 0.1. -- a couple of bugs
fixed, plus much additional material and more example scripts
added.This document is both a tutorial and a reference on shell
scripting with Bash. It assumes no previous knowledge of
scripting or programming, but progresses rapidly toward an
intermediate/advanced level of instruction. The exercises and
heavily-commented examples invite active reader participation.
Still, it is a work in progress. The intention is to add much
supplementary material in future updates to this HOWTO, so that
it will gradually evolve into an LDP "guide", i.e.,
a complete book.Table of Contents1. Why Shell Programming?2. Starting Off With a Sha-Bang2.1. Invoking the script2.2. Shell wrapper, self-executing script3. Tutorial / Reference3.1. exit and exit status3.2. Special characters used in shell scripts3.3. Introduction to Variables and Parameters3.4. Quoting3.5. Tests3.6. Operations and Related Topics3.7. Variables Revisited3.8. Loops3.9. Internal Commands and Builtins3.10. External Filters, Programs and Commands3.11. System and Administrative Commands3.12. Backticks (`...`)3.13. I/O Redirection3.14. Recess Time3.15. Regular Expressions3.16. Subshells3.17. Process Substitution3.18. Functions3.19. List Constructs3.20. Arrays3.21. Files3.22. Here Documents3.23. Of Zeros and Nulls3.24. Debugging3.25. Options3.26. Gotchas3.27. Miscellany3.28. Bash, version 24. CreditsBibliographyA. Contributed ScriptsB. CopyrightList of Tables3-1. bash optionsList of Examples2-1. cleanup: A script to clean up the log
files in /var/log2-2. cleanup: An enhanced
and generalized version of above script.2-3. shell wrapper2-4. A slightly more complex shell wrapper3-1. exit / exit status3-2. Code blocks and I/O redirection3-3. Saving the results of a code block to a file3-4. Backup of all files changed in last day3-5. Variable assignment and substitution3-6. Using param substitution and :3-7. Renaming file extensions:3-8. Using pattern matching to parse arbitrary strings3-9. What is truth?3-10. Equivalence of [ ] and test3-11. Tests, command chaining, redirection3-12. arithmetic and string comparisons3-13. zmost3-14. Compound Condition Tests Using && and ||3-15. Representation of numerical constants:3-16. Variable Assignment3-17. Variable Assignment, plain and fancy3-18. Positional Parameters3-19. wh, whois domain name lookup3-20. Using shift3-21. Using declare to type variables3-22. Indirect References3-23. Generating random numbers3-24. Simple for loops3-25. Missing in [list] in a
for loop3-26. Using efax in batch mode3-27. Simple while loop3-28. Another while loop3-29. until loop3-30. Effects of break and
continue in a loop3-31. Using case3-32. Creating menus using case3-33. Creating menus using select3-34. Creating menus using select in a function3-35. Using getopts to read the flags/options
passed to a script3-36. Using set with positional
parameters3-37. basename and dirname3-38. Variable assignment, using read3-39. Changing the current working directory3-40. "Including" a data file3-41. Waiting for a process to finish before proceeding3-42. Using ls to create a table of contents
for burning a CDR disk3-43. Badname, eliminate file names
in current directory containing
bad characters and white space.3-44. Log file using xargs to monitor system log3-45. copydir, copying files in current
directory to another, using xargs3-46. Showing the effect of eval3-47. Forcing a log-off3-48. Using expr3-49. Letting let do some arithmetic.3-50. printf in action3-51. Using cpio to move a directory tree3-52. toupper: Transforms a file to all uppercase.3-53. lowercase: Changes all filenames in working directory to lowercase.3-54. nl: A self-numbering script.3-55. Formatted file listing.3-56. Using date3-57. uuencoding encoded files3-58. Using seq to generate loop arguments3-59. Effects of exec3-60. killall, from /etc/rc.d/init.d3-61. Perl embedded in a bash script3-62. Variable scope in a subshell3-63. Running parallel processes in subshells3-64. Simple function3-65. Function Taking Parameters3-66. Converting numbers to Roman numerals3-67. Local variable visibility3-68. Recursion, using a local variable3-69. Using an "and list" to test for command-line arguments3-70. Using "or lists" in combination with an "and list"3-71. Simple array usage3-72. Some special properties of arrays3-73. An old friend:
The Bubble Sort3-74. Complex array application:
Sieve of Erastosthenes3-75. dummyfile: Creates a 2-line dummy file3-76. broadcast: Sends message to everyone logged in3-77. Multi-line message using cat3-78. upload: Uploads a file pair to "Sunsite"
incoming directory3-79. Setting up a swapfile using /dev/zero3-80. test23, a buggy script3-81. test24, another buggy script3-82. Trapping at exit3-83. Cleaning up after Control-C3-84. String expansion3-85. Indirect variable references - the new way3-86. Using arrays and other miscellaneous trickery
to deal four random hands from a deck of cardsA-1. manview: A script for viewing formatted man pagesA-2. manview: A script for uploading to an ftp site,
using a locally encrypted passwordA-3. behead: A script for removing mail and news message headersA-4. ftpget: A script for downloading files via ftpChapter 1. Why Shell Programming?The shell is a command interpreter. It is the insulating layer between
the operating system kernel and the user. Yet, it is 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 to administrative system tasks and other routine repetitive jobs
not requiring the bells and whistles of a full-blown tightly structured
programming language.A working knowledge of shell scripting is essential to everyone wishing
to become reasonably adept 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 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, even if slowly,
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 classical 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 factorcomplex applications, where structured programming is
a necessityfile handling (Bash is limited to serial file access,
and that only in a particularly clumsy and inefficient
line-by-line fashion)need to generate or manipulate graphics or GUIsneed direct access to system hardwareneed port or socket I/Oneed to use libraries or interface with legacy codeIf any of the above applies, consider a more powerful scripting
language, perhaps Perl, Tcl, Python, or even 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 "Born-Again
Shell" and a pun on Stephen Bourne's now classic
Bourne Shell. Bash has become the de facto standard
for shell scripting on all flavors of UNIX. Most of the
principles dealt with in this document 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 a news
group posting by Tom Christiansen in October of 1993).
The following is a tutorial in shell scripting. It relies
heavily on examples to illustrate features of the shell. As far
as possible, the example scripts have been tested, and some of them
may actually be useful in real life. The reader should cut out and
save the examples, assign them appropriate names, give them execute
permission (chmod u+x scriptname), then
run them to see what happens. Note that some of the scripts below
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 document wrote
the example scripts that follow.Chapter 2. Starting Off With a Sha-BangIn 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, just 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 can easily be modified, customized, or generalized for a
particular application.Example 2-2. cleanup: An enhanced
and generalized version of above script.#!/bin/bash
# cleanup, version 2
# Run as root, of course.
if [ -n $1 ]
# Test if command line argument present.
then
lines=$1
else
lines=50
# default, if not specified on command line.
fi
cd /var/log
tail -$lines messages > mesg.temp
# Saves last section of message log file.
mv mesg.temp messages
# cat /dev/null > messages
# No longer needed, as the above method is safer.
cat /dev/null > wtmp
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 variant of
the first 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 an
executable shell script (man magic gives
more info 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
enables the specific commands and directives embedded in the
shell or program called.#!/bin/sh
#!/bin/bash #!/bin/awk #!/usr/bin/perl #!/bin/sed
#!/usr/bin/tclEach 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 may have to sacrifice a few bash-specific features
(the script will 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. Example 2, above, requires the initial
#!, since the variable assignment line,
lines=50, uses a shell-specific construct.
Note that #!/bin/sh invokes the default
shell interpreter, which defaults to /bin/bash
on a Linux machine.2.1. Invoking the scriptHaving written the script, you can invoke it by sh
scriptname, or alternately bash
scriptname. (Not recommended is using sh
<scriptname, since this effectively disables
reading from input within the script.) Much more convenient is
to make the script itself directly executable by
Either:chmod 755 scriptname (gives
everyone execute permission)orchmod +x scriptname (gives
everyone execute permission)chmod
u+x scriptname (gives only the
script owner execute permission)
In this case, you could try calling the script by
./scriptname.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 [return]
from the command line.2.2. Shell wrapper, self-executing scriptA
sed or
awk script would
normally be invoked from the command line by a
sed -e 'commands' or
awk -e 'commands'.
Embedding such a script in a bash script permits calling it more simply,
and makes it "reusable". This also permits combining the
functionality of sed and awk, for
example piping the output of a set of sed commands to
awk. As a saved executable file, you can then
repeatedly invoke it in its original form or modified, without retyping
it on the command line.Example 2-3. shell wrapper#!/bin/bash
# This is a simple script
# that removes blank lines
# from a file.
# No argument checking.
# Same as
# sed -e '/^$/d $1' filename
# invoked from the command line.
sed -e /^$/d $1
# '^' is beginning of line,
# '$' is end,
# and 'd' is delete.Example 2-4. A slightly more complex shell wrapper#!/bin/bash
# "subst", a script that substitutes one pattern for
# another in a file,
# i.e., "subst Smith Jones letter.txt".
if [ $# -ne 3 ]
# Test number of arguments to script
# (always a good idea).
then
echo "Usage: `basename $0` old-pattern new-pattern filename"
exit 1
fi
old_pattern=$1
new_pattern=$2
if [ -f $3 ]
then
file_name=$3
else
echo "File \"$3\" does not exist."
exit 2
fi
# Here is where the heavy work gets done.
sed -e "s/$old_pattern/$new_pattern/" $file_name
# 's' is, of course, the substitute command in sed,
# and /pattern/ invokes address matching.
# Read the literature on 'sed' for a more
# in-depth explanation.
exit 0
# Successful invocation of the script returns 0.Exercise. Write a shell script that performs a simple task.Chapter 3. Tutorial / Reference ...there are dark corners in the Bourne shell, and people use all
of them. --Chet Ramey 3.1. exit and exit statusThe
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 shell.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.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 nn
command may be used to deliver an
nn exit status to
the shell (nn
must be a decimal number in the 0 -
255 range).
$? reads the exit status of script or function.Example 3-1. exit / exit status#!/bin/bash
echo hello
echo $?
# exit status 0 returned
# because command successful.
lskdf
# bad command
echo $?
# non-zero exit status returned.
echo
exit 143
# Will return 143 to shell.
# To verify this, type $? after script terminates.
# By convention, an 'exit 0' shows success,
# while a non-zero exit value indicates an error or anomalous condition.
# It is also appropriate for the script to use the exit status
# to communicate with other processes, as when in a pipe with other scripts.3.2. Special characters used in shell scripts#Comments. Lines beginning with a # (with the exception
of #!) are comments.# This line is a comment.Comments may also occur at the end of a command.echo "A comment will follow." # Comment here.Comments may also follow white space at the beginning of a line. # A tab precedes this comment.;Command separator. Permits putting two or more commands on the same lineecho hello; echo thereNote that the ; sometimes needs to be escaped (\).."dot" command. Equivalent to source, explained further on:null command. Exit status 0, alias for
trueEndless loop:while :
do
operation-1
operation-2
...
operation-n
done
Placeholder in if/then test:if condition
then : # Do nothing and branch ahead
else
take-some-action
fi
Provides a placeholder where a binary operation is
expected, see Section 3.3.1.: ${username=`whoami`}
# ${username=`whoami`} without the leading : gives an error
Evaluate string of variables using
"parameter substitution", see Example 3-6:: ${HOSTNAME?} ${USER?} ${MAIL?}
Prints error message if one or more of essential environmental
variables not set.${}Parameter substitution. See Section 3.3 for more details.()command group. (a=hello; echo $a)Note: A listing of commands within
parentheses starts a
subshell (see Section 3.16).{}block of code. This, in effect, creates an anonymous function.The code block enclosed in braces may have I/O redirected to
and from it.Example 3-2. Code blocks and I/O redirection#!/bin/bash
{
read fstab
} < /etc/fstab
echo "First line in /etc/fstab is:"
echo "$fstab"
exit 0Example 3-3. Saving the results of a code block to a file#!/bin/bash
# rpm-check
# ---------
# 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.
NOARGS=1
if [ -z $1 ]
then
echo "Usage: `basename $0` rpm-file"
exit $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 [ ! $? ]
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/{}file pathname. Mostly used in 'find' constructs.> >& >> <redirection. scriptname >filename redirects the output of
scriptname to file
filename. Overwrite
filename if it already exists.command >&2 redirects output of
command to stderr.scriptname >>filename appends
the output of scriptname
to file filename. If
filename does not already exist,
it will be created.For a more detailed explanation, see Section 3.13.<<redirection used in "here document". See Section 3.22.|pipe. Passes the output of previous command to next one, or to shell.
This is a method of chaining commands together.echo ls -l | sh
passes the output of "ls -l" to the shell, with the
same result as a simple "ls -l".cat *.lst | sort | uniq
sorts the output of all the .lst files and
deletes duplicate lines.Note: 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. (See Section 3.24 for more detail
on signals.)>|force redirection (even if
noclobber environmental variable is in effect). This will forcibly overwrite an existing file.-redirection from/to stdin or stdout. (cd /source/directory && tar cf - . ) | (cd /dest/directory && tar xvfp -)
# Move entire file tree from one directory to another
# [courtesy Alan Cox, a.cox@swansea.ac.uk]
#
# More elegant than, but equivalent to:
# cd source-directory
# tar cf - . | (cd ../target-directory; tar xzf -)bunzip2 linux-2.2.15.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 the "-" is not itself a Bash operator,
but rather an option recognized by certain UNIX utilities.Where a filename is expected, redirects output to stdout
(mostly seen with tar cf)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 tarred and gzipped file.
if [ $# = 0 ]
then
echo "Usage: `basename $0` filename"
exit 1
fi
tar cvf - `find . -mtime -1 -type f -print` > $1.tar
gzip $1.tar
exit 0~home directory. ~bozo is bozo's home directory,
and ls ~bozo lists the contents of it.
~/ is the current user's home directory,
and ls ~/ lists the contents of
it.White spacefunctions as a separator, separating commands or variables. White space consists of either spaces, tabs, blank lines, or any
combination thereof. In some contexts, such as variable assignment,
white space is not permitted, and results in a syntax error.Blank linesBlank lines have no effect on the action of a script, and are therefore useful
for visually separating functional sections of the script.3.3. Introduction to Variables and ParametersVariables are at the heart of every programming and scripting
language. They are used for arithmetic operations and manipulation of
quantities, string parsing, and working in the abstract with symbols -
tokens that represent something else. A variable is nothing more than a
location or set of locations in computer memory that holds an item of
data.$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 $,
is when declared or assigned. 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. Using single
quotes (' ') causes the variable name to be
used literally, and no substitution will take place. This
is full quoting.Note that $variable is actually a
simplified alternate form of
${variable}. In contexts
where the $variable syntax
causes an error, the longer form may work (see Section 3.3.1 below).Example 3-5. Variable assignment and substitution#!/bin/bash
# Variables: assignment and substitution
a=37.5
hello=$a
# No space permitted on either side of = sign when initializing variables.
echo hello
# Not a reference.
echo $hello
echo ${hello} #Identical to above.
echo "$hello"
echo "${hello}"
echo '$hello'
# Variable referencing disabled by single quotes,
# because $ interpreted literally.
# Notice the effect of different types of quoting.
# ------------------------------------------------
echo; echo
numbers="one two three"
other_numbers="1 2 3"
# If whitespace within variables, then quotes necessary.
echo "numbers = $numbers"
echo "other_numbers = $other_numbers"
echo
echo "uninitialized variable = $uninitialized_variable"
# Uninitialized variable has null value (no value at all).
echo
exit 0Note: An uninitialized variable has a
"null" value - no assigned value at all
(not zero!). Using a variable before assigning a value
to it will inevitably cause problems.3.3.1. Parameter Substitution${parameter}Same as $parameter, i.e.,
value of the variable parameterMay be used for concatenating variables with stringsyour_id=${USER}-on-${HOSTNAME}
echo "$your_id"${parameter-default}If parameter not set, use defaultecho ${username-`whoami`}
# Echoes the result of `whoami`, but variable "username" is still unset.Note: This is almost equivalent to
${parameter:-default}. The
extra : makes a difference only when
parameter has been declared,
but is null. #!/bin/bash
username0=
echo "username0 = ${username0-`whoami`}"
# username0 has been declared, but is set to null.
# Will not echo.
echo "username1 = ${username1-`whoami`}"
# username1 has not been declared.
# Will echo.
username2=
echo "username2 = ${username2:-`whoami`}"
# username2 has been declared, but is set to null.
# Will echo because of :- rather than just - in condition test.
exit 0${parameter=default}, ${parameter:=default}If parameter not set, set it to defaultBoth 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+otherwise}, ${parameter:+otherwise}If parameter set, use 'otherwise", else use null stringBoth forms nearly equivalent. The :
makes a difference only when parameter
has been declared and is null, as above.${parameter?err_msg}, ${parameter:?err_msg}If parameter set, use it, else print err_msgBoth forms nearly equivalent. The :
makes a difference only when parameter
has been declared and is null, as above.Example 3-6. Using param substitution and :#!/bin/bash
# Let's check some of the system's environmental variables.
# 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 $MAIL."
echo
echo "If you are reading this message,"
echo "critical environmental variables have been set."
echo
echo
# The ':' operator seems fairly error tolerant.
# This script works even if the '$' omitted in front of
# {HOSTNAME}, {USER?}, {HOME?}, and {MAIL?}. Why?
# ------------------------------------------------------
# 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
# If ZZXy23AB has not been set...
: ${ZZXy23AB?}
# This will give you an error message and terminate.
echo "You will not see this message."
exit 0 Parameter substitution and/or expansion. The following expressions are the complement to the
match in
expr string operations (see Example 3-48). These particular ones are used mostly
in parsing file path names.${var#pattern}, ${var##pattern}Strip off shortest/longest part of
pattern if it matches the front end of
variable.
${var%pattern}, ${var%%pattern}Strip off shortest/longest part of
pattern if it matches the back end of
variable.
Version 2 of bash adds additional options.Example 3-7. Renaming file extensions:#!/bin/bash
# rfe
# ---
# Renaming file extensions.
#
# rfe old_extension new_extension
#
# Example:
# To rename all *.gif files in working directory to *.jpg,
# rfe gif jpg
if [ $# -ne 2 ]
then
echo "Usage: `basename $0` old_file_suffix new_file_suffix"
exit 1
fi
for filename in *.$1
# Traverse list of files ending with 1st argument.
do
mv $filename ${filename%$1}$2
# Strip off part of filename matching 1st argument,
# then append 2nd argument.
done
exit 0${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.
${var/patt/replacement}first match of patt,
within var replaced with
replacement.If replacement is
omitted, then the first match of
patt is replaced by
nothing, that is, deleted.${var//patt/replacement}all matches of patt,
within var replaced with
replacement.Similar to above, if
replacement is omitted,
then all occurrences patt
are replaced by nothing, that
is, deleted.Example 3-8. 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%*-*}
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##/*/}
# Same effect as t=`basename $path_name`
echo "path_name, stripped of prefixes = $t"
t=${path_name%/*.*}
# Same effect as t=`dirname $path_name`
echo "path_name, stripped of suffixes = $t"
echo
t=${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 03.4. QuotingQuoting 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,
*.)When referencing a variable, it is generally advisable in
enclose it in double quotes (" "). This preserves
all special characters within the variable name, except
$, ', and \. This
allows referencing it, that is, replacing the variable with
its value (see Example 3-5, above). Enclosing the
arguments to an echo statement in double
quotes is usually a good practice.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").Escaping is a method of quoting single
characters. The escape (\)
preceding a character will either toggle on or turn off a special
meaning for that character, depending on context.\nmeans newline\rmeans return\tmeans tab\v\vmeans vertical tab\bmeans backspace\ameans "alert" (beep or flash)\0xxtranslates to the octal ASCII
equivalent of 0xx# Use the -e option with 'echo' to print these.
echo -e "\v\v\v\v" # Prints 4 vertical tabs.
echo -e "\042" # Prints " (quote).
\" gives the quote its literal meaningecho "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 $variable01The 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 continues the
command sequence onto the next line.(cd /source/directory && tar cf - . ) | \
(cd /dest/directory && tar xvfp -)
# Repeating Alan Cox's directory tree copy command,
# but split into two lines for increased legibility.
3.5. TestsThe if/then construct tests whether a condition is
true, and if so, executes one or more commands. Note that in this
context, 0 (zero) will evaluate as true, as will
a random string of alphanumerics. Puzzling out the logic of this is
left as an exercise for the reader.Example 3-9. What is truth?#!/bin/bash
if [ 0 ]
#zero
then
echo "0 is true."
else
echo "0 is false."
fi
if [ ]
#NULL (empty condition)
then
echo "NULL is true."
else
echo "NULL is false."
fi
if [ xyz ]
#string
then
echo "Random string is true."
else
echo "Random string is false."
fi
if [ $xyz ]
#string
then
echo "Undeclared variable is true."
else
echo "Undeclared variable is false."
fi
exit 0Exercise. Explain the behavior of Example 3-9, above.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
Add a semicolon when 'if' and 'then' are on same line.if [ -x filename ]; thenelif This is a contraction for else if.
The effect is to nest an inner if/then construction
within an outer one.if [ condition ]
then
command
command
command
elif
# Same as else if
then
command
command
else
default-command
fi
The test condition-true construct is the exact
equivalent of if [condition-true ]. The left bracket
[ is, in fact, an alias for test. (The
closing right bracket ] in a test should not therefore
be strictly necessary, however newer versions of bash detect it as a
syntax error and complain.)Example 3-10. Equivalence of [ ] and test#!/bin/bash
echo
if test -z $1
then
echo "No command-line arguments."
else
echo "First command-line argument is $1."
fi
# Both code blocks are functionally identical.
if [ -z $1 ]
# if [ -z $1
# also works, but outputs an error message.
then
echo "No command-line arguments."
else
echo "First command-line argument is $1."
fi
echo
exit 03.5.1. File test operatorsReturns true if...-efile exists-ffile is a regular file-sfile is not zero size-dfile is a directory-bfile is a block device (floppy, cdrom, etc.)
-cfile is a character device (keyboard, modem, sound
card, etc.)-pfile is a pipe-Lfile is a symbolic link-Sfile is a socket-rfile is readable (has read permission)-wfile has write permission-xfile has execute permission-ggroup-id flag set on file-uuser-id flag set on file-k"sticky bit" set (if user does not
own a directory that has the sticky bit set, she cannot delete
files in it, not even files she owns)-Oyou are owner of file-Ggroup-id of file same as yours-t nfile descriptor n is openThis usually refers to stdin,
stdout, and stderr
(file descriptors 0 - 2).f1 -nt f2file f1 is newer than
f2f1 -ot f2file f1 is older than
f2f1 -ef f2files f1 and
f2 are links to the same
file!"not" -- reverses the sense of the
tests above (returns true if condition absent).Example 3-11. Tests, command chaining, redirection#!/bin/bash
# This line is a comment.
filename=sys.log
if [ ! -f $filename ]
then
touch $filename; echo "Creating file."
else
cat /dev/null > $filename; echo "Cleaning out file."
fi
# Of course, /var/log/messages must have
# world read permission (644) for this to work.
tail /var/log/messages > $filename
echo "$filename contains tail end of system log."
exit 03.5.2. Comparison operators (binary)integer comparison-eqis equal to ($a -eq $b)-neis not equal to ($a -ne $b)-gtis greater than ($a -gt $b)-geis greater than or equal to ($a -ge $b)-ltis less than ($a -lt $b)-leis less than or equal to ($a -le $b)string comparison=is equal to ($a = $b)!=is not equal to ($a != $b)\<is less than, in ASCII alphabetical order ($a
\< $b)Note that the "<" needs to be
escaped.\>is greater than, in ASCII alphabetical order
($a \> $b)Note that the ">" needs to be
escaped.See Example 3-73 for an application of this
comparison operator.-zstring is "null", that is, has zero length-nstring in not "null". Note that this test
does not work reliably (a bash bug?). Use
! -z instead.Example 3-12. 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 integer comparisons.
# Be careful.
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)"
fi
echo
exit 0Example 3-13. zmost#!/bin/bash
#View gzipped files with 'most'
NOARGS=1
if [ $# = 0 ]
# same effect as: if [ -z $1 ]
then
echo "Usage: `basename $0` filename" >&2
# Error message to stderr.
exit $NOARGS
# Returns 1 as exit status of script
# (error code)
fi
filename=$1
if [ ! -f $filename ]
then
echo "File $filename not found!" >&2
# Error message to stderr.
exit 2
fi
if [ ${filename##*.} != "gz" ]
# Using bracket in variable substitution.
then
echo "File $1 is not a gzipped file!"
exit 3
fi
zcat $1 | most
exit 0
# Uses the file viewer 'most'
# (similar to 'less')compound comparison-alogical andexp1 -a exp2 returns true if
both exp1 and exp2 are true.-ological or exp1 -o exp2 returns
true if either exp1 or exp2 are
true.These are simpler forms of the comparison operators
&& and ||,
which require brackets to separate the target expressions.Refer to Example 3-14 to see compound comparison operators
in action.3.6. Operations and Related Topics3.6.1. Operations=All-purpose assignment operator, which works for both arithmetic and
string assignments. var=27
category=minerals
May also be used in a string comparison test. if [ $string1 = $string2 ]
then
command
fi
The following are normally used in combination with
expr or let.arithmetic operators+plus-minus*multiplication/division%modulo, or mod (returns the remainder of an integer
division)+="plus-equal" (increment variable by a constant)`expr $var+=5` results in
var being incremented by
5.-="minus-equal" (decrement variable by a constant)*="times-equal" (multiply variable by a constant)`expr $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)The bitwise logical operators seldom make an appearance in shell scripts.
Their chief use seems to be manipulating and testing values read from
ports or sockets. "Bit flipping" is more relevant to compiled languages,
such as C and C++, which run fast enough to permit its use on the fly.<<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"relational tests<less than>greater than<=less than or equal to>=greater than or equal to==equal to (test)!=not equal to&&and (logical)if [ $condition1 ] && [ $condition2 ]
# if both condition1 and condition2 hold true...||or (logical)if [ $condition1 ] || [ $condition2 ]
# if both condition1 or condition2 hold true...Example 3-14. 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 ]
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 03.6.2. 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 # is evaluated as
BASE#NUMBER (this option is of limited
usefulness because of range restrictions).Example 3-15. Representation of numerical constants:#!/bin/bash
# Representation of numbers.
# Decimal
let "d = 32"
echo "d = $d"
# Nothing out of the ordinary here.
# Octal: numbers preceded by '0'
let "o = 071"
echo "o = $o"
# Expresses result in decimal.
# Hexadecimal: numbers preceded by '0x' or '0X'
let "h = 0x7a"
echo "h = $h"
# Other bases: BASE#NUMBER
# BASE between 2 and 64.
let "b = 32#77"
echo "b = $b"
# This notation only works for a very limited range of numbers.
let "c = 2#47" # Error: out of range.
echo "c = $c"
exit 03.7. Variables RevisitedUsed properly, variables can add power and flexibility
to scripts. This requires learning their subtleties and
nuances.Internal (builtin) variablesenvironmental variables affecting bash script behavior$IFSinput field separatorThis defaults to white space, but may be
changed, for example, to parse a comma-separated data file.
$HOMEhome directory of the user (usually /home/username)$PATHpath to binaries (usually
/usr/bin/,
/usr/X11R6/bin/,
/usr/local/bin, etc.)Note that the "working directory",
./, is usually omitted from
the $PATH as a security measure.$PS1This is the main prompt, seen at the command line.$PS2The secondary prompt, seen when additional input is
expected. It is represented as an ">".$PWDworking directory (directory you are in at the time)$EDITORthe default editor invoked by a script, usually
vi or emacs.$TMOUTIf 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.Note: Unfortunately, this works only while waiting
for input at the shell prompt console or in an xterm. While
it would be nice to speculate on the uses of this internal
variable for timed input, for example in combination with
read, TMOUT
does not work in that context and is virtually useless for
shell scripting.Implementing timed input in a script is certainly
possible, but hardly seems worth the effort. It requires
setting up a timing loop to signal the script when it times
out. Additionally, a signal handling routine is necessary
to trap (see Example 3-82) the interrupt generated
by the timing loop (whew!).#!/bin/bash
# TMOUT=3 useless in a script
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 $$ &
# Waits XXX seconds, then sends sigalarm to script.
}
Int14Vector()
{
answer="TIMEOUT"
PrintAnswer
exit 14
}
trap Int14Vector 14
# Timer interrupt - 14 - subverted for our purposes.
echo "What is your favorite vegetable "
TimerOn
read answer
PrintAnswer
# Admittedly, this is a kludgy implementation of timed input,
# but pretty much as good as can be done with Bash.
# (Challenge to reader: come up with something better.)
# If you need something a bit more elegant...
# consider writing the application in C or C++,
# using appropriate library functions, such as 'alarm' and 'setitimer'.
exit 0$SECONDSThe number of seconds the script has been running.#!/bin/bash
ENDLESS_LOOP=1
echo
echo "Hit Control-C to exit this script."
echo
while [ $ENDLESS_LOOP ]
do
if [ $SECONDS -eq 1 ]
then
units=second
else
units=seconds
fi
echo "This script has been running $SECONDS $units."
sleep 1
done
exit 0$REPLYThe default value when a variable is not
supplied to read. Also applicable to
select menus, but only supplies the item
number of the variable chosen, not the value of the variable
itself.#!/bin/bash
echo
echo -n "What is your favorite vegetable? "
read
echo "Your favorite vegetable is $REPLY."
# REPLY holds the value of last "read" if and only if no variable supplied.
echo
echo -n "What is your favorite fruit? "
read fruit
echo "Your favorite fruit is $fruit."
echo "but..."
echo "Value of \$REPLY is still $REPLY."
# $REPLY is still set to its previous value because
# the variable $fruit absorbed the new "read" value.
echo
exit 0$BASHthe path to the bash binary itself,
usually /bin/bash$BASH_ENVan environmental variable pointing to a bash startup
file to be read when a script is invoked$0, $1,
$2, etc.positional parameters (passed from command line to script, passed to
a function, or set to a variable)$#number of command line arguments or positional parameters$$process id of script, often used in scripts to construct temp file names$?exit status of command, function, or the script itself$*All of the positional parameters$@Same as $*, but each parameter is a
quoted string, that is, the parameters are passed on intact,
without interpretation or expansion$-Flags passed to script$!PID of last job run in backgroundvariable assignmentInitializing or changing the value of a variable=the assignment operator (no space before &
after)Do not confuse this with = and
-eq, which test, rather than assign!Note: = can be either an
assignment or a test operator, depending on
context.Example 3-16. Variable Assignment#!/bin/bash
echo
# When is a variable "naked", i.e., lacking the '$' in front?
# Assignment
a=879
echo "The value of \"a\" is $a"
# Assignment using 'let'
let a=16+5
echo "The value of \"a\" is now $a"
echo
# In a 'for' loop (really, a type of disguised assignment)
echo -n "The values of \"a\" in the loop are "
for a in 7 8 9 11
do
echo -n "$a "
done
echo
echo
# In a 'read' statement
echo -n "Enter \"a\" "
read a
echo "The value of \"a\" is now $a"
echo
exit 0Example 3-17. Variable Assignment, plain and fancy#!/bin/bash
a=23
# Simple case
echo $a
b=$a
echo $b
# Now, getting a little bit fancier...
a=`echo Hello!`
# Assigns result of 'echo' command to 'a'
echo $a
a=`ls -l`
# Assigns result of 'ls -l' command to 'a'
echo $a
exit 0Variable assignment using the $() mechanism
(a newer method than back quotes)# From /etc/rc.d/rc.local
R=$(cat /etc/redhat-release)
arch=$(uname -m)local variablesvariables visible only within a code block or function
(see Section 3.18)environmental variablesvariables that affect the behavior of the shell and
user interface, such as the path and the promptIf 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.Note: 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 parametersarguments 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, etc.)Example 3-18. Positional Parameters#!/bin/bash
echo
echo The name of this script is $0
# Adds ./ for current directory
echo The name of this script is `basename $0`
# Strip out path name info (see 'basename')
echo
if [ $1 ]
then
echo "Parameter #1 is $1"
# Need quotes to escape #
fi
if [ $2 ]
then
echo "Parameter #2 is $2"
fi
if [ $3 ]
then
echo "Parameter #3 is $3"
fi
echo
exit 0Some 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 also have to be symbolic
links present to all the alternate names of the same
script.Note: If a script expects a command line parameter
but is invoked without one, this may cause a null variable
assignment, certainly 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.
variable1x=$1x
# This will prevent an error, even if positional parameter is absent.
# The extra character can be stripped off later, if desired, like so.
variable1=${variable1x/x/}
# This uses one of the parameter substitution templates previously discussed.
# Leaving out the replacement pattern results in a deletion.---Example 3-19. wh, whois domain name lookup#!/bin/bash
# Does a 'whois domain-name' lookup
# on any of 3 alternate servers:
# ripe.net, cw.net, radb.net
# Place this script, named '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
if [ -z $1 ]
then
echo "Usage: `basename $0` [domain-name]"
exit 1
fi
case `basename $0` in
# Checks script name and calls proper server
"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 0---
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 does not change. If you use a large number
of positional parameters to a script, shift
lets you access those past 10.Example 3-20. Using shift#!/bin/bash
# Name this script something like shift000,
# and invoke it with some parameters, for example
# ./shift000 a b c def 23 skidoo
# Demo of using 'shift'
# to step through all the positional parameters.
until [ -z "$1" ]
do
echo -n "$1 "
shift
done
echo
# Extra line feed.
exit 03.7.1. Typing variables: declare or typesetThe declare or typeset
keywords (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 not available in version 1 of bash.-r readonlydeclare -r var1(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 integerdeclare -i var2The script treats subsequent occurrences of
var2 as an integer.
Note that certain arithmetic operations are permitted for declared
integer variables without the need for
expr or
let.-a arraydeclare -a indicesThe variable indices will be treated as
an array.-f functionsdeclare -f # (no arguments)A declare -f line within a script causes
a listing of all the functions contained in that script.-x exportdeclare -x var3This declares a variable as available for exporting outside the
environment of the script itself.Example 3-21. Using declare to type variables#!/bin/bash
declare -f
# Lists the function below.
func1 ()
{
echo This is a function.
}
declare -r var1=13.36
echo "var1 declared as $var1"
# Attempt to change readonly variable.
var1=13.37
# Generates error message.
echo "var1 is still $var1"
echo
declare -i var2
var2=2367
echo "var2 declared as $var2"
var2=var2+1
# Integer declaration eliminates the need for 'let'.
echo "var2 incremented by 1 is $var2."
# Attempt to change variable declared as integer
echo "Attempting to change var2 to floating point value, 2367.1."
var2=2367.1
# results in error message, with no change to variable.
echo "var2 is still $var2"
exit 03.7.2. Indirect References to VariablesAssume that the value of a variable is the name of a second
variable. Is it somehow possible to retrieve the value
of this second variable from the first one? For example,
if a=letter_of_alphabet
and letter_of_alphabet=z,
can a reference to a return
z? This can indeed be done, and
it is called an indirect reference. It
uses the unusual eval var1=\$$var2
notation.Example 3-22. Indirect References#!/bin/bash
# Indirect variable referencing.
a=letter_of_alphabet
letter_of_alphabet=z
# Direct reference.
echo "a = $a"
# Indirect reference.
eval a=\$$a
echo "Now a = $a"
echo
# Now, let's try changing the second order reference.
t=table_cell_3
table_cell_3=24
eval t=\$$t
echo "t = $t"
# So far, so good.
table_cell_3=387
eval t=\$$t
echo "Value of t changed to $t"
# ERROR!
# Cannot indirectly reference changed value of variable this way.
# For this to work, must use ${!t} notation.
exit 0Note: This method of indirect referencing has its limitations. If
the second order variable changes its value, an indirect reference to
the first order variable produces an error. Fortunately, this flaw has
been fixed in the newer ${!variable}
notation introduced with version 2 of Bash (see Example 3-85).3.7.3. $RANDOM: generate random integerNote: $RANDOM is an internal Bash function (not a constant) that
returns a pseudorandom integer in the range
0 - 32767. $RANDOM should not be used
to generate an encryption key.Example 3-23. Generating random numbers#!/bin/bash
# $RANDOM returns a different random integer at each invocation.
# Nominal range: 0 - 32767 (signed integer).
MAXCOUNT=10
count=1
echo
echo "$MAXCOUNT random numbers:"
echo "-----------------"
while [ $count -le $MAXCOUNT ] # Generate 10 ($MAXCOUNT) random integers.
do
number=$RANDOM
echo $number
let "count += 1" # Increment count.
done
echo "-----------------"
# If you need a random int within a certain range, then use the 'modulo' operator.
RANGE=500
echo
number=$RANDOM
let "number %= $RANGE"
echo "Random number less than $RANGE --> $number"
echo
# If you need a random int 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
# May combine above two techniques to retrieve random number between two limits.
number=0 #initialize
while [ $number -le $FLOOR ]
do
number=$RANDOM
let "number %= $RANGE"
done
echo "Random number between $FLOOR and $RANGE --> $number"
echo
# May generate binary choice, that is, "true" or "false" value.
BINARY=2
number=$RANDOM
let "number %= $BINARY"
if [ $number -eq 1 ]
then
echo "TRUE"
else
echo "FALSE"
fi
echo
# May generate toss of the dice.
SPOTS=7
DICE=2
die1=0
die2=0
# Tosses each die separately, and so gives correct odds.
while [ $die1 -eq 0 ] #Can't have a zero come up.
do
let "die1 = $RANDOM % $SPOTS"
done
while [ $die2 -eq 0 ]
do
let "die2 = $RANDOM % $SPOTS"
done
let "throw = $die1 + $die2"
echo "Throw of the dice = $throw"
echo
exit 03.8. Loopsfor (in)This is the basic looping construct. It differs significantly
from its C counterpart.for [arg] in [list] do command... done Note that list may contain wild cards.Note further that if do is on same line as
for, there needs to be a semicolon before list.for [arg] in [list] ; do Example 3-24. Simple for loops#!/bin/bash
for planet in Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto
do
echo $planet
done
echo
# Entire 'list' enclosed in quotes creates a single variable.
for planet in "Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto"
do
echo $planet
done
exit 0Omitting 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.Example 3-25. Missing in [list] in a
for loop#!/bin/bash
# Invoke both with and without arguments,
# and see what happens.
for a
do
echo $a
done
# 'in list' missing, therefore
# operates on '$#'
# (command-line argument list)
exit 0Example 3-26. Using efax in batch mode#!/bin/bash
if [ $# -ne 2 ]
# Check for proper no. of command line args.
then
echo "Usage: `basename $0` phone# text-file"
exit 1
fi
if [ ! -f $2 ]
then
echo "File $2 is not a text file"
exit 2
fi
# Create fax formatted files from text files.
fax make $2
for file in $(ls $2.0*)
# Concatenate the converted files.
# Uses wild card in variable list.
do
fil="$fil $file"
done
# Do the work.
efax -d /dev/ttyS3 -o1 -t "T$1" $fil
exit 0whileThis construct tests for a condition at the top of a loop, and keeps
looping as long as that condition is true.while [condition] do command... done As is the case with for/in loops, placing the
do on the same line as the condition test
requires a semicolon.while [condition] ; do Note that certain specialized while loops, as,
for example, a getopts construct, deviate
somewhat from the standard template given here.Example 3-27. Simple while loop#!/bin/bash
var0=0
while [ "$var0" -lt 10 ]
do
echo -n "$var0 "
# -n suppresses newline.
var0=`expr $var0 + 1`
# var0=$(($var0+1)) also works.
done
echo
exit 0Example 3-28. Another while loop#!/bin/bash
while [ "$var1" != end ]
do
echo "Input variable #1 "
echo "(end to exit)"
read var1
# It's not 'read $var1'
# because value of var1 is set.
echo "variable #1 = $var1"
# Need quotes because of #
done
# Note: Echoes 'end' because
# termination condition
# tested for at top of loop.
exit 0untilThis construct tests for a condition at the top of a loop, and keeps
looping as long as that condition is false (opposite of
while loop).until [condition-is-true] do command... done Note that an until loop tests for the
terminating condition at the top of the loop, differing from a
similar construct in some programming languages.As is the case with for/in loops, placing the
do on the same line as the condition test
requires a semicolon.until [condition-is-true] ; do Example 3-29. until loop#!/bin/bash
until [ "$var1" = end ]
# Tests condition at top of loop.
do
echo "Input variable #1 "
echo "(end to exit)"
read var1
echo "variable #1 = $var1"
done
exit 0break, continueThe 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 of the loop, skipping all the
remaining commands in that particular loop cycle.Example 3-30. Effects of break and
continue in a loop#!/bin/bash
echo
echo Printing Numbers 1 through 20.
a=0
while [ $a -le 19 ]
do
a=$(($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 "
done
# Exercise for reader:
# 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 [ $a -le 19 ]
do
a=$(($a+1))
if [ $a -gt 2 ]
then
break
# Skip entire rest of loop.
fi
echo -n "$a "
done
echo
echo
exit 0case (in) / esacThe case construct is the shell equivalent
of 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... ;; "$condition2" ) command... ;; esac Note: Quoting the variables is recommended.Each test line ends with a left paren ).Each condition block ends with a double
semicolon ;;.The entire case block terminates with an
esac (case spelled
backwards).
Example 3-31. Using case#!/bin/bash
echo
echo "Hit a key, then hit return."
read Keypress
case "$Keypress" in
[a-z] ) echo "Lowercase letter";;
[A-Z] ) echo "Uppercase letter";;
[0-9] ) echo "Digit";;
* ) echo "Punctuation, whitespace, or other";;
esac
# Allows ranges of characters in [square brackets].
exit 0Example 3-32. Creating menus using case#!/bin/bash
# Crude rolodex-type 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 "[Smith], 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 "Girlfriend"
echo "Birthday: Feb. 11"
;;
# Add info for Smith & Zane later.
* )
# Default option.
echo
echo "Not yet in database."
;;
esac
echo
exit 0selectThe 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 3-33. 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 $vegetable."
echo "Yuck!"
echo
break
# if no 'break' here, keeps looping forever.
done
exit 0If in list is
omitted, then select uses the list of command
line arguments ($@) 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 3-34. 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 03.9. Internal Commands and BuiltinsA builtin is a command contained in the bash tool
set, literally built in.getoptsThis powerful tool parses command line arguments passed to the
script. This is the bash analog of the getopt
library function familiar to C programmers. It permits passing and
concatenating multiple flags[1] and options 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 a flag. A colon following the flag
name in the declaration tags that flag as having an option.A getopts construct usually comes packaged
in a while loop, which processes the flags and
options one at a time, then decrements the implicit
$OPTIND variable to step to the next.Note: The arguments must be passed from the command line to the
script preceded by a minus (-) or a plus
(+), else getopts will
not process them, and will, in fact, terminate option
processing at the first argument encountered lacking these
modifiers.The getopts template
differs slightly from the standard while
loop, in that it lacks condition brackets.The getopts
construct replaces the obsolete getopt
command.
while getopts ":abcde:fg" Option
# Initial declaration.
# a, b, c, d, e, f, and g are the flags expected.
# The : after flag 'e' shows it will have an option 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 '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 3-35. Using getopts to read the flags/options
passed to a script#!/bin/bash
# 'getopts' processes command line args to script.
# Usage: scriptname -options
# Note: dash (-) necessary
# Try invoking this script with
# 'scriptname -mn'
# 'scriptname -oq qOption'
# (qOption can be some arbitrary string.)
OPTERROR=33
if [ -z $1 ]
# Exit and complain if no argument(s) given.
then
echo "Usage: `basename $0` options (-mnopqrs)"
exit $OPTERROR
fi
while getopts ":mnopq:rs" Option
do
case $Option in
m ) echo "Scenario #1: option -m-";;
n | o ) echo "Scenario #2: option -$Option-";;
p ) echo "Scenario #3: option -p-";;
q ) echo "Scenario #4: option -q-, with argument \"$OPTARG\"";;
# Note that option 'q' must have an additional argument,
# otherwise nothing happens.
r | s ) echo "Scenario #5: option -$Option-"'';;
* ) echo "Unimplemented option chosen.";;
esac
done
shift $(($OPTIND - 1))
# Decrements the argument pointer
# so it points to next argument.
exit 0exitUnconditionally 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 a good practice to end all
but the simplest scripts with an exit 0,
indicating a successful run.setThe 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 (see Section 3.25). 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 3-36. 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"
echo
set `uname -a`
# Sets the positional parameters to the output
# of the command `uname -a`
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
exit 0unsetThe unset command deletes an
internal script variable. It is a way of negating a previous
set. Note that this command does not affect
positional parameters.exportThe 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 export
command is in startup files, to initialize and make
accessible environmental variables to subsequent user
processes (see Section 3.21). readonlySame 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.basenameStrips 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"
dirnameStrips the basename from a file
name, printing only the path information.Note: basename and dirname
can operate on any arbitrary string. The filename given as an
argument does not need to refer to an existing file.Example 3-37. basename and dirname#!/bin/bash
a=/home/heraclius/daily-journal.txt
echo "Basename of /home/heraclius/daily-journal.txt = `basename $a`"
echo "Dirname of /home/heraclius/daily-journal.txt = `dirname $a`"
exit 0read"Reads" the value of a variable from stdin,
that is, interactively fetches input from the keyboard. The -a
option lets read get array variables (see
Example 3-72).Example 3-38. Variable assignment, using read#!/bin/bash
echo -n "Enter the value of variable 'var1': "
# -n option to echo suppresses newline
read var1
# Note no '$' in front of var1, since it is being set.
echo "var1 = $var1"
# Note that a single 'read' statement can set multiple variables.
echo
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 0The read command may also
"read" its variable value from a file
redirected to stdin (see Section 3.13). If the file contains more
than one line, only the first line is assigned to the
variable. If there is more than one parameter to the
read, then each variable gets assigned
a successive whitespace delineated string. Caution!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 = $var3"
# Note inconsistent behavior of "read" here.
# 1) Rewinds back to the beginning of input file.
# 2) Each variable is now set to a corresponding string, separated by whitespace,
# rather than to an entire line of text.
# 3) The final variable gets the remainder of the line.
# 4) If there are more variables to be set than whitespace-terminated strings
# on the first line of the file, then the excess variable remain unset.trueA command that returns a successful
(zero) exit status, but does nothing else.
# Endless loop
while true
# alias for :
do
operation-1
operation-2
...
operation-n
# Need a way to break out of loop.
donefalseA command that returns an unsuccessful exit status,
but does nothing else.# Null loop
while false
do
# The following code will not execute.
operation-1
operation-2
...
operation-n
# Nothing happens!
done factorFactor an integer into prime factors. bash$ factor 27417
27417: 3 13 19 37
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.
pwdPrint Working Directory. This gives the user's (or script's) current
directory.pushd, popd, dirsThis 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-namepopd 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. 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 3-39. 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 0source, . (dot command), dirsThis command, when invoked from the command line, executes a script. Within
a script, a source file-name loads the file
file-name. This is the
shell scripting equivalent of a C/C++ #include
directive. It is useful in situations when multiple scripts use a
common data file or function library.Example 3-40. "Including" a data file#!/bin/bash
# Load a data file.
. data-file
# Same effect as "source data-file"
# Note that the file "data-file", given below
# must be present in working directory.
# 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 0File data-file for Example 3-40, 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
} 3.9.1. Job Control CommandswaitStop script execution until all jobs running in
background have terminated, or until the job number
specified as an option terminates. Sometimes used to
prevent a script from exiting before a background
job finishes executing (this would create a dreaded
orphan process).Example 3-41. Waiting for a process to finish before proceeding#!/bin/bash
if [ -z $1 ]
then
echo "Usage: `basename $0` find-string"
exit 1
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
# Lacking the wait command, in the worse case scenario,
# the script would exit while 'updatedb' was still running,
# leaving it as an orphan process.
exit 0suspendThis has the same effect as
Control-Z,
pausing a foreground job.stopThis has the same effect as suspend, but
for a background job.disownRemove job(s) from the shell's table of active jobs.jobsLists the jobs running in the background, giving the job number.
Not as useful as ps.timesGives statistics on the system time used in executing commands, in the
following form:
0m0.020s 0m0.020s
This capability is of very limited value, since it is uncommon to
profile and benchmark shell scripts.killForcibly terminate a process by sending it an
appropriate terminate signal. Note
that kill -l lists all the
"signals". (See Section 3.24 for more detail on signals). 3.10. External Filters, Programs and CommandsThis is a descriptive listing of standard UNIX commands useful in shell
scripts.lsThe 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.Example 3-42. Using ls to create a table of contents
for burning a CDR disk#!/bin/bash
# Script to automate burning a CDR.
# Uses Joerg Schilling's "cdrecord" package
# (http://www.fokus.gmd.de/nthp/employees/schilling/cdrecord.html)
# If this script invoked as an ordinary user, need to suid cdrecord
# (chmod u+s /usr/bin/cdrecord, as root).
if [ -z $1 ]
then
IMAGE_DIRECTORY=/opt
# Default directory, if not specified on command line.
else
IMAGE_DIRECTORY=$1
fi
ls -lRF $IMAGE_DIRECTORY > $IMAGE_DIRECTORY/contents
# The "l" option gives a "long" file listing.
# The "R" option makes the listing recursive.
# The "F" option marks the file types (directories suffixed by a /).
echo "Creating table of contents."
mkisofs -r -o cdimage.iso $IMAGE_DIRECTORY
echo "Creating ISO9660 file system image (cdimage.iso)."
cdrecord -v -isosize speed=2 dev=0,0 cdimage.iso
# Change speed parameter to speed of your burner.
echo "Burning the disk."
echo "Please be patient, this will take a while."
exit 0chmodChanges the attributes of a file.chmod +x filename
# Makes "filename" executable for all users.chmod 644 filename
# Makes "filename" readable/writable to owner, readable to
# others
# (octal mode).chmod 1777 directory-name
# Gives everyone read, write, and execute permission in directory,
# however also sets the "sticky bit", which means that
# only the directory owner can change files in the directory.umaskSet the default file attributes (for a particular user).findexec COMMANDCarries out COMMAND on each file
that find scores a hit on.
COMMAND is followed by {} \;
(the ; is escaped to make certain the shell
reads it literally and terminates the command sequence).
This causes COMMAND to bind
to and act on the path name of the files found.
Example 3-43. Badname, eliminate file names
in current directory containing
bad characters and white space.#!/bin/bash
# Delete filenames in current directory containing bad characters.
for filename in *
do
badname=`echo "$filename" | sed -n /[\+\{\;\"\\\=\?~\<\>\&\*\|\$]/p`
# Files containing those nasties: + { ; " \ = ? ~ ( ) < > & * | $
rm $badname 2>/dev/null
# So error messages deep-sixed.
done
# Now, take care of files containing all manner of whitespace.
find . -name "* *" -exec rm -f {} \;
# The "{}" references the paths of all the files that "find" finds.
# The '\' ensures that the ';' is interpreted literally, as end of command.
exit 0See the man page for find for more
detail.xargsA 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 backquotes
fail 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.ls | xargs -p -l gzip gzips every file in
current directory, one at a time, prompting before each operation.One of the more interesting xargs options is
-n XX, which limits
the number of arguments passed to XX.ls | xargs -n 8 echo lists the files in the
current directory in 8 columns.Example 3-44. Log file 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 readable by ordinary users
# if invoked by same (#root chmod 755 /var/log/messages).
( date; uname -a ) >>logfile
# Time and machine name
echo --------------------------------------------------------------------- >>logfile
tail -5 /var/log/messages | xargs | fmt -s >>logfile
echo >>logfile
echo >>logfile
exit 0Example 3-45. copydir, copying files in current
directory to another, using xargs#!/bin/bash
# Copy (verbose) all files in current directory
# to directory specified on command line.
if [ -z $1 ]
# Exit if no argument given.
then
echo "Usage: `basename $0` directory-to-copy-to"
exit 1
fi
ls . | xargs -i -t cp ./{} $1
# This is the exact equivalent of
# cp * $1
exit 0eval arg1, arg2, ...Translates into commands the arguments in a list
(useful for code generation within a script).Example 3-46. Showing the effect of eval#!/bin/bash
y=`eval ls -l`
echo $y
y=`eval df`
echo $y
# Note that LF's not preserved
exit 0Example 3-47. Forcing a log-off#!/bin/bash
y=`eval ps ax | sed -n '/ppp/p' | awk '{ print $1 }'`
# Finding the process number of 'ppp'
kill -9 $y
# Killing it
# Restore to previous state...
chmod 666 /dev/ttyS3
# Doing a SIGKILL on ppp changes the permissions
# on the serial port. Must be restored.
rm /var/lock/LCK..ttyS3
# Remove the serial port lock file.
exit 0expr arg1 operation arg2 ...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 + 5returns 8expr 5 % 3returns 2y=`expr $y + 1`incrementing variable, same as let y=y+1 and
y=$(($y+1)), as discussed elsewherez=`expr substr $string28 $position $length`Note that external programs, such as
sed and Perl have far superior
string parsing facilities, and it might well be advisable to use
them instead of the built-in bash ones.Example 3-48. 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
echo Logical Operators
echo
a=3
echo a = $a
b=`expr $a \> 10`
echo 'b=`expr $a \> 10`, therefore...'
echo "If a > 10, b = 0 (false)"
echo b = $b
b=`expr $a \< 10`
echo "If a < 10, b = 1 (true)"
echo b = $b
echo
echo
# Comparison Operators
echo Comparison Operators
echo
a=zipper
echo a is $a
if [ `expr $a = snap` ]
# Force re-evaluation of variable 'a'
then
echo "a is not zipper"
fi
echo
echo
# String Operators
echo String Operators
echo
a=1234zipper43231
echo The string being operated upon is $a.
# index: position of substring
b=`expr index $a 23`
echo Numerical position of first 23 in $a is $b.
# substr: print 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.
# length: length of string
b=`expr length $a`
echo Length of $a is $b.
# 'match' operations similarly to 'grep'
b=`expr match $a [0-9]*`
echo Number of digits at the beginning of $a is $b.
b=`expr match $a '$[0-9]*$'`
echo The digits at the beginning of $a are $b.
echo
exit 0Note that : can substitute for match.
b=`expr $a : [0-9]*` is an exact equivalent of
b=`expr match $a [0-9]*` in the above example.letThe let command carries out arithmetic
operations on variables. In many cases, it functions as a less
complex version of expr.Example 3-49. Letting let do some arithmetic.#!/bin/bash
echo
let a=11
# Same as 'a=11'
let a=a+5
# Equivalent to let "a = a + 5"
# (double quotes makes it more readable)
echo "a = $a"
let "a <<= 3"
# Equivalent of let "a = a << 3"
echo "a left-shifted 3 places = $a"
let "a /= 4"
# Equivalent to let "a = a / 4"
echo $a
let "a -= 5"
# Equivalent to let "a = a - 5"
echo $a
let "a = a * 10"
echo $a
let "a %= 8"
echo $a
exit 0printfThe printf, formatted print, command is an
enhanced echo. It is a limited variant of the
C language printf, and the syntax is somewhat
different.printf format-string... parameter... See the printf man page for in-depth coverage.Note: Older versions of bash may not
support printf.Example 3-50. 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
# Note correct round off.
printf "\n"
# Prints a line feed, equivalent to 'echo'.
printf "Constant = \t%d\n" $DecimalConstant
# Insert tab (\t)
printf "%s %s \n" $Message1 $Message2
echo
exit 0atThe at job control command executes a given
set of commands at a specified time. This is a user version of
cron.at 2pm January 15 prompts for a set of
commands to execute at that time. These commands may include
executable shell scripts.Using either the -f option or input
redirection (<), at
reads a command list from a file. This file can
include shell scripts, though they should, of course,
be noninteractive. bash$ at 2:30 am Friday < at-jobs.list
job 2 at 2000-10-27 02:30
psLists currently executing jobs by owner and process id. This is usually
invoked with ax options, and may be piped to
grep to search for a specific process.ps ax | grep sendmail results in:
295 ? S 0:00 sendmail: accepting connections on port 25
batchThe 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.sleepThis is the shell equivalent of a wait loop. It pauses for a
specified number of seconds, doing nothing. This can be useful for
timing or in processes running in the background, checking for a
specific event every so often.
sleep 3
# Pauses 3 seconds.
ddThis is the somewhat obscure and much feared "data duplicator"
command. It 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.The dd command can copy raw data and disk
images to and from devices, such as floppies. It can even be used
to create boot floppies.
dd if=kernel-image of=/dev/fd0H1440
One important use for dd is initializing
temporary swap files (see Example 3-79).sortFile sorter, often used as a filter in a pipe. See the man page
for options.diffSimple file comparison utility. The files must be sorted (this may, if
necessary be accomplished by filtering the files through sort before
passing 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.
A common use for diff is to generate difference
files to be used with patch (see below).
The -e option outputs files suitable for ed or
ex scripts.patch -p1 <patch-file
# Takes all the changes listed in 'patch-file' and applies them
# to the files referenced therein.
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?).commVersatile file comparison utility. The files must be sorted for this to be
useful.comm
-options
first-file
second-filecomm file-1 file-2 outputs three columns:
column 1 = lines unique to file-1column 2 = lines unique to file-2column 3 = lines common to both.The options allow suppressing output of one or more columns.
-1 suppresses column
1-2 suppresses column
2-3 suppresses column
3-12 suppresses both columns
1 and 2, etc.
uniqThis 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
expandA filter than converts tabs to spaces, often seen in a pipe.cutA 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:
cat /etc/mtab | cut -d ' ' -f1,2Using cut to list the OS and kernel version:
uname -a | cut -d" " -f1,3,11,12cut -d ' ' -f2,3 filename is equivalent to
awk '{ print $2, $3 }' filename
colrmColumn removal filter. This removes columns (characters) from a file
and writes them, lacking the specified columns, back to stdout.
colrm 2 3 <filename removes the second and
third characters from each line of the text file filename.
pasteTool for merging together different files into a single, multi-column file.
In combination with cut, useful for creating system log files.
joinConsider this a more flexible version of
paste. It works on exactly two files, but
permits specifying which fields to paste together, and in which
order.cpioThis specialized archiving copy command is rarely used any more,
having been supplanted by
tar/gzip. It still has its
uses, such as moving a directory tree.Example 3-51. Using cpio to move a directory tree#!/bin/bash
# Copying a directory tree using cpio.
if [ $# -ne 2 ]
then
echo Usage: `basename $0` source destination
exit 1
fi
source=$1
destination=$2
find "$source" -depth | cpio -admvp "$destination"
exit 0cdThe 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 xvfp -)
[from the previously cited example by Alan Cox]touchUtility 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.
splitUtility for splitting a file into smaller chunks. Usually used
for splitting up large files in order to back them up on floppies or
preparatory to e-mailing or uploading them.rmDelete (remove) a file or files. The -f
forces removal of even readonly files. When used with
the recursive flag -r, this command
removes files all the way down the directory tree (very
dangerous!).rmdirRemove directory. The directory must be empty of all files,
including dotfiles, for this command to succeed.lnCreates links to pre-existings files. Most often used
with the -s, symbolic or
"soft" link flag. This permits referencing
the linked file by more than one name and is a superior
alternative to aliasing.ln -s oldfile newfile
links the previously existing
oldfile to the newly created link,
newfile.mkdirMake directory, creates a new directory.
mkdir -p project/programs/December
creates the named directory. The
-p option automatically creates
any necessary parent directories.cpThis is the file copy command. cp file1
file2 copies file1 to
file2, overwriting
file2 if it already exists.mvThis 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.rcp"Remote copy", copies files between two different networked machines.
Using rcp and similar utilities with security
implications in a shell script may not be advisable. Consider
instead, using an expect script.yesIn 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.echoprints (to stdout) an expression or variable ($variable).
echo Hello
echo $aNormally, each echo command prints a terminal newline, but
the -n option suppresses this.cat, taccat, an acronym for
concatenate,
lists a file to stdout. When combined with redirection
(> or >>), it is commonly
used to concatenate files. cat filename
cat file.1 file.2 file.3 > file.123
The -n option to
cat inserts consecutive numbers before
each line of the target file(s).tac, is the inverse of
cat, listing a file backwards from its end.headlists the first 10 lines of a file to stdout.taillists the end of a file to stdout (the default is
10 lines, but this can be changed).
Commonly used to keep track of changes to a system logfile,
using the -f option, which outputs lines
appended to the file.Example 3-44 and Example 3-83 show
tail in action.tee[UNIX borrows an idea here from the plumbing trade.]This is a redirection operator, but with a difference. Like the
plumber's tee, it permits "siponing
off" 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. tee
|------> to file
|
===============|===============
command--->----|-operator-->---> result of command(s)
===============================
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.)sed, awkmanipulation scripting languages in order to parse text and command
outputsedNon-interactive "stream editor", permits using
many ex commands in batch mode.awkProgrammable file extractor and formatter, good for manipulating and/or
extracting fields (columns) in text files. Its syntax is similar to C.wcwc gives a "word count" on a file or I/O stream:
$ wc /usr/doc/sed-3.02/README
20 127 838 /usr/doc/sed-3.02/README
[20 lines 127 words 838 characters]wc -w gives only the word count.wc -l gives only the line count.wc -c 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
trcharacter translation filter.Note: must use quoting and/or brackets, as appropriate.tr "A-Z" "*" <filename
changes all the uppercase letters in
filename to asterisks (writes to stdout).tr -d [0-9] <filename deletes all digits
from the file filename.Example 3-52. toupper: Transforms a file to all uppercase.#!/bin/bash
# Changes a file to all uppercase.
if [ -z $1 ]
# Standard check whether command line arg is present.
then
echo "Usage: `basename $0` filename"
exit 1
fi
tr [a-z] [A-Z] <$1
exit 0Example 3-53. 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 into bash by Chet Ramey,
# and considerably simplified by Mendel Cooper,
# author of this HOWTO.
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 0foldA filter that wraps inputted lines to a specified width.fmtSimple-minded file formatter.nlLine 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.Example 3-54. nl: A self-numbering script.#!/bin/bash
# This file echoes itself twice to stdout with its lines numbered.
# 'nl' sees this as line 3 since it does not number blank lines.
# 'cat -n' sees the above line as number 5.
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.
exit 0prPrint formatting filter. This will paginate a file (or stdout) into
sections suitable for hard copy printing. A particularly useful option
is -d, forcing double-spacing.Example 3-55. Formatted file listing.#!/bin/bash
# Get a file listing...
b=`ls /usr/local/bin`
# ...40 columns wide.
echo $b | fmt -w 40
# Could also have been done by
# echo $b | fold - -s -w 40
exit 0dateSimply invoked, date prints the date and
time to stdout. Where this command gets interesting is in its
formatting and parsing options.Example 3-56. Using date#!/bin/bash
#Using the 'date' command
# Needs a leading '+' to invoke formatting.
echo "The number of days since the year's beginning is `date +%j`."
# %j gives day of year.
echo "The number of seconds elapsed since 01/01/1970 is `date +%s`."
# %s yields number of seconds since "UNIX epoch" began,
# but how is this useful?
prefix=temp
suffix=`eval date +%s`
filename=$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 0timeOutputs 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.grepA multi-purpose file search tool that uses regular expressions.
Originally a command/filter in the ancient ed line editor, g/re/p, or global - regular expression - print.grep pattern [file...]
search the files file, etc. for
occurrences of pattern.ls -l | grep '.txt' has the same effect as ls -l *.txt.The -i option to
grep causes a case-insensitive
search.Example 3-83 demonstrates how to use
grep to search for a keyword in a system
log file.whichwhich <command> gives the full path
to the command. This is useful for finding out whether
a particular command or utility is installed on the
system.$bash which pgp
/usr/bin/pgp
scriptThis utility records (saves to a file) all the user keystrokes at
the command line in a console or an xterm window. This, in effect,
create a record of a session.tarThe standard UNIX archiving utility. Originally a Tape ARchiving
program, from whence it derived its name, 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. GNU tar has long since been patched to accept
gzip options, see below.gzipThe standard GNU/UNIX compression utility, replacing the inferior and
proprietary compress.sharShell 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.fileA 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.uuencodeThis 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.uudecodeThis reverses the encoding, decoding uuencoded files back into the
original binaries.Example 3-57. uuencoding encoded files#!/bin/bash
lines=35
# Allow 35 lines for the header (very generous).
for File in *
# Test all the files in the current working directory...
do
search1=`head -$lines $File | grep begin | wc -w`
search2=`tail -$lines $File | grep end | wc -w`
# Files which are uuencoded 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
exit 0more, lessPagers that display a text file or text streaming to stdout, one page at a
time.jot, seqThese utilities emit a sequence of integers, with a user selected
increment. This can be used to advantage in a for loop.Example 3-58. Using seq to generate loop arguments#!/bin/bash
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
echo
exit 0clearThe 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 3-32.3.11. System and Administrative CommandsThe startup and shutdown scripts in
/etc/rc.d illustrate the uses
(and usefulness) 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.unameOutput system specifications (OS, kernel version, etc.) to stdout.
Invoked with the -a option, gives verbose system info.uname -a outputs something like:
Linux localhost.localdomain 2.2.15-2.5.0 #1 Sat Feb 5 00:13:43 EST 2000 i586 unknownulimitSets an upper limit on system
resources. Usually invoked with the -f
option, which sets a limit on file size (ulimit
-f 1000 sets a 1-meg limit on files).uptimeShows 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, 0.27envRuns a program or script with certain environmental variables set or changed
(without changing the overall system environment).shoptThis command permits changing shell options on the fly.
Works with version 2 of bash only.
shopt -s cdspell
# Allows misspelling directory names with 'cd' command.lockfileThis 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
permitting 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 create a lock file that already exists,
the script will likely hang.cronAdministrative 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. It runs as a daemon (background
process) and executes scheduled entries from /etc/crontab.
chrootCHange 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). A chroot /opt
would cause references to /usr/bin to be
translated to /opt/usr/bin,
for example. 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. Note that after a chroot, the execution path for
system binaries is no longer valid.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). Invoke only as root, and
use with caution.lddShow 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)whoShow all users logged on to the system.whoami is a variant of
who that lists only the current
user.wShow 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
grendel tty1 - 4:22pm 6:41 4.47s 0.45s startxttyEchoes the name of the current user's terminal.
Note that each separate xterm window counts as a different
terminal.bash# tty
/dev/pts/1wallThis is an acronym for "write all", i.e.,
sending a message to all users every terminal logged on in 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.wall System going down for maintenance in 5 minutes!fuserIdentifies 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.loggerAppends 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'.jobsLists all jobs running in the background.psShows currently running processes owned by user.
Usually invoked as ps ax,
showing all running processes.nohupKeeps 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.freeShows memory and cache usage in tabular form. The output of this command
lends itself to parsing, using grep, awk or Perl.
bash$ free
total used free shared buffers cached
Mem: 30504 28624 1880 15820 1608 16376
-/+ buffers/cache: 10640 19864
Swap: 68540 3128 65412syncForces writing all updated data from buffers to hard drive. 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
was a useful precautionary measure before a system reboot.initThe 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.telinitSymlinked 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!runlevelShows 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).halt, shutdown, rebootCommand set to shut the system down, usually just prior to a power down.execThis is actually a system call that replaces the current process with
a specified command. It is mostly seen in combination with find,
to execute a command on the files found. When used as a standalone in
a script, this forces an exit from the script when the exec'ed command
terminates. An exec is also used to reassign file descriptors.
exec <zzz-file replaces stdin with the file
zzz-file.Example 3-59. Effects of exec#!/bin/bash
exec echo "Exiting $0."
# Exit from script.
# The following lines never execute.
echo "Still here?"
exit 0ifconfigNetwork interface configuration utility.routeShow info about or make changes to the kernel routing table.netstatShow current network information and statistics, such as routing tables and
active connections.mknodCreates block or character device files (may be necessary when installing
new hardware on the system).mountMount a filesystem, usually on an external device, such as a floppy
or CDROM. The file /etc/fstab provides a
handy listing of available filesystems, including options, that
may be automatically or manually mounted. The file
/etc/mtab shows the currently mounted filesystems
(including the virtual ones, such as /proc).
mount -t iso9660 /dev/cdrom /mnt/cdrom
# Mounts CDROM
mount /mnt/cdrom
# Shortcut, if /mnt/cdrom listed in /etc/fstabumountUnmount a currently mounted filesystem. Before physically removing a
previously mounted floppy or CDROM disk, the device must be
umount'ed, else filesystem corruption may result.
umount /mnt/cdromlsmodList installed kernel modules.insmodForce insertion of a kernel module. Must be invoked as root.modprobeModule loader that is normally invoked automatically in a startup script.depmodCreates module dependency file, usually invoked from startup script.rdevGet 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 another dangerous command, if misused.
Using 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 3-60. killall, from /etc/rc.d/init.d#!/bin/sh
# --> Comments added by the author of this HOWTO 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 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, and since 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
# --> using the 'stop' shell builtin.
fi
doneThat wasn't so bad. Aside from a little fancy footwork with variable
matching, there is no new material there.Exercise. In /etc/rc.d/init.d,
analyze the halt script. It is a bit longer than
killall, but similar in concept. Make a copy of this script somewhere in
your home directory and experiment with it (do not run it as root). Do
a simulated run with the -vn flags
(sh -vn scriptname). Add extensive comments. Change
the "action" commands to "echos".Now, look at some of the more complex scripts in
/etc/rc.d/init.d. See if
you can understand parts of them. Follow the above procedure
to analyze them. For some additional insight, you might also
examine the file sysvinitfiles in /usr/doc/initscripts-X.XX, which
is part of the "initscripts" documentation.For those scripts needing a single do-it-all tool, a Swiss army knife,
there is Perl. Perl combines the capabilities of
sed, awk, and throws
in a large subset of C, to boot. It is modular and contains support for
everything ranging from object oriented programming up to and including
the kitchen sink. Short Perl scripts can be effectively embedded in
shell scripts, and there may even be some substance to the claim that
Perl can totally replace shell scripting.Example 3-61. Perl embedded in a bash script#!/bin/bash
perl -e 'print "This is an embedded Perl script\n"'
# Some shell commands may follow.
exit 03.12. Backticks (`...`)Command substitutionCommands within backticks generate command line text.The output of commands within backticks can be used
as arguments to another command or to load a variable.
rm `cat filename`
# "filename" contains a list of files to delete.
textfile_listing=`ls *.txt`
# Variable contains names of all *.txt files in current working directory.
echo $textfile_listing
Incrementing / decrementing variables (commonly used with
expr)z=`expr $z + 3`
Note that this particular use of backticks
has been superseded by double parentheses
$((...)) or the very convenient
let construction.
z=$(($z+3))
let z=z+3
let "z += 3" #If quotes, then spaces and special operators allowed.
All these are equivalent. You may use whichever one
"rings your chimes".
3.13. I/O RedirectionThere 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 the output of a file, command, program,
or script and sending it as input to another file, command,
program, or script.Each open file gets assigned a file descriptor.
[2]
The file descriptors for stdin,
stdout, and stderr
are 0, 1, and 2, respectively. Opening additional files assigns
descriptors 3 to 9. It is sometimes useful to assign one of these
additional file descriptors to stdout or
stderr as a temporary duplicate copy. This
simplifies restoration to normal after complex redirection and
reshuffling. >
# 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.
>>
# Redirect stdout to a file.
# Creates the file if not present, otherwise appends to it.
2> &1
# Redirects stderr to stdout.
# Has the effect of making visible error messages that might otherwise not be seen.
i> &j
# Redirects file descriptor i to j
# All output of file pointed to by i gets sent to file pointed to by j
<
# Accept input from a file.
# Companion command to ">", and often used in combination with it.
grep search-word <filename
|
# 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".Note: 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-filen<&-close input file descriptor
n<&-close stdinn>&-close output file descriptor n>&-close stdout3.14. 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 a something
which will bring you luck and good fortune. Just e-mail a copy
of this document to 10 of your friends. Before you make the
copies, send a 100-line Bash script to the first person on
the list given 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
xbill. 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 3.15. Regular ExpressionsIn order to fully utilize the power of shell scripting, you need to
master regular expressions.3.15.1. A Brief Introduction to Regular ExpressionsAn expression is a set of characters that has an
interpretation above and beyond its literal meaning. 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 that UNIX
endows with special features.The main uses for regular expressions (REs) are text
searches and string manipulation. An RE
matches a single character or a set
of characters.The asterisk * matches any number of
characters, including zero.The dot . matches any one character,
except a newline.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.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 a an
RE matches the end of a line.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-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).The backslash \ escapes a special character,
which means that character gets interpreted literally.A \$ reverts back to its
literal meaning of "dollar sign", rather than
its RE meaning of end-of-line.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
a different special character meaning otherwise.[0-9]\{5\} matches exactly five
digits (characters in the range of 0 to 9).Note: Curly brackets are not available as an RE in
awk."Sed & Awk", by Dougherty and Robbins (see
Bibliography) gives a very complete and lucid
treatment of REs.3.15.2. Using REs in scriptsSed, awk, and Perl, used as filters in scripts, take REs as arguments
when "sifting" or transforming files or I/O streams.3.16. SubshellsRunning 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.( command1; command2; command3; ... )A command list embedded between
parentheses runs as a
subshell.Note: Variables in a subshell are not
visible outside the block of code in the subshell. These are,
in effect, local variables.Example 3-62. Variable scope in a subshell#!/bin/bash
echo
outer_variable=Outer
(
inner_variable=Inner
echo "From subshell, \"inner_variable\" = $inner_variable"
echo "From subshell, \"outer\" = $outer_variable"
)
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".
echo
exit 0Example 3-63. 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.
wait #Don't execute the next command until subshells finish.
diff list123 list456
Note: A command block between curly
braces does not launch
a subshell.{ command1; command2; command3; ... }3.17. 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)><(command)These initiate process substitution. This uses a
named pipe (temp file) to send the results
of the process within parentheses to another process.Note: There are no spaces between the
parentheses and the "<" or ">".
Space there would simply cause redirection from a subshell,
rather than 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'.
3.18. FunctionsLike "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. Whenever there is
repetitive code, when a task repeats with only slight variations,
then writing a function should be investigated.function function-name { command... }
or
function-name () { command... }
This second form will cheer the hearts of C programmers.The opening bracket in the function may optionally be placed on the
second line, to more nearly resemble C function syntax. function-name () { command... }
Functions are called, triggered, simply by
invoking their names.Note that the function definition must precede the first
call to it. There is no method of "declaring"
the function, as, for example, in C.Example 3-64. Simple function#!/bin/bash
funky ()
{
echo This is a funky function.
echo Now exiting funky function.
}
# Note: function must precede call.
# Now, call the function.
funky
exit 0More complex functions may have arguments passed to them
and return exit values to the script for further
processing.function-name $arg1 $arg2The function refers to the passed arguments by position (as if they were
positional parameters), that is, $1,
$2, and so forth.Example 3-65. Function Taking Parameters#!/bin/bash
func2 () {
if [ -z $1 ]
# Checks if any params.
then
echo "No parameters passed to function."
return 0
else
echo "Param #1 is $1."
fi
if [ $2 ]
then
echo "Parameter #2 is $2."
fi
}
func2
# Called with no params
echo
func2 first
# Called with one param
echo
func2 first second
# Called with two params
echo
exit 0Note: In contrast to certain other programming languages,
shell scripts permit passing only value parameters to functions.
Variable names (which are actually pointers), if passed as
parameters to functions, will be treated as string literals
and cannot be dereferenced. Functions interpret their
arguments literally.exit statusFunctions 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
$?.returnTerminates a function. The return statement
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 3-66. 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 for the reader.
# Usage: roman number-to-convert
ARG_ERR=1
OUT_OF_RANGE=200
if [ -z $1 ]
then
echo "Usage: `basename $0` number-to-convert"
exit $ARG_ERR
fi
num=$1
if [ $num -gt $OUT_OF_RANGE ]
then
echo "Out of range!"
exit $OUT_OF_RANGE
fi
to_roman ()
{
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
}
# Note: must declare function
# before first call to it.
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 0local variablesA variable declared as local is one
that is visible only within the block of code in which it
appears. In a shell script, this means the variable has
meaning only within its own function.Example 3-67. Local variable visibility#!/bin/bash
func ()
{
local a=23
echo
echo "a in function is $a"
echo
}
func
# Now, see if local 'a'
# exists outside function.
echo "a outside function is $a"
echo
# Nope, 'a' not visible globally.
exit 0Local variables permit recursion (a recursive function
is one that calls itself), but this practice usually
involves much computational overhead and is definitely
not recommended in a shell
script.Example 3-68. Recursion, using a local variable#!/bin/bash
# factorial
# ---------
# Does bash permit recursion?
# Well, yes, but...
# You gotta have rocks in your head to try it.
MAX_ARG=5
WRONG_ARGS=1
RANGE_ERR=2
if [ -z $1 ]
then
echo "Usage: `basename $0` number"
exit $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 $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
else
let "decrnum = number - 1"
fact $decrnum # Recursive function call.
let "factorial = $number * $?"
fi
return $factorial
}
fact $1
echo "Factorial of $1 is $?."
exit 03.19. List ConstructsThe "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. Note that the exit status
of an "and list" or an "or list" is
the exit status of the last command executed.and listcommand-1 && command-2 && command-3 && ... command-n
Each command executes in turn provided that the previous command
has given a return value of true. At the
first false return,
the command chain terminates (the first command returning false
is the last one to execute).Example 3-69. 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 to script."
# All the chained commands return true.
else
echo "Less than 2 arguments 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. This is a bug, not a feature.
# This accomplishes the same thing, coded 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 to script."
else
echo "Less than 2 arguments to script."
fi
# It's longer and less elegant than using an "and list".
exit 0or listcommand-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 3-70. Using "or lists" in combination with an "and list"#!/bin/bash
# "Delete", not-so-cunning file deletion utility.
# Usage: delete filename
if [ -z $1 ]
then
file=nothing
else
file=$1
fi
# Fetch file name (or "nothing") for deletion message.
[ ! -f $1 ] && echo "$1 not found. Can't delete a nonexistent file."
# AND LIST, to give error message if file not present.
[ ! -f $1 ] || ( rm -f $1; echo "$file deleted." )
# OR LIST, to delete file if present.
# ( command1 ; command2 ) is, in effect, an AND LIST variant.
# Note logic inversion above.
# AND LIST executes on true, OR LIST on false.
[ ! -z $1 ] || echo "Usage: `basename $0` filename"
# OR LIST, to give error message if no command line arg (file name).
exit 0Clever combinations of "and" and "or"
lists are possible, but the logic may easily become convoluted and
require extensive debugging.3.20. ArraysNewer versions of bash support one-dimensional
arrays. Arrays may be declared with the variable[xx]
notation or explicitly by a declare -a variable
statement. To dereference (find the contents of) an array variable, use
curly bracket notation, that is, ${variable[xx]}.Example 3-71. Simple array usage#!/bin/bash
area[11]=23
area[13]=37
area[51]=UFOs
# Note that array members need not be consecutive
# or contiguous.
# Some members of the array can be left uninitialized.
# Gaps in the array are o.k.
echo -n "area[11] = "
echo ${area[11]}
echo -n "area[13] = "
echo ${area[13]}
# Note that {curly brackets} needed
echo "Contents of area[51] are ${area[51]}."
# Contents of uninitialized array variable print blank.
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 doesn't work because
# adding an integer to a string is not permitted.
exit 0Arrays variables have a syntax all their own, and even standard bash operators
have special options adapted for array use.Example 3-72. Some special properties of arrays#!/bin/bash
declare -a colors
# Permits declaring an array without specifying size.
echo "Enter your favorite colors (separated from each other by a space)."
read -a colors
# Special option to 'read' command,
# allowing it to assign elements in an array.
echo
element_count=${#colors[@]} # Special syntax to extract number of elements in array.
# element_count=${#colors[*]} works also.
index=0
# List all the elements in the array.
while [ $index -lt $element_count ]
do
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]} "
echo
# Again, list all the elements in the array, but using a more elegant method.
echo ${colors[@]}
# echo ${colors[*]} works also.
echo
exit 0As 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[*]}.--Arrays permit deploying old familiar algorithms as shell scripts.
Whether this is necessarily a good idea is left to the reader to
decide.Example 3-73. An old friend:
The Bubble Sort#!/bin/bash
# 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.
Countries=(Netherlands Ukraine Zair Turkey Russia Yemen Syria Brazil Argentina Nicaragua Japan Mexico Venezuela Greece England Israel Peru Canada Oman Denmark Wales France Kashmir Qatar Liechtenstein Hungary)
# Couldn't think of one starting with X (darn).
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.
then
exchange $index `expr $index + 1` # Swap.
fi
let "index += 1"
done # End of inner loop
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--Arrays enable implementing a shell script version of the Sieve of
Erastosthenes. 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 3-74. Complex array application:
Sieve of Erastosthenes#!/bin/bash
# sieve.sh
# Sieve of Erastosthenes
# Ancient algorithm for finding prime numbers.
# This runs a couple of orders of magnitude
# slower than equivalent C program.
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.
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
}
# Invoke the functions sequentially.
initialize
sift
print_primes
echo
# This is what they call structured programming.
exit 03.21. Files/etc/profilesystem-wide defaults (all shells, not just Bash)$HOME/.bashrcuser-specific Bash init file, found in each user's home
directoryThese are the startup files for Bash. They
contain the aliases and environmental variables made available
to Bash running as a user shell and to all Bash scripts invoked
after system initialization.3.22. Here DocumentsA here document is a way of feeding a command
script to an interactive program, such as ftp,
telnet, or ex. Typically, it
consists of a command list to the program, delineated by a limit
string. The special symbol << precedes the limit string.
This has the same effect as redirecting the output of a file into the
program, that is,
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
...
LimitStringChoose 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.Example 3-75. dummyfile: Creates a 2-line dummy file#!/bin/bash
# Non-interactive use of 'vi' to edit a file.
# Emulates 'sed'.
if [ -z $1 ]
then
echo "Usage: `basename $0` filename"
exit 1
fi
TARGETFILE=$1
vi $TARGETFILE <<x23LimitStringx23
i
This is line 1 of the example file.
This is line 2 of the example file.
^[
ZZ
x23LimitStringx23
# Note that ^[ above is a literal escape
# typed by Control-V Escape
exit 0The 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.Example 3-76. broadcast: Sends message to everyone logged in#!/bin/bash
wall <<zzz23EndOfMessagezzz23
Dees ees a message frrom Central Headquarters:
Do not keel moose!
# Other message text goes here.
# Note: Comment lines printed by 'wall'.
zzz23EndOfMessagezzz23
# Could have been done more efficiently by
# wall <message-file
exit 0Example 3-77. 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
exit 0Example 3-78. upload: Uploads a file pair to "Sunsite"
incoming directory#!/bin/bash
# upload
# upload file pair (filename.lsm, filename.tar.gz)
# to incoming directory at Sunsite
if [ -z $1 ]
then
echo "Usage: `basename $0` filename"
exit 1
fi
Filename=`basename $1`
# Strips pathname out of file name
Server="metalab.unc.edu"
Directory="/incoming/Linux"
# These need not be hard-coded into script,
# 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 0Note: Some utilities will not work in a
"here document". The pagers, more and
less are among these.For those tasks too complex for a "here document",
consider using the expect scripting language, which
is specifically tailored for feeding input into non-interactive programs.3.23. Of Zeros and NullsUses of /dev/nullThink 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 or stderr (from Example 3-80):
rm $badname 2>/dev/null
# So error messages [stderr] deep-sixed.
Deleting contents of a file, but preserving the file itself, with
all attendant permissions (from Example 2-1 and Example 2-2):
cat /dev/null > /var/log/messages
cat /dev/null > /var/log/wtmp
Automatically emptying the contents of a log file (especially good for
dealing with those nasty "cookies" sent by Web commercial sites):
rm -f ~/.netscape/cookies
ln -s /dev/null ~/.netscape/cookies
# All cookies now get sent to a black hole, rather than saved to disk.
Uses of /dev/zeroLike /dev/null,
/dev/zero is a pseudo file, but it actually
contains nulls (numerical zeros, not the ASCII kind). Output written to it
disappears, and it is fairly difficult to actually read the nulls in
/dev/zero, though it can be done with od or
a hex editor. The chief use for /dev/zero is
in creating an initialized dummy file of specified length intended
as a temporary swap file.Example 3-79. Setting up a swapfile using /dev/zero#!/bin/bash
# Creating a swapfile.
# This script must be run as root.
FILE=/swap
BLOCKSIZE=1024
PARAM_ERROR=33
SUCCESS=0
if [ -z $1 ]
then
echo "Usage: `basename $0` swapfile-size"
# Must be at least 40 blocks.
exit $PARAM_ERROR
fi
dd if=/dev/zero of=$FILE bs=$BLOCKSIZE count=$1
echo "Creating swapfile of size $1 blocks (KB)."
mkswap $FILE $1
swapon $FILE
echo "Swapfile activated."
exit $SUCCESS3.24. DebuggingThe Bash shell contains no debugger, nor even any debugging-specific
commands or constructs. Syntax errors or outright typos in the script
generate cryptic error messages that are often of no help in debugging
a non-functional script.Example 3-80. test23, a buggy script#!/bin/bash
a=37
if [$a -gt 27 ]
then
echo $a
fi
exit 0Output from script:
./test23: [37: command not found
What's wrong with the above script (hint: after the if)?What if the script executes, but does not work as expected? This is the
all too familiar logic error.Example 3-81. test24, another buggy script#!/bin/bash
# This is supposed to delete all filenames
# containing embedded spaces in current directory,
# but doesn't. Why not?
badname=`ls | grep ' '`
# echo "$badname"
rm "$badname"
exit 0To find out what's wrong with Example 3-81, uncomment the
echo "$badname" line. Echo statements are useful for
seeing whether what you expect is actually what you get.Summarizing the symptoms of a buggy script,
It bombs with an error message syntax error, orIt runs, but does not work as expected
(logic error)It runs, works as expected, but has nasty side effects
(logic bomb).
Tools for debugging non-working scripts include
echo statements at critical points in the script to trace the variables,
and otherwise give a snapshot of what is going on.using the tee filter to check processes or
data flows at critical points.setting option flags -n -v -xsh -n scriptname checks for syntax errors
without actually running the script. This is the equivalent of
inserting set -n or
set -o noexec into the script. Note that
certain types of syntax errors can slip past this check.sh -v scriptname echoes each command before
executing it. This is the equivalent of inserting set -v or
set -o verbose in the script.sh -x scriptname echoes the result each
command, but in an abbreviated manner. This is the equivalent of
inserting set -x or
set -o xtrace in the script.Inserting set -u or
set -o nounset in the script runs it, but
gives an unbound variable error message
at each attempt to use an undeclared variable.trapping at exitThe exit command in a script actually sends a
signal 0, terminating
the process, that is, the script itself. It is often useful to trap
the exit, forcing a "printout" of
variables, for example. The trap must be the
first command in the script.
trapSpecifies an action on receipt of a signal; also useful for debugging.
Note: A signal is simply
a message sent to a process, either by the kernel or
another process, telling it to take some specified
action (usually to terminate). For example, hitting a
Control-C,
sends a user interrupt, an INT signal, to a running
program.
trap 2 #ignore interrupts (no action specified)
trap 'echo "Control-C disabled."' 2
Example 3-82. Trapping at exit#!/bin/bash
trap 'echo Variable Listing --- a = $a b = $b' EXIT
# EXIT is the name of the signal generated upon exit from a script.
a=39
b=36
exit 0
# Note that commenting out the 'exit' command makes no difference,
# since the script exits anyhow after running out of commands.Example 3-83. Cleaning up after Control-C#!/bin/bash
# logon.sh
# A quick 'n dirty script to check whether you are on-line yet.
TRUE=1
LOGFILE=/var/log/messages
# Note that $LOGFILE must be readable (chmod 644 /var/log/messages).
TEMPFILE=temp.$$
# Create a "unique" temp file name, using process id of the script.
KEYWORD=address
# At logon, the line "remote IP address xxx.xxx.xxx.xxx" appended to /var/log/messages.
ONLINE=22
USER_INTERRUPT=13
trap 'rm -f $TEMPFILE; exit $USER_INTERRUPT' TERM INT
# Cleans up the temp file if script interrupted by control-c.
echo
while [ $TRUE ] #Endless loop.
do
tail -1 $LOGFILE> $TEMPFILE
# Saves last line of system log file as temp file.
search=`grep $KEYWORD $TEMPFILE`
# Checks for presence of the "IP address" phrase,
# indicating a successful logon.
if [ ! -z "$search" ] # Quotes necessary because of possible spaces.
then
echo "On-line"
rm -f $TEMPFILE # Clean up temp file.
exit $ONLINE
else
echo -n "." # -n option to echo suppresses newline,
# so you get continuous rows of dots.
fi
sleep 1
done
# Note: if you change the KEYWORD variable to "Exit",
# this script can be used while on-line to check for an unexpected logoff.
# Exercise: Change the script, as per the above note,
# and prettify it.
exit 0Note: trap '' SIGNAL (two adjacent
apostrophes) disables SIGNAL for the remainder of the
script. trap SIGNAL restores
the functioning of SIGNAL once more. This is useful to
protect a critical portion of a script from an undesirable
interrupt. trap '' 2 # Signal 2 is Control-C, now disabled.
command
command
command
trap 2 # Reenables Control-C
3.25. OptionsOptions are settings that change shell and/or script
behavior.The set command (see Section 3.9) enables options within a script. At the
point in the script where you want the options to take effect,
use set -o option-name or, in short form,
set -option-abbrev. These two forms are
equivalent. #!/bin/bash
set -o verbose
# Echoes all commands before executing.
#!/bin/bash
set -v
# Exact same effect as above.
Note: To disable an option within a script,
use set +o option-name or set
+option-abbrev. #!/bin/bash
set -o verbose
# Command echoing on.
command
...
command
set +o verbose
# Command echoing off.
command
# Not echoed.
set -v
# Command echoing on.
command
...
command
set +v
# Command echoing off.
command
exit 0
An alternate method of enabling script options is to invoke
the script with the option(s) from the command line. Some
options are available only this way. Among these are
-i, force script to run interactive,
and -r, restricted. bash -v script-name
or
bash -o verbose script-name
The following is a listing of some useful options. They may be
specified in either abbreviated form or by complete name.Table 3-1. bash optionsAbbreviationNameEffect-CnoclobberPrevent overwriting of files by redirection (may be
overridden by >|)-aallexportExport all defined variables-bnotifyNotify when jobs running in background terminate (not of
much use in a script)-fnoglobFilename expansion disabled-pprivilegedScript runs as "suid" (caution!)-unounsetAttempt to use undefined variable
outputs error message-vverbosePrint each command to stdout before executing it-xxtraceSimilar to -v, but expands commands-eerrexitAbort script at first error (when a command exits with
non-zero status)-nnoexecRead commands in script, but do not execute them-t(none)Exit after first command-(none)End of options flag. All other arguments
are positional parameters.--(none)Unset positional parameters.
If arguments given (--arg1arg2),
positional parameters set to arguments.3.26. GotchasAssigning reserved words or characters to variable names.
var1=case
# Causes problems.
var2=23skidoo
# Also problems. Variable names starting with a digit are reserved by the shell.
# Try var2=_23skidoo. Starting variables with an underscore is o.k.
var3=xyz((!*
# Causes even worse problems.
Using a hyphen or other reserved characters in a variable name.
var-1=23
# Use 'var_1' instead.
Using white space inappropriately (in contrast to other programming
languages bash can be finicky about white space).
var1 = 23
# 'var1=23' is correct.
let c = $a - $b
# 'let c=$a-$b' or 'let "c = $a - $b"' are correct.
if [ $a -le 5]
# 'if [ $a -le 5 ]' is correct.
Using uninitialized variables (that is, using variables before a value is
assigned to them). An uninitialized variable has a value of
"null", not zero.Mixing up = and -eq in
a test. Remember, = is for comparing literal
variables and -eq is for numbers.
if [ $a = 273 ] # Wrong!
if [ $a -eq 273 ] # Correct.
Commands issued from a script may fail to execute because the script owner
lacks execute permission for them. If a user cannot invoke a command from
the command line, then putting it into a script will likewise fail. Try
changing the attributes of the command in question, perhaps setting the
suid bit (as root, of course).Using bash version 2 functionality (see below)
in a script headed with #!/bin/bash may cause a
bailout with error messages. Your system may still have an older version
of bash as the default installation. Try changing the header of the
script to #!/bin/bash2.Making scripts "suid" is generally a bad idea, as it
may compromise system security. Administrative scripts should be run by
root, not regular users.3.27. Miscellany Nobody really knows what the Bourne shell's grammar is. Even
examination of the source code is little help.
--Tom Duff 3.27.1. Interactive and non-interactive scriptsAn interactive script is one that
requires input from the user, usually with
read statements (see Example 3-38).Init and startup scripts are necessarily non-interactive, since
they must run without human intervention. Many administrative
and system maintenance scripts are likewise non-interactive.
Unvarying repetitive tasks may usually be automated by
non-interactive scripts.Non-interactive scripts can run in the background, but
interactive ones hang, waiting for input that never comes.
Handle that difficulty by having an expect
script or embedded here document
(see Section 3.22) feed input to an interactive
script running as a background job. In the simplest case,
redirect a file to supply input to a read
statement (read variable <file).
These particular workarounds make possible general purpose
scripts that run in either interactive or non-interactive
modes.If a script needs to test whether it is running in interactive
mode, it is simply a matter of finding whether the
prompt variable,
$PS1 is set. (If the user is being
prompted for input, it is, of course, necessary to display
the prompt.)
if [-z $PS1] # no prompt?
then
# non-interactive
...
else
# interactive
...
fi3.27.2. OptimizationsMost shell scripts are quick 'n dirty solutions to non-complex
problems. As such, optimizing them for speed is not much of an
issue. Consider the case, though, where a script carries out
an important task, does it well, but runs too slowly. Rewriting
it in a compiled language may not be a palatable option. The
simplest fix would be to rewrite the parts of the script
that slow it down. Is it possible to apply principles of code
optimization even to a lowly shell script?Check the loops in the script. Time consumed by repetitive
operations adds up quickly. Use the time and
times tools to profile computation-intensive
commands. Consider rewriting time-critical code sections in C, or
even in assembler.Try to minimize file i/o. Bash is not particularly efficient at
handling files, so consider using more appropriate tools for
this within the script, such as awk or Perl.Try to write your scripts in a structured, coherent form, so
they can be reorganized and tightened up as necessary. Some of the
optimization techniques applicable to high-level languages may work
for scripts, but others, such as loop unrolling, are mostly
irrelevant. Above all, use common sense.3.27.3. Assorted TipsTo keep a record of which user scripts have run
during a particular sesssion or over a number of sessions,
add the following lines to each script you want to keep track
of. This will keep a continuing file record of the script
names and invocation times. # Append (>>) following to end of save file.
date>> $SAVE_FILE #Date and time.
echo $0>> $SAVE_FILE #Script name.
echo>> $SAVE_FILE #Blank line as separator.
# Of course, SAVE_FILE defined and exported as environmental variable in ~/.bashrc
# (something like ~/.scripts-run)
3.28. Bash, version 2The current version of bash, the one you have
running on your machine, is actually version 2. This update of the
classic bash scripting language added array
variables, string and parameter expansion, and a better method
of indirect variable references, among other features.Example 3-84. String expansion#!/bin/bash
# String expansion.
# Introduced in version 2 of bash.
# Strings of the form $'xxx'
# have the standard escaped characters interpreted.
echo $'Ringing bell 3 times \a \a \a'
echo $'Three form feeds \f \f \f'
echo $'10 newlines \n\n\n\n\n\n\n\n\n\n'
exit 0Example 3-85. Indirect variable references - the new way#!/bin/bash
# Indirect variable referencing.
# This has a few of the attributes of references in C++.
a=letter_of_alphabet
letter_of_alphabet=z
# Direct reference.
echo "a = $a"
# Indirect reference.
echo "Now a = ${!a}"
# The ${!variable} notation is greatly superior to the old "eval var1=\$$var2"
echo
t=table_cell_3
table_cell_3=24
echo "t = ${!t}"
table_cell_3=387
echo "Value of t changed to ${!t}"
# Useful for referencing members
# of an array or table,
# or for simulating a multi-dimensional array.
# An indexing option would have been nice (sigh).
exit 0Example 3-86. Using arrays and other miscellaneous trickery
to deal four random hands from a deck of cards#!/bin/bash2
# Must specify version 2 of bash, else might not work.
# Cards:
# deals four random hands from a deck of cards.
UNPICKED=0
PICKED=1
DUPE_CARD=99
LOWER_LIMIT=0
UPPER_LIMIT=51
CARDS_IN_SUITE=13
CARDS=52
declare -a Deck
declare -a Suites
declare -a Cards
# It would have been easier and more intuitive
# with a single, 3-dimensional array. Maybe
# a future version of bash will support
# multidimensional arrays.
initialize_Deck ()
{
i=$LOWER_LIMIT
until [ $i -gt $UPPER_LIMIT ]
do
Deck[i]=$UNPICKED
let "i += 1"
done
# Set each card of "Deck" as unpicked.
echo
}
initialize_Suites ()
{
Suites[0]=C #Clubs
Suites[1]=D #Diamonds
Suites[2]=H #Hearts
Suites[3]=S #Spades
}
initialize_Cards ()
{
Cards=(2 3 4 5 6 7 8 9 10 J Q K A)
# Alternate method of initializing array.
}
pick_a_card ()
{
card_number=$RANDOM
let "card_number %= $CARDS"
if [ ${Deck[card_number]} -eq $UNPICKED ]
then
Deck[card_number]=$PICKED
return $card_number
else
return $DUPE_CARD
fi
}
parse_card ()
{
number=$1
let "suite_number = number / CARDS_IN_SUITE"
suite=${Suites[suite_number]}
echo -n "$suite-"
let "card_no = number % CARDS_IN_SUITE"
Card=${Cards[card_no]}
printf %-4s $Card
# Print cards in neat columns.
}
seed_random ()
{
# Seed random number generator.
seed=`eval date +%s`
let "seed %= 32766"
RANDOM=$seed
}
deal_cards ()
{
echo
cards_picked=0
while [ $cards_picked -le $UPPER_LIMIT ]
do
pick_a_card
t=$?
if [ $t -ne $DUPE_CARD ]
then
parse_card $t
u=$cards_picked+1
# Change back to 1-based indexing (temporarily).
let "u %= $CARDS_IN_SUITE"
if [ $u -eq 0 ]
then
echo
echo
fi
# Separate hands.
let "cards_picked += 1"
fi
done
echo
return 0
}
# Structured programming:
# entire program logic modularized in functions.
#================
seed_random
initialize_Deck
initialize_Suites
initialize_Cards
deal_cards
exit 0
#================
# Exercise 1:
# Add comments to thoroughly document this script.
# Exercise 2:
# Revise the script to print out each hand sorted in suites.
# You may add other bells and whistles if you like.
# Exercise 3:
# Simplify and streamline the logic of the script.Chapter 4. CreditsPhilippe Martin
translated this document into DocBook/SGML. While not on
the job at a small French company as a software developer,
he enjoys working on GNU/Linux documentation and software,
reading literature, playing music, and for his peace of mind
making merry with friends. You may run across him somewhere
in France or in the Basque Country, or email him at feloy@free.fr.I would like to especially thank Patrick
Callahan, Mike Novak, and
Pal Domokos for catching bugs, pointing
out ambiguities, and for suggesting clarifications and changes.
Their lively discussion of shell scripting and general documentation
issues inspired me to try to make this HOWTO more readable.Others making helpful suggestions were Gabor Kiss and Leopold
Toetsch.BibliographyDale Dougherty and Arnold Robbins, Sed and Awk, 2nd edition, O'Reilly and Associates, 1997, 1-156592-225-5. To unfold the full power of shell scripting, you need at least a passing
familiarity with sed and
awk. This is the standard tutorial. It
includes an excellent introduction to "regular expressions". Read this
book.*Aeleen Frisch, Essential System Administration, 2nd edition, O'Reilly and Associates, 1995, 1-56592-127-5.This excellent sys admin manual has a decent introduction to shell
scripting for sys administrators and does a nice job of explaining the
startup and initialization scripts. The book is long overdue for a third
edition (are you listening, Tim O'Reilly?).*Stephen Kochan and Patrick Woods, Unix Shell Programming, Hayden, 1990, 067248448X.The standard reference, though a bit dated by now.*Cameron Newham and Bill Rosenblatt, Learning the Bash Shell, 2nd edition, O'Reilly and Associates, 1998, 1-56592-347-2.This is a valiant effort at a decent shell primer, but somewhat deficient
in coverage on programming topics and lacking sufficient examples.*Anatole Olczak, Bourne Shell Quick Reference Guide, ASP, Inc., 1991, 093573922X.A very handy pocket reference, despite lacking
coverage of Bash-specific features.*Jerry Peek, Tim O'Reilly, and Mike Loukides, Unix Power Tools, 2nd edition, O'Reilly and Associates, Random House, 1997, 1-56592-260-3.Contains a couple of sections of very informative
in-depth articles on shell programming, but falls short of being
a tutorial. It reproduces much of the regular expressions tutorial
from the Dougherty and Robbins book, above.*Ellen Siever, Linux in a Nutshell, 2nd edition, O'Reilly and Associates, 1999, 1-56592-585-8.The all-around best Linux command reference, even has a Bash section.*The O'Reilly books on Perl. (Actually, any O'Reilly books.)The man pages for bash and
bash2, date,
expect, expr,
find, grep,
gzip, ln,
patch, tar, tr,
xargs. The texinfo documentation on
bash, dd, gawk,
and sed.The excellent "Bash Reference Manual", by Chet Ramey and Brian Fox,
distributed as part of the "bash-2-doc" package (available as an rpm).Ben Okopnik's well-written introductory Bash
scripting articles in issues 53, 54, 55, 57, and 58
of the Linux Gazette
.Chet Ramey's Bash F.A.Q.The sed
F.A.Q.Appendix A. Contributed ScriptsThese scripts, while not fitting into the text of this document, do
illustrate some interesting shell programming techniques. They are useful,
too. Have fun analyzing and running them.Example A-1. manview: A script for viewing formatted man pages
#!/bin/bash
# Formats the source of a man page for viewing in a user directory.
# This is useful when writing man page source and you want to
# look at the intermediate results on the fly while working on it.
if [ -z $1 ]
then
echo "Usage: `basename $0` [filename]"
exit 1
fi
groff -Tascii -man $1 | less
# From the man page for groff.
exit 0Example A-2. manview: A script for uploading to an ftp site,
using a locally encrypted password
#!/bin/bash
# Example 3-71 modified to use encrypted password.
if [ -z $1 ]
then
echo "Usage: `basename $0` filename"
exit 1
fi
Username=bozo
# Change to suit.
Filename=`basename $1`
# Strips pathname out of file name
Server="XXX"
Directory="YYY"
# Change above to actual server name & directory.
password=`cruft <pword`
# "pword" is the file containing encrypted password.
# Uses the author's own "cruft" file encryption package,
# based on onetime pad algorithm,
# and obtainable from:
# Primary-site: ftp://metalab.unc.edu /pub/Linux/utils/file
# cruft-0.2.tar.gz [16k]
ftp -n $Server <<End-Of-Session
# -n option disables auto-logon
user $Username $Password
binary
bell
# Ring 'bell' after each file transfer
cd $Directory
put $Filename
bye
End-Of-Session
exit 0+The following two scripts are by Mark Moraes of the University
of Toronto. See the enclosed file "Moraes-COPYRIGHT"
for permissions and restrictions.Example A-3. behead: A script for removing mail and news message headers
#! /bin/sh
# Strips off the header from a mail/News message i.e. till the first
# empty line
# Mark Moraes, University of Toronto
# --> These comments added by author of HOWTO.
if [ $# -eq 0 ]; then
# --> If no command line args present, then works on file redirected to stdin.
sed -e '1,/^$/d' -e '/^[ ]*$/d'
# --> Delete empty lines and all lines until
# --> first one beginning with white space.
else
# --> If command line args present, then work on files named.
for i do
sed -e '1,/^$/d' -e '/^[ ]*$/d' $i
# --> Ditto, as above.
done
fi
# --> Exercise for the reader: Add error checking and other options.
# -->
# --> Note that the small sed script repeats, except for the arg passed.
# --> Does it make sense to embed it in a function? Why or why not?Example A-4. ftpget: A script for downloading files via ftp
#! /bin/sh
# $Id: ftpget,v 1.2 91/05/07 21:15:43 moraes Exp $
# Script to perform batch anonymous ftp. Essentially converts a list of
# of command line arguments into input to ftp.
# Simple, and quick - written as a companion to ftplist
# -h specifies the remote host (default prep.ai.mit.edu)
# -d specifies the remote directory to cd to - you can provide a sequence
# of -d options - they will be cd'ed to in turn. If the paths are relative,
# make sure you get the sequence right. Be careful with relative paths -
# there are far too many symlinks nowadays.
# (default is the ftp login directory)
# -v turns on the verbose option of ftp, and shows all responses from the
# ftp server.
# -f remotefile[:localfile] gets the remote file into localfile
# -m pattern does an mget with the specified pattern. Remember to quote
# shell characters.
# -c does a local cd to the specified directory
# For example,
# ftpget -h expo.lcs.mit.edu -d contrib -f xplaces.shar:xplaces.sh \
# -d ../pub/R3/fixes -c ~/fixes -m 'fix*'
# will get xplaces.shar from ~ftp/contrib on expo.lcs.mit.edu, and put it in
# xplaces.sh in the current working directory, and get all fixes from
# ~ftp/pub/R3/fixes and put them in the ~/fixes directory.
# Obviously, the sequence of the options is important, since the equivalent
# commands are executed by ftp in corresponding order
#
# Mark Moraes (moraes@csri.toronto.edu), Feb 1, 1989
# --> Angle brackets changed to parens, so Docbook won't get indigestion.
#
# --> These comments added by author of HOWTO.
# PATH=/local/bin:/usr/ucb:/usr/bin:/bin
# export PATH
# --> Above 2 lines from original script probably superfluous.
TMPFILE=/tmp/ftp.$$
# --> Creates temp file, using process id of script ($$)
# --> to construct filename.
SITE=`domainname`.toronto.edu
# --> 'domainname' similar to 'hostname'
# --> May rewrite this to parameterize this for general use.
usage="Usage: $0 [-h remotehost] [-d remotedirectory]... [-f remfile:localfile]... \
[-c localdirectory] [-m filepattern] [-v]"
ftpflags="-i -n"
verbflag=
set -f # So we can use globbing in -m
set x `getopt vh:d:c:m:f: $*`
if [ $? != 0 ]; then
echo $usage
exit 1
fi
shift
trap 'rm -f ${TMPFILE} ; exit' 0 1 2 3 15
echo "user anonymous ${USER-gnu}@${SITE} > ${TMPFILE}"
# --> Added quotes (recommended in complex echoes).
echo binary >> ${TMPFILE}
for i in $*
# --> Parse command line args.
do
case $i in
-v) verbflag=-v; echo hash >> ${TMPFILE}; shift;;
-h) remhost=$2; shift 2;;
-d) echo cd $2 >> ${TMPFILE};
if [ x${verbflag} != x ]; then
echo pwd >> ${TMPFILE};
fi;
shift 2;;
-c) echo lcd $2 >> ${TMPFILE}; shift 2;;
-m) echo mget "$2" >> ${TMPFILE}; shift 2;;
-f) f1=`expr "$2" : "$[^:]*$.*"`; f2=`expr "$2" : "[^:]*:$.*$"`;
echo get ${f1} ${f2} >> ${TMPFILE}; shift 2;;
--) shift; break;;
esac
done
if [ $# -ne 0 ]; then
echo $usage
exit 2
fi
if [ x${verbflag} != x ]; then
ftpflags="${ftpflags} -v"
fi
if [ x${remhost} = x ]; then
remhost=prep.ai.mit.edu
# --> Rewrite to match your favorite ftp site.
fi
echo quit >> ${TMPFILE}
# --> All commands saved in tempfile.
ftp ${ftpflags} ${remhost} < ${TMPFILE}
# --> Now, tempfile batch processed by ftp.
rm -f ${TMPFILE}
# --> Finally, tempfile deleted (you may wish to copy it to a logfile).
# --> Exercises for reader:
# --> 1) Add error checking.
# --> 2) Add bells & whistles.Appendix B. CopyrightThe "Advanced Bash-Scripting HOWTO" is copyright,
(c) 2000, by Mendel Cooper. This document may only be
distributed subject to the terms and conditions set forth in
the LDP
License.If this document is incorporated into a printed book, the author
requests a courtesy copy. This is a request, not a requirement.Notes[1]A flag is an argument
that acts as a signal, switching script behaviors on or
off.[2]A file descriptor
is simply a number that the operating system assigns
to an open file to keep track of it. Consider it
a simplified version of a file pointer. It is
analogous to a file handle in
C. �� | 2 | ��

Используются технологии uCoz