Bash-Beginners-Guide

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Bash Guide for Beginners if [ -o noclobber ] then echo "Your files are protected against accidental overwriting using redirection." fi The environment The above example will work when entered on the command line: anny ~> if [ -o noclobber ] ; then echo ; echo "your files are protected against overwriting." ; echo ; fi your files are protected against overwriting. anny ~> However, if you use testing of conditions that depend on the environment, you might get different results when you enter the same command in a script, because the script will open a new shell, in which expected variables and options might not be set automatically. 7.1.2. Simple applications of if 7.1.2.1. Testing exit status The ? variable holds the exit status of the previously executed command (the most recently completed foreground process). The following example shows a simple test: anny ~> if [ $? -eq 0 ] More input> then echo 'That was a good job!' More input> fi That was a good job! anny ~> The following example demonstrates that TEST-COMMANDS might be any UNIX command that returns an exit status, and that if again returns an exit status of zero: anny ~> if ! grep $USER /etc/passwd More input> then echo "your user account is not managed locally"; fi your user account is not managed locally anny > echo $? 0 anny > The same result can be obtained as follows: anny > grep $USER /etc/passwd anny > if [ $? -ne 0 ] ; then echo "not a local account" ; fi not a local account anny > Chapter 7. Conditional statements 82
Bash Guide for Beginners 7.1.2.2. Numeric comparisons The examples below use numerical comparisons: anny > num=`wc -l work.txt` anny > echo $num 201 anny > if [ "$num" -gt "150" ] More input> then echo ; echo "you've worked hard enough for today." More input> echo ; fi you've worked hard enough for today. anny > This script is executed by cron every Sunday. If the week number is even, it reminds you to put out the garbage cans: #!/bin/bash # Calculate the week number using the date command: WEEKOFFSET=$[ $(date +"%V") % 2 ] # Test if we have a remainder. If not, this is an even week so send a message. # Else, do nothing. if [ $WEEKOFFSET -eq "0" ]; then echo "Sunday evening, put out the garbage cans." | mail -s "Garbage cans out" your@your_domain. fi 7.1.2.3. String comparisons An example of comparing strings for testing the user ID: if [ "$(whoami)" != 'root' ]; then echo "You have no permission to run $0 as non-root user." exit 1; fi With Bash, you can shorten this type of construct. The compact equivalent of the above test is as follows: [ "$(whoami)" != 'root' ] && ( echo you are using a non-privileged account; exit 1 ) Similar to the "&&" expression which indicates what to do if the test proves true, "||" specifies what to do if the test is false. Regular expressions may also be used in comparisons: anny > gender="female" anny > if [[ "$gender" == f* ]] More input> then echo "Pleasure to meet you, Madame."; fi Pleasure to meet you, Madame. anny > Chapter 7. Conditional statements 83
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Bash Guide for Beginners Machtelt G
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Bash Guide for Beginners Table of C
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Bash Guide for Beginners Table of C
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Table of Contents Glossary C.......
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Introduction 1. Why this guide? The
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Revision 1.6 2005-03-01 Revised by:
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Bash Guide for Beginners See Chapte
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Bash Guide for Beginners mia:~> cat
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Bash Guide for Beginners When invok
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More information about the file des
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Built-in commands are necessary to
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• Brace expansion • Tilde expan
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A number of methods can be used; on
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Bash Guide for Beginners stop in ea
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Chapter 2. Writing and debugging sc
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the documentation of your favorite
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Bash Guide for Beginners Comments a
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Bash Guide for Beginners + set +x I
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Chapter 3. The Bash environment In
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Page 73 and 74: sandy ~> sed -n '/a text/,/This/p'
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The commands after && are only exec
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zip2 "$TAR" echo "...done." >> "$LO
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Chapter 12. Catching signals In thi
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Bash Guide for Beginners maud: ~> k
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Bash Guide for Beginners 2. Write a
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Bash Guide for Beginners jobs kill
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if [ $i -eq 5 ] ArrowUp+Enter or !!
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cd Change directory. cdp/cdplay An
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Bash Guide for Beginners gimp grep
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Send and receive mail. man Read man
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Bash Guide for Beginners rsh RPM Pa
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W Bash Guide for Beginners w Show w
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Index A aliases Section 3.5.1 ANSI-
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Bash Guide for Beginners Section 3.
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J Bash Guide for Beginners K kill k
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eadonly Section 10.1.3 regular expr
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Bash Guide for Beginners vi(m) Sect
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