Bash-Beginners-Guide

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Real Programmers Bash Guide for Beginners Most programmers will prefer to use the test built-in command, which is equivalent to using square brackets for comparison, like this: test "$(whoami)" != 'root' && (echo you are using a non-privileged account; exit 1) No exit? If you invoke the exit in a subshell, it will not pass variables to the parent. Use { and } instead of ( and ) if you do not want Bash to fork a subshell. See the info pages for Bash for more information on pattern matching with the "(( EXPRESSION ))" and "[[ EXPRESSION ]]" constructs. 7.2. More advanced if usage 7.2.1. if/then/else constructs 7.2.1.1. Dummy example This is the construct to use to take one course of action if the if commands test true, and another if it tests false. An example: freddy scripts> gender="male" freddy scripts> if [[ "$gender" == "f*" ]] More input> then echo "Pleasure to meet you, Madame." More input> else echo "How come the lady hasn't got a drink yet?" More input> fi How come the lady hasn't got a drink yet? freddy scripts> [] vs. [[]] Contrary to [, [[ prevents word splitting of variable values. So, if VAR="var with spaces", you do not need to double quote $VAR in a test - eventhough using quotes remains a good habit. Also, [[ prevents pathname expansion, so literal strings with wildcards do not try to expand to filenames. Using [[, == and != interpret strings to the right as shell glob patterns to be matched against the value to the left, for instance: [[ "value" == val* ]]. Like the CONSEQUENT-COMMANDS list following the then statement, the ALTERNATE-CONSEQUENT-COMMANDS list following the else statement can hold any UNIX-style command that returns an exit status. Another example, extending the one from Section 7.1.2.1: anny ~> su - Password: [root@elegance root]# if ! grep ^$USER /etc/passwd 1> /dev/null > then echo "your user account is not managed locally" > else echo "your account is managed from the local /etc/passwd file" > fi your account is managed from the local /etc/passwd file Chapter 7. Conditional statements 84
[root@elegance root]# We switch to the root account to demonstrate the effect of the else statement - your root is usually a local account while your own user account might be managed by a central system, such as an LDAP server. 7.2.1.2. Checking command line arguments Instead of setting a variable and then executing a script, it is frequently more elegant to put the values for the variables on the command line. We use the positional parameters $1, $2, ..., $N for this purpose. $# refers to the number of command line arguments. $0 refers to the name of the script. The following is a simple example: Bash Guide for Beginners Figure 7-1. Testing of a command line argument with if Here's another example, using two arguments: anny ~> cat weight.sh #!/bin/bash # This script prints a message about your weight if you give it your # weight in kilos and height in centimeters. weight="$1" height="$2" idealweight=$[$height - 110] if [ $weight -le $idealweight ] ; then echo "You should eat a bit more fat." else Chapter 7. Conditional statements 85
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Bash Guide for Beginners Machtelt G
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Bash Guide for Beginners Table of C
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Bash Guide for Beginners Table of C
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Table of Contents Glossary C.......
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Introduction 1. Why this guide? The
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Revision 1.6 2005-03-01 Revised by:
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Bash Guide for Beginners See Chapte
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Bash Guide for Beginners mia:~> cat
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Bash Guide for Beginners When invok
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More information about the file des
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Built-in commands are necessary to
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• Brace expansion • Tilde expan
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A number of methods can be used; on
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Bash Guide for Beginners stop in ea
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Chapter 2. Writing and debugging sc
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the documentation of your favorite
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Bash Guide for Beginners Comments a
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Bash Guide for Beginners + set +x I
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Chapter 3. The Bash environment In
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Bash Guide for Beginners local pid
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zip2 "$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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