Editing Bash (Unix shell) (section)

=== Brace Expansion ===
Brace expansion, also called alternation, is a feature copied from the [[C shell]]. It generates a set of alternative combinations. Generated results need not exist as files. The results of each expanded string are not sorted and left to right order is preserved:

<syntaxhighlight lang="console">
$ echo a{p,c,d,b}e
ape ace ade abe
$ echo {a,b,c}{d,e,f}
ad ae af bd be bf cd ce cf
</syntaxhighlight>
Users should not use brace expansions in portable shell scripts, because the [[Bourne shell]] does not produce the same output.

<syntaxhighlight lang="console">
$ # bash shell
$/bin/bash -c 'echo a{p,c,d,b}e'
ape ace ade abe
$ # A traditional shell does not produce the same output
$ /bin/sh -c 'echo a{p,c,d,b}e'
a{p,c,d,b}e
</syntaxhighlight>

When brace expansion is combined with wildcards, the braces are expanded first, and then the resulting wildcards are substituted normally. Hence, a listing of JPEG and PNG images in the current directory could be obtained using:

<syntaxhighlight lang="bash">
ls *.{jpg,jpeg,png}    # expands to *.jpg *.jpeg *.png - after which,
                       # the wildcards are processed
echo *.{png,jp{e,}g}   # echo just shows the expansions -
                       # and braces in braces are possible.
</syntaxhighlight>

In addition to alternation, brace expansion can be used for sequential ranges between two integers or characters separated by double dots. Newer versions of Bash allow a third integer to specify the increment.

<syntaxhighlight lang="console">
$ echo {1..10}
1 2 3 4 5 6 7 8 9 10
$ echo {01..10}
01 02 03 04 05 06 07 08 09 10
$ echo file{1..4}.txt
file1.txt file2.txt file3.txt file4.txt
$ echo {a..e}
a b c d e
$ echo {1..10..3}
1 4 7 10
$ echo {a..j..3}
a d g j
</syntaxhighlight>

When brace expansion is combined with variable expansion (A.K.A. ''parameter expansion'' and ''parameter substitution'') the variable expansion is performed ''after'' the brace expansion, which in some cases may necessitate the use of the <code>eval</code> built-in, thus:

<syntaxhighlight lang="console">
$ start=1; end=10
$ echo {$start..$end} # fails to expand due to the evaluation order
{1..10}
$ eval echo {$start..$end} # variable expansion occurs then resulting string is evaluated
1 2 3 4 5 6 7 8 9 10
</syntaxhighlight>

<!--
=== Tilde Expansion ===
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=== Parameter and Variable Expansion ===
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'''+++ Deprecated <code>`...`</code>'''
--><!--
=== Process Substitution (again) ===
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--><!--
=== Arithmetic Expansion ===
'''+++ <code>((..))</code>'''

'''+++ <code>$((...))</code>'''

'''+++ Other Arithmetic Contexts'''

'''+++ Integer Variables'''
--><!--
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