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<title>execline: the case command</title>
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<h1> The <tt>case</tt> program </h1>
<p>
<tt>case</tt> compares a value against a series of regular expressions,
and executes into a program depending on the first expression the value
matches.
</p>
<h2> Interface </h2>
<p>
In an <a href="execlineb.html">execlineb</a> script:
</p>
<pre>
case [ -E | -e ] [ -i ] [ -n | -N ] <em>value</em>
{
[ regex { <em>prog...</em> } ]
[ regex { <em>prog...</em> } ]
...
}
<em>progdefault...</em>
</pre>
<ul>
<li> <tt>case</tt> reads an argument <em>value</em> and a sequence of
directives in a <a href="el_semicolon.html">block</a>. </li>
<li> Each directive is a regular expression followed by a block. </li>
<li> <tt>case</tt> matches <em>value</em> against the regular expressions
in the order they are given. </li>
<li> As soon as <em>value</em> matches a <em>regex</em>, <tt>case</tt>
executes into the <em>prog...</em> command line that immediately follows
the matched regex. </li>
<li> If <em>value</em> matches no <em>regex</em>, <tt>case</tt>
eventually execs into <em>progdefault...</em>, or exits 0 if <em>progdefault...</em>
is empty. </li>
</ul>
<h2> Options </h2>
<ul>
<li> <tt>-e</tt> : Interpret the <em>regex</em> words as
<a href="https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_03">basic
regular expressions</a>. </li>
<li> <tt>-E</tt> : Interpret the <em>regex</em> words as
<a href="https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_04">extended
regular expressions</a>. This is the default. </li>
<li> <tt>-i</tt> : Perform case-insensitive matches. </li>
<li> <tt>-N</tt> : Make the matching expression and
subexpressions available to <em>prog</em>'s environment. See the "Subexpression
matching" section below. </li>
<li> <tt>-n</tt> : Do not transmit the matching expression and
subexpressions to <em>prog...</em> via the environment. This is the default. </li>
</ul>
<h2> Subexpression matching </h2>
<p>
If the <tt>-N</tt> option has been given, and <em>value</em> matches a <em>regex</em>,
then <tt>case</tt> will run <em>prog</em> with a modified environment:
</p>
<ul>
<li> The <tt>0</tt> variable will contain the <em>regex</em> that <em>value</em> matched. </li>
<li> The <tt>#</tt> variable will contain the number of subexpressions in <em>regex</em>. </li>
<li> For every integer <em>i</em> between 1 and the number of subexpressions (included), the
variable <em>i</em> contains the part of <em>value</em> that matched the <em>i</em>th subexpression
in <em>regex</em>. </li>
</ul>
<p>
To retrieve that information into your command line in an execline script, you can use the
<a href="elgetpositionals.html">elgetpositionals</a> program.
</p>
<h3> An example </h3>
<p>
Consider the following script; say it's named <tt>match</tt>.
</p>
<pre>#!/bin/execlineb -S1
emptyenv
case -N -- $1
{
"\\([fo]+\\)bar\\(baz\\)" { /usr/bin/env }
}
</pre>
<p>
Running <tt>match foooobarbaz</tt> will print the following lines, corresponding
to the output of the <tt>/usr/bin/env</tt> command:
</p>
<pre>0=\([fo]+\)bar\(baz\)
#=2
1=foooo
2=baz
</pre>
<h2> Notes </h2>
<ul>
<li> <em>value</em> must match <em>regex</em> as a full word. If only a substring of <em>value</em>
matches <em>regex</em>, it is not considered a match. </li>
<li> If <em>value</em> matches no <em>regex</em>, <em>progdefault...</em> is always executed with an
unmodified environment, whether subexpression matching has been requested or not. </li>
</ul>
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