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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 [ -S | -s ] [ -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>-s</tt> : Shell matching. The <em>regex</em> words will not be
interpreted as regular expressions, but as shell expressions to be interpreted
via <a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/fnmatch.html">fnmatch()</a>.
The other options also change meanings, see the <em>Shell matching</em> section below. </li>
<li> <tt>-S</tt> : Regular expression matching. This is the default. This
section, and all of the sections below except the <em>Shell matching</em> one,
assumes that it is the case. </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_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>#=2
0=([fo]+)bar(baz)
1=foooo
2=baz
</pre>
<h2> Shell matching </h2>
<p>
If the <tt>-s</tt> option has been given to <tt>case</tt>, then the <em>regex</em>
words are not interpreted as regular expressions, but as shell patterns, as is
performed by the shell's
<a href="https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_04_05">case</a>
conditional construct. This has the following consequences:
</p>
<ul>
<li> Subexpression matching is always disabled. </li>
<li> <em>prog...</em> is always executed with an unmodified environment. </li>
<li> The options to the <tt>case</tt> command change meanings: instead of
controlling how the <em>regex</em> regular expressions are interpreted by the
<a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/regcomp.html">regcomp()</a>
primitive, they now control how <em>value</em> is matched against the <em>regex</em> patterns
(which are not regular expressions!) via the
<a href="https://pubs.opengroup.org/onlinepubs/9699919799/functions/fnmatch.html">fnmatch()</a>
primitive. Namely:
<ul>
<li> <tt>-e</tt> : Treat a backslash as an ordinary character; do not allow
character escaping in patterns. (This sets the <tt>FNM_NOESCAPE</tt> flag.) </li>
<li> <tt>-E</tt> : Allow backslash escaping in patterns. This is the default.
(This clears the <tt>FNM_NOESCAPE</tt> flag.) </li>
<li> <tt>-i</tt> : Treat a period (<tt>.</tt>) as a special character for
matching (set <tt>FNM_PERIOD</tt>). By default, the period is not a special character
(<tt>FNM_PERIOD</tt> is cleared). </li>
<li> <tt>-N</tt> : Treat patterns as pathnames: make slashes character special.
(This sets the <tt>FNM_PATHNAME</tt> flag.) </li>
<li> <tt>-n</tt> : Do not treat patterns as pathnames (clear the
<tt>FNM_PATHNAME</tt> flag). This is the default. </li>
</ul> </li>
</ul>
<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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