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<p>
<a href="index.html">execline</a><br />
<a href="//skarnet.org/software/">Software</a><br />
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<h1> The <tt>trap</tt> program </h1>
<p>
<tt>trap</tt> traps signals and runs a variety of commands according
to the signals it catches.
</p>
<h2> Interface </h2>
<p>
In an <a href="execlineb.html">execlineb</a> script:
</p>
<pre>
trap [ -x ]
{
[ SIGTERM { <em>progsigterm...</em> } ]
[ quit { <em>progsigquit...</em> } ]
[ 1 { <em>progsighup</em>... } ]
[ default { <em>progdefault</em>... } ]
...
}
<em>prog...</em>
</pre>
<ul>
<li> <tt>trap</tt> reads a sequence of directives in a
<a href="el_semicolon.html">block</a>. It expects at least one
directive. </li>
<li> Each directive is a keyword followed by a block. </li>
<li> The keyword can be the special word <tt>default</tt>, a signal
name (case-insensitive, with or without the <tt>SIG</tt> prefix),
or a signal number. The block following it is a command line to
run when the specified event occurs. </li>
<li> <tt>trap</tt> sets traps for the various directives it reads.
A trap for <tt>SIGTERM</tt> will be triggered when the <tt>trap</tt>
program receives a SIGTERM. A <tt>default</tt>
trap will be used for any signal that is not caught by another trap. </li>
<li> It spawns a child executing <em>prog...</em>. </li>
<li> It sets the <tt>!</tt> environment
variable to the pid of the <em>prog...</em> process, and the <tt>SIGNAL</tt>
environment variable to the trapped signal. </li>
<li> Whenever it catches a signal, it spawns the program described in the
corresponding directive. It will not spawn a program for the same signal
twice: if the first subprocess is still active when another instance of the
same signal arrives, this second instance is ignored. </li>
<li> When <em>prog...</em> exits, <tt>trap</tt> exits with an
<a href="exitcodes.html">approximation</a> of the same exit code. </li>
</ul>
<h2> Options </h2>
<ul>
<li> <tt>-x</tt> : forward signals. If this option is given,
any signal that <tt>trap</tt> receives and that is not explicitly
trapped will be sent to <em>prog</em>. By default, <tt>trap</tt> does
not forward any signals, and does not ignore them either - for instance a
SIGTERM, unless caught by a <tt>SIGTERM</tt> directive, will kill the
<tt>trap</tt> process (and leave <em>prog</em> running). With the
<tt>-x</tt> option, without a <tt>SIGTERM</tt> directive, a SIGTERM
will still be caught by <tt>trap</tt>, that will send it to
<em>prog</em>. Note that if a <tt>default</tt> directive is present,
this option does nothing. </li>
</ul>
<h2> Notes </h2>
<ul>
<li> Programs defined in command line directives can start with
<code><a href="importas.html">importas</a> ! !</code> to retrieve the pid of
<em>prog</em> in <tt>$!</tt>. If they need the signal number, which
can be the case in <tt>default</tt> directives, they can for instance use
<code><a href="multisubstitute.html">multisubstitute</a> { importas ! ! importas SIGNAL SIGNAL }</code>
to get both <tt>$!</tt> and <tt>$SIGNAL</tt> substitutions. </li>
<li> The <tt>-x</tt> option is basically a shortcut for a <code>default {
multisubstitute { importas ! ! importas SIGNAL SIGNAL } kill -$SIGNAL $! }</code> directive. </li>
<li> <tt>trap</tt> is a standard shell builtin, with similar
functionality. It is more idiomatic, and probably more efficient,
to use that builtin in shell scripts, and to only use the
<tt>trap</tt> program in execline scripts. </li>
</ul>
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