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<title>execline: the trap command</title>
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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>
<tt>trap</tt> traps signals and runs a variety of commands according
to the signals it catches.
<h2> Interface </h2>
<p>
In an <a href="execlineb.html">execlineb</a> script:
</p>
<pre>
trap [ -x ] [ -t <em>millisecs</em> ]
{
[ timeout { <em>progtimeout...</em> } ]
[ SIGTERM { <em>progsigterm...</em> } ]
[ quit { <em>progsigquit...</em> } ]
[ 1 { <em>progsighup</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>timeout</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 trap for <tt>timeout</tt> will be triggered when <em>millisecs</em>
milliseconds elapse without a signal being caught. </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. </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 be still be caught by <tt>trap</tt>, that will send it to
<em>prog</em>. </li>
<li> <tt>-t <em>millisecs</em></tt> : if a <tt>timeout</tt>
directive exists, the corresponding <em>progtimeout...</em> will be run
every time <em>millisecs</em> milliseconds elapse without <tt>trap</tt>
receiving a signal. By default, <em>millisecs</em> is 0, which means
infinite (i.e. a <tt>timeout</tt> directive will never trigger). </li>
</ul>
<h2> Notes </h2>
<ul>
<li> Programs defined in command line directives can start with
<tt><a href="importas.html">$</a> ! !</tt> to retrieve the pid of
<em>prog</em> in <tt>${!}</tt> </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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