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<title>s6: the s6-ftrig-listen1 program</title>
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<h1> The s6-ftrig-listen1 program </h1>
<p>
s6-ftrig-listen1 subscribes to a <a href="fifodir.html">fifodir</a>, then
spawns a program, then waits for a pattern of events to occur on the fifodir.
</p>
<p>
s6-ftrig-listen1 acts just as <a href="s6-ftrig-wait.html">s6-ftrig-wait</a>,
except it can make sure that the process sending the notifications is actually
started <em>after</em> there is a listener for those events.
</p>
<h2> Interface </h2>
<pre>
s6-ftrig-listen1 [ -t <em>timeout</em> ] <em>fifodir</em> <em>regexp</em> <em>prog...</em>
</pre>
<ul>
<li> s6-ftrig-listen1 subscribes to <em>fifodir</em> </li>
<li> It then forks and exec <em>prog...</em> with all its arguments </li>
<li> It waits for the series of events received on <em>fifodir</em>
to match <em>regexp</em>. <em>regexp</em> must be an
<a href="https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_04">Extended
Regular Expression</a>. </li>
<li> When the series of read events matches <em>regexp</em>,
s6-ftrig-listen1 prints the last event it received to stdout (one byte followed by a newline),
then exits 0. </li>
</ul>
<h2> Options </h2>
<ul>
<li> <tt>-t <em>timeout</em></tt> : if the events on <em>fifodir</em> have not
matched <em>regexp</em> after <em>timeout</em> milliseconds, print an error message on
stderr and exit 1. By default, s6-ftrig-listen1 waits indefinitely for a matching series
of events. </li>
</ul>
<h2> Usage example </h2>
<p>
The following sequence of shell commands has a race condition:
</p>
<p> <em>In terminal 1:</em> </p>
<pre>
s6-mkfifodir /tmp/toto
s6-ftrig-wait /tmp/toto "message"
</pre>
<p> <em>Then in terminal 2</em> </p>
<pre>
s6-ftrig-notify /tmp/toto message
</pre>
<p>
Depending on the operating system's scheduler, there is the possibility that
the s6-ftrig-notify process starts sending "message" <em>before</em> the
s6-ftrig-wait process has actually subscribed to <tt>/tmp/toto</tt>, in which
case the notification will be missed. The following sequence of shell commands
accomplishes the same goal in a reliable way, without the race condition:
</p>
<pre>
s6-mkfifodir /tmp/toto
s6-ftrig-listen1 /tmp/toto "message" s6-ftrig-notify /tmp/toto message
</pre>
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