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<title>s6: the s6-notifywhenup program</title>
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<meta name="Keywords" content="s6 command s6-notifywhenup fifodir notification event notifier send service daemon ready" />
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<p>
<a href="index.html">s6</a><br />
<a href="http://skarnet.org/software/">Software</a><br />
<a href="http://skarnet.org/">skarnet.org</a>
</p>
<h1> The s6-notifywhenup program </h1>
<p>
s6-notifywhenup launches a daemon while listening to a file descriptor,
and sends a 'U' event to a <a href="fifodir.html">fifodir</a> when it
receives something on that file descriptor.
</p>
<p>
<a href="notifywhenup.html">This page</a> explains why this program is
needed.
</p>
<h2> Interface </h2>
<pre>
s6-notifywhenup [ -d <em>fd</em> ] [ -e <em>fifodir</em> ] [ -f ] [ -t <em>timeout</em> ] <em>prog...</em>
</pre>
<ul>
<li> s6-notifywhenup forks and executes <em>prog...</em> as the
parent, with a pipe from <em>prog...</em>'s stdout to the child. </li>
<li> The child waits for a newline (<tt>\n</tt>) to be written
on the pipe. When it gets it, it creates an empty
<tt>./supervise/ready</tt> file then sends a 'U' event to the
<tt>./event</tt> fifodir. </li>
<li> The child exits 0. </li>
</ul>
<h2> Options </h2>
<ul>
<li> <tt>-d <em>fd</em></tt> : listen to
<em>prog</em>'s output on descriptor <em>fd</em>. The default is 1. </li>
<li> <tt>-e <em>fifodir</em></tt> : send a 'U' event to fifodir
<em>fifodir</em>. Default is <tt>./event</tt>. </li>
<li> <tt>-f</tt> : simple fork. Normally, s6-notifywhenup doubleforks,
in order to accommodate for a
<em>prog</em> that does not expect to have a child and avoid a
pending zombie. This option avoids the doublefork, but it should only be
set if <em>prog</em> reaps even children it doesn't know it has. </li>
<li> <tt>-t <em>timeout</em></tt> : if no EOF has been received
after <em>timeout</em> milliseconds, exit without sending the event.
Default is 0, meaning infinite. </li>
</ul>
<h2> Notes </h2>
<ul>
<li> s6-notifywhenup executes <em>prog...</em> as the parent in order
for <em>prog</em> to keep the same pid, which is vital for supervised
processes. </li>
<li> s6-notifywhenup can be used, for instance, with
<a href="http://skarnet.org/software/s6-networking/s6-tcpserver.html">s6-tcpserver</a>
and its <tt>-1</tt> option, so that reliable startup notification is
achieved. <tt>s6-notifywhenup -f s6-tcpserver -1 <em>args</em></tt> will
send a 'U' event to <tt>./event</tt> when s6-tcpserver is actually
listening to its network socket. </li>
<li> The <a href="s6-svwait.html">s6-svwait</a> program can be used
to wait for 'U' events. </li>
</ul>
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