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<title>s6: the s6-svlisten program</title>
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<p>
<a href="index.html">s6</a><br />
<a href="//skarnet.org/software/">Software</a><br />
<a href="//skarnet.org/">skarnet.org</a>
</p>
<h1> The s6-svlisten program </h1>
<p>
s6-svlisten runs a program while listening on notifications from
a collection of supervised services, and blocks until they all go up, or down.
</p>
<p>
s6-svlisten only waits for notifications; it never polls.
</p>
<h2> Interface </h2>
<p>
In an execline script:
</p>
<pre>
s6-svlisten [ -U | -u | -D | -d | -r | -R ] [ -a | -o ] [ -t <em>timeout</em> ] { <em>servicedir</em> <em>servicedir...</em> } <em>prog...</em>
</pre>
<p>
Outside of an execline script:
</p>
<pre>
s6-svlisten [ -U | -u | -D | -d | -r | -R ] [ -a | -o ] [ -t <em>timeout</em> ] <em>servicedir</em> <em>servicedir...</em> "" <em>prog...</em>
</pre>
<ul>
<li> s6-svlisten checks the state of one or more <a href="servicedir.html">service
directories</a> given as arguments in the first block and monitor
their state changes. </li>
<li> It spawns <em>prog...</em> as a child right after getting the
initial state of all the monitored services. </li>
<li> It then blocks until the wanted state happens. </li>
<li> If no service directories are listed (the block is empty), then
instead of doing all that, it immediately execs into <em>prog...</em>.
</ul>
<h2> Exit codes </h2>
<ul>
<li> 0: success, the wanted state has been reached </li>
<li> 99: timed out </li>
<li> 100: wrong usage </li>
<li> 102: the <a href="s6-supervise.html">s6-supervise</a> process monitoring the service died </li>
<li> 111: system call failed </li>
<li> <em>n</em>: services were expected to come up, but <em>n</em> of them
reported permanent failure </li>
</ul>
<h2> Options </h2>
<ul>
<li> <tt>-u</tt> : up. s6-svlisten will wait until the services are up, as
reported by s6-supervise.
This is the default; it is not reliable, but it does not depend on specific
support in the service programs. See <a href="notifywhenup.html">this page</a>
for details. </li>
<li> <tt>-U</tt> : really up. s6-svlisten will wait until the services are
up <em>and</em> ready as reported by the services themselves. This requires
specific support in the service programs, and the use of the
<tt>notification-fd</tt> file in the
<a href="servicedir.html">service directory</a>.
See the explanation on <a href="notifywhenup.html">this page</a>. </li>
<li> <tt>-d</tt> : down. s6-svlisten will wait until the services are down. </li>
<li> <tt>-D</tt> : really down. s6-svlisten will wait until the
services are down and the cleanup scripts in <tt><em>servicedir</em>/finish</tt>
for every <em>servicedir</em>
have finished executing (or have timed out and been killed). </li>
<li> <tt>-r</tt> : restart. s6-svlisten will wait until all
the services have been started or restarted, i.e. they have been in
the down state, then the up state. </li>
<li> <tt>-R</tt> : restart and ready. s6-svlisten will wait until
all the services have been started or restarted and have notified
readiness. </li>
<li> <tt>-o</tt> : or. s6-svlisten will wait until <em>one</em> of the
given services comes up or down. </li>
<li> <tt>-a</tt> : and. s6-svlisten will wait until <em>all</em> of the
given services come up or down. This is the default. </li>
<li> <tt>-t <em>timeout</em></tt> : if the requested events have not
happened after <em>timeout</em> milliseconds, s6-svlisten will print a message
to stderr and exit 99. By default, <em>timeout</em> is 0, which means no time
limit. </li>
</ul>
<h2> Notes </h2>
<ul>
<li> s6-svlisten is the service-specific version of
<a href="s6-ftrig-listen.html">s6-ftrig-listen</a>. The point of s6-svlisten
is to use it to spawn a program such as <a href="s6-svc.html">s6-svc</a>,
in order to send signals to services while making sure to catch their
state changes - thus avoiding the race condition that occurs when running
<a href="s6-svc.html">s6-svc</a> then <a href="s6-svwait.html">s6-svwait</a>
sequentially. </li>
<li> s6-svlisten needs to handle a variable length list of service directories.
For that, it uses an encoding provided by
<a href="//skarnet.org/software/execline/">execline</a>, so it's best
to only use it in execline scripts (only the execline syntax is guaranteed
not to change). There is a variant of s6-svlisten that does not use execline
syntax, but only handles one service directory:
<a href="s6-svlisten1.html">s6-svlisten1</a>. </li>
<li> The <tt>-R</tt> or <tt>-r</tt> options imply the <tt>-a</tt> option.
It is not possible to wait for one of the listed services to restart. </li>
</ul>
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