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<title>s6: How to run s6-svscan under another init process</title>
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<p>
<a href="index.html">s6</a><br />
<a href="http://skarnet.org/software/">Software</a><br />
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<h1> How to run s6-svscan under another init process </h1>
<p>
You can have a reliable supervision tree even if s6-svscan is not your process 1.
The supervision tree just has to be <em>rooted</em> in process 1: that means that
your process 1 will have to supervise your s6-svscan process somehow. That way,
if s6-svscan dies, it will be restarted, and your set of services will always
be maintained.
</p>
<p>
Be aware, though, that pipes between services and loggers are maintained
by the s6-svscan process; if this process dies, the pipes will be closed and
some logs may be lost.
</p>
<a name="log">
<h2> Logging the supervision tree's output </h2>
</a>
<p>
s6-svscan and the various s6-supervise processes might produce error or
warning messages; those messages are written to s6-svscan's stderr (which
is inherited by the s6-supervise processes). To log these messages:
</p>
<ul>
<li> You can use your init system's logging tools, and make your init
system launch s6-svscan as is; its stderr should already be taken care
of by the logging tools. </li>
<li> You can use a trick similar to the <a href="s6-svscan-1.html#log">process
1 output logging trick</a> so the supervision tree's messages are logged via
a service that's maintained by the supervision tree itself. Then your init
system should not launch s6-svscan directly, but a wrapper script that performs
the proper redirections. The
<tt>examples/s6-svscanboot</tt> file in the s6 distribution gives an example of
such a script. Make sure that your initial <a href="scandir.html">scan
directory</a> contains a <a href="servicedir.html">service directory</a> for your
initial logging service, that must read on the logging FIFO. </li>
</ul>
<p>
In the following examples, we'll assume that <tt>/command/s6-svscanboot</tt>
is the name of the script you are using to start s6-svscan. Adjust this accordingly.
</p>
<a name="sysv">
<h2> System V init </h2>
</a>
<p>
Put an appropriate line in your <tt>/etc/inittab</tt> file, then reload this
config file with <tt>telinit q</tt>.
</p>
<h3> Example </h3>
<pre> SV:123456:respawn:/command/s6-svscanboot </pre>
<a name="upstart">
<h2> Upstart </h2>
</a>
<p>
Put an appropriate configuration file in the <tt>/etc/init</tt> folder,
for instance <tt>/etc/init/s6-svscan.conf</tt>, then start the service
with <tt>start s6-svscan</tt>.
</p>
<h3>Example </h3>
<pre># s6-svscan
start on runlevel [2345]
stop on runlevel [!2345]
oom never
respawn
exec /command/s6-svscanboot
</pre>
<a name="systemd">
<h2> systemd </h2>
<p>
systemd has
<a href="http://www.freedesktop.org/software/systemd/man/daemon.html">its
own way</a> of supervising services. If you are a systemd user, chances
are you do not need s6. If you are interested in using s6, I encourage
you to also stop using systemd.
</p>
<p>
</p>
<a name="bsd">
<h2> BSD init </h2>
</a>
<p>
Put an appropriate line in your <tt>/etc/ttys</tt> file, then reload this
file with <tt>kill -s HUP 1</tt>.
</p>
<h3> Example </h3>
<pre> sv /command/s6-svscanboot "" on </pre>
<a name="launchd">
<h2> MacOS X launchd </h2>
</a>
<p>
Like systemd, launchd comes with its own
<a href="https://developer.apple.com/library/mac/documentation/macosx/conceptual/bpsystemstartup/chapters/CreatingLaunchdJobs.html">way
of supervising services</a>; if you are a launchd user, you probably do
not need s6.
</p>
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