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<title>s6: the s6-svc program</title>
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<h1> The s6-svc program </h1>
<p>
s6-svc sends commands to a running <a href="s6-supervise.html">s6-supervise</a>
process. In other words, it's used to control a supervised process; among
other benefits, it allows an administrator to send signals to daemons without
knowing their PIDs, and without using horrible hacks such as .pid files.
</p>
<h2> Interface </h2>
<pre>
s6-svc [ -wu | -wU | -wd | -wD | -wr | -wR ] [ -T <em>timeout</em> ] [ -abqhkti12pcyoduxOr ] <em>servicedir</em>
</pre>
<p>
s6-svc sends the given series of commands to the
<a href="s6-supervise.html">s6-supervise</a> process monitoring the
<em>servicedir</em> directory, then exits 0. It exits 111 if it cannot send
a command, or 100 if no s6-supervise process is running on <em>servicedir</em>.
</p>
<h2> Options </h2>
<ul>
<li> <tt>-a</tt> : send a SIGALRM to the supervised process </li>
<li> <tt>-b</tt> : send a SIGABRT to the supervised process </li>
<li> <tt>-q</tt> : send a SIGQUIT to the supervised process </li>
<li> <tt>-h</tt> : send a SIGHUP to the supervised process </li>
<li> <tt>-k</tt> : send a SIGKILL to the supervised process </li>
<li> <tt>-t</tt> : send a SIGTERM to the supervised process </li>
<li> <tt>-i</tt> : send a SIGINT to the supervised process </li>
<li> <tt>-1</tt> : send a SIGUSR1 to the supervised process </li>
<li> <tt>-2</tt> : send a SIGUSR2 to the supervised process </li>
<li> <tt>-p</tt> : send a SIGSTOP to the supervised process </li>
<li> <tt>-c</tt> : send a SIGCONT to the supervised process </li>
<li> <tt>-y</tt> : send a SIGWINCH to the supervised process </li>
<li> <tt>-o</tt> : once. Equivalent to "-uO". </li>
<li> <tt>-d</tt> : down. If the supervised process is up, send it
a SIGTERM (by default) then a SIGCONT (to make sure even stopped processes
receive the signal aimed to kill them) and do not restart it.
The SIGTERM default can be changed by editing the <tt>./down-signal</tt>
file in the <a href="servicedir.html">service directory</a>. </li>
<li> <tt>-u</tt> : up. If the supervised process is down, start it.
Automatically restart it when it dies. </li>
<li> <tt>-x</tt> : exit. When the service is asked to be down and
the supervised process dies, s6-supervise will exit too. This command should
normally never be used on a working system. Note that if this command is
sent and a <tt>./finish</tt> script exists for the service, the last
<tt>./finish</tt> invocation before
<a href="s6-supervise.html">s6-supervise</a> exits will run with its stdin
and stdout redirected to <tt>/dev/null</tt>. </li>
<li> <tt>-O</tt> : mark the service to run once at most. iow: do not
restart the supervised process when it dies. If it is down when the command
is received, do not even start it. </li>
<li> <tt>-r</tt> : If the service is up, restart it, by sending it a
signal to kill it and letting <a href="s6-supervise.html">s6-supervise</a>
start it again. By default, the signal is a SIGTERM; this can be configured
via the <tt>./down-signal</tt> file in the <a href="servicedir.html">service
directory</a>. </li>
<li> <tt>-T <em>timeout</em></tt> : if the <tt>-w<em>state</em></tt>
option has been given, <tt>-T</tt> specifies a timeout
(in milliseconds) after which s6-svc will exit 1 with an error message if
the service still hasn't reached the desired state. By default, the
timeout is 0, which means that s6-svc will block indefinitely. </li>
<li> <tt>-wd</tt> : s6-svc will not exit until the service is down,
i.e. until the <tt>run</tt> process has died. </li>
<li> <tt>-wD</tt> : s6-svc will not exit until the service is down
<em>and</em> ready to be brought up, i.e. a possible <tt>finish</tt> script has
exited. </li>
<li> <tt>-wu</tt> : s6-svc will not exit until the service is up,
i.e. there is a process running the <tt>run</tt> executable. </li>
<li> <tt>-wU</tt> : s6-svc will not exit until the service is up <em>and</em>
<a href="notifywhenup.html">ready</a> as notified by the daemon itself.
If the <a href="servicedir.html">service directory</a> does not contain
a <tt>notification-fd</tt> file to tell
<a href="s6-supervise.html">s6-supervise</a> to accept readiness
notification, s6-svc will print a warning and act as if the <tt>-wu</tt>
option had been given instead. </li>
<li> <tt>-wr</tt> : s6-svc will not exit until the service has been
started or restarted. </li>
<li> <tt>-wR</tt> : s6-svc will not exit until the service has been
started or restarted and has notified readiness. </li>
</ul>
<h2> Usage examples </h2>
<pre> s6-svc -h /service/httpd </pre>
<p>
Send a SIGHUP to the process represented by the <tt>/service/httpd</tt>
service directory. Traditionally, this makes web servers reload their
configuration file.
</p>
<pre> s6-svc -r /service/sshd </pre>
<p>
Kill (and automatically restart, if the wanted state of the service is up)
the process represented by the <tt>/service/sshd</tt> service directory -
typically the sshd server.
</p>
<pre> s6-svc -wD -d /service/ftpd </pre>
<p>
Take down the ftpd server and block until the process is down and
the finish script has completed.
</p>
<pre> s6-svc -wU -T 5000 -u /service/ftpd </pre>
<p>
Bring up the ftpd server and block until it has sent notification that it
is ready. Exit 1 if it is still not ready after 5 seconds.
</p>
<pre> s6-svc -wR -t /service/ftpd </pre>
<p>
Send a SIGTERM to the ftpd server; wait for
<a href="s6-supervise.html">s6-supervise</a> to restart it, and block
until it has notified that it is ready to serve again.
</p>
<pre> s6-svc -a /service/httpd/log </pre>
<p>
Send a SIGALRM to the logger process for the httpd server. If this logger
process is <a href="s6-log.html">s6-log</a>, this triggers a log rotation.
</p>
<h2> Internals </h2>
<ul>
<li> s6-svc writes control commands into the <tt><em>servicedir</em>/supervise/control</tt>
FIFO. An s6-supervise process running on <em>servicedir</em> will be listening to this FIFO,
and will read and interpret those commands. </li>
<li> When invoked with one of the <tt>-w</tt> options, s6-svc executes into
<a href="s6-svlisten1.html">s6-svlisten1</a>, which will listen to service state
changes and spawn another s6-svc instance (without the <tt>-w</tt> option)
that will send the commands to the service. Any error message written during
the waiting period will mention it is being written by s6-svlisten1; this is normal. </li>
</ul>
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