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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. </li>
<li> <tt>-X</tt> : close fds and exit. Like <tt>-x</tt>, but
<a href="s6-supervise.html">s6-supervise</a> will immediately close its
stdin, stdout and stderr. This is useful when s6-supervise has descriptors
open to the service it is supervising and the service is waiting for them
to close before exiting. Note that if this option is used, the last
execution of the service's <tt>finish</tt> script will be run with
stdin, stdout and stderr 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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