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<title>s6-rc: the s6-rc program</title>
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<p>
<a href="index.html">s6-rc</a><br />
<a href="http://skarnet.org/software/">Software</a><br />
<a href="http://skarnet.org/">skarnet.org</a>
</p>
<h1> The s6-rc program </h1>
<p>
s6-rc is a <em>service manager</em>, a.k.a <em>machine state manager</em>:
a program to manage the <em>live state</em> of a machine -
what services are currently up and what services are currently down. It
can list active services, or change the live state.
</p>
<p>
s6-rc is meant to be the one-stop shop of service management: once
service definitions have been made into a compiled service database
via <a href="s6-rc-compile.html">s6-rc-compile</a>, and the machine
state has been initialized via <a href="s6-rc-init.html">s6-rc-init</a>,
any state change - be it initial startup, shutdown, or anything else -
should be achieved by a single <tt>s6-rc change</tt> invocation.
</p>
<p>
Except in test installations with specifically made compiled
databases and live directories, s6-rc should only be run as root -
especially when asking for a state change.
</p>
<h2> Interface </h2>
<pre>
s6-rc help
s6-rc [ -l live ] [ -a ] [ -u | -d ] list <em>servicenames...</em>
s6-rc [ -l live ] [ -a ] [ -u | -d ] listall <em>servicenames...</em>
s6-rc [ -l live ] [ -a ] [ -u | -d ] [ -p ] [ -v <em>verbosity</em> ] [ -n <em>dryrunthrottle</em> ] [ -t timeout ] change [ <em>servicenames...</em> ]
</pre>
<ul>
<li> s6-rc expects to find a functional <em>live state</em> in
<em>live</em>. The live state is normally created at boot time by
<a href="s6-rc-init.html">s6-rc-init</a>. If the live state is
invalid, s6-rc complains and exits 4. </li>
<li> Depending on the arguments given, it prints the requested
information to stdout, then exits 0; or it performs a machine
state change. </li>
</ul>
<h2> Exit codes </h2>
<ul>
<li> 0: success </li>
<li> 1: some of the state transitions could not be performed </li>
<li> 2: timed out during state change </li>
<li> 3: unknown service name in the arguments </li>
<li> 4: invalid service database </li>
<li> 100: wrong usage </li>
<li> 111: system call failed </li>
</ul>
<h2> Service selection </h2>
<p>
The arguments <em>servicenames...</em> may be atomic services or
bundles; they are resolved into a set of atomic services, which is
called the <em>selection</em>. If the <tt>-a</tt> option is present,
the current set of active services (i.e. services that are up) is
added to the selection.
<strong>The s6-rc command always operates on the selection</strong>.
</p>
<h2> Options </h2>
<h3> General options </h3>
<ul>
<li> <tt>-l <em>live</em></tt> : look for the
live state in <em>live</em>. Default is
<tt>/run/s6-rc</tt>. The default can be changed at compile-time by
giving the <tt>--livedir=<em>live</em></tt> option to <tt>./configure</tt>. </li>
<li> <tt>-a</tt> : add the current set of active services to
the selection. This is useful for instance at
shutdown time: <tt>s6-rc -da change</tt>
will stop all services. </li>
</ul>
<h3> Up or down </h3>
<p>
These options control what is to be done: bring selected services
up or down (for <tt>s6-rc change</tt>) or whether to use the
forward or reverse dependency graph (for <tt>s6-rc listall</tt>).
Default is <em>up</em>.
</p>
<ul>
<li> <tt>-u</tt> : up. </li>
<li> <tt>-d</tt> : down. </li>
</ul>
<h3> <tt>s6-rc change</tt> control </h3>
<ul>
<li> <tt>-v <em>verbosity</em></tt> : be more or less
verbose. Default is 1: warning and error messages will be printed to
stderr. 0 silences warnings. 2 writes information messages whenever
s6-rc performs a transition. 3 or more is debug info. </li>
<li> <tt>-n <em>dryruntimeout</em></tt> : dry run.
s6-rc will pretend to perform transitions, but will replace all its
program invocations by a call to
<a href="s6-rc-dryrun.html">s6-rc-dryrun</a>, which will do nothing but
print the command line s6-rc would have executed, then sleep for
<em>dryruntimeout</em> milliseconds before reporting success. </li>
<li> <tt>-t <em>timeout</em></tt> : timeout. If s6-rc
isn't done after <em>timeout</em> milliseconds, it will exit, leaving
the live state as it is at exit time. It does not kill its children, so
a child may successfully complete afterwards and the live state will
not be updated; in that case, subsequent <tt>s6-rc change</tt>
invocations will notice
and correctly update it. </li>
<li> <tt>-p</tt> : prune. The state will be brought to
<em>exactly</em> the set of selected services, plus their dependencies, and
the other services will be brought down. With the <tt>-d</tt> option,
the meaning is reversed: the state will be brought to the maximum
possible set that does not include selected services. The change, however,
is always performed in the same manner: first the unwanted services are
all brought down, then the wanted services are all brought up. </li>
</ul>
<h2> Subcommands </h2>
<h3> s6-rc help <h3>
<p>
Prints a short help message.
</p>
<h3> s6-rc list <em>servicenames...</em> </h3>
<p>
Prints the selection. If the <tt>-d</tt> option has been given,
the selection is inverted before it is printed, i.e. all the
services but <em>servicenames...</em> will be printed.
</p>
<p>
This is mostly useful as <tt>s6-rc -a list</tt>, which simply prints
the list of currently active services. <tt>s6-rc -da list</tt>
will print the list of currently down services.
</p>
<h3> s6-rc listall <em>servicenames...</em> </h3>
<ul>
<li> s6-rc computes the complete dependency tree of the
selection: the selection, plus the dependencies of every service
in the selection, and recursively down. With the <tt>-d</tt>
option, it computes the reverse dependency tree: the selection,
plus what depends on it, and recursively up. The result is
called the <em>closed selection</em>. </li>
<li> It prints the closed selection to its standard output.
Note that the closed selection may be a very long list of services,
including the most basic ones. </li>
</ul>
<h3> s6-rc change </h3>
<p>
<tt>s6-rc change</tt> is the service state engine. It will bring the
machine to a state where:
</p>
<ul>
<li> <tt>-u</tt> (default): all the services in the selection are up </li>
<li> <tt>-d</tt>: all the services in the selection are down </li>
<li> <tt>-pu</tt>: all the services in the selection, and <em>only</em> them, are up </li>
<li> <tt>-pd</tt>: all the services in the selection, and <em>only</em> them, are down </li>
</ul>
<p>
To do so:
</p>
<ul>
<li> From the selection, s6-rc computes the closed selection </li>
<li> It then computes the necessary transitions so that all the
services in the closed selection can match the desired state </li>
<li> It then performs the transitions - or simulates them if the
<tt>-n</tt> option has been given. </li>
<li> s6-rc processes services as soon as they can be processed:
as soon as a service that needs to change state has all its dependencies
fulfilled, it undergoes its transition. Independent services are processed
in parallel: s6-rc parallelizes the transitions as much as it can, and
there is no useless waiting time. </li>
<li> s6-rc updates the live state in <em>live</em> after every
successful transition. </li>
<li> If every transition completes successfully, the live state
eventually matches the desired state, and s6-rc exits 0. </li>
<li> If a transition fails, s6-rc will not perform the transitions
that depend on it. It will keep processing all the other independent
transitions, though, until there is no more work it can complete; then
it will exit 1. </li>
</ul>
<h4> Longrun transitions </h4>
<p>
Transitions for longrun services are simple: s6-rc removes or creates
a <tt>./down</tt> file in the live service directory for the longrun
service, then sends a command
to the supervision tree to start or stop the service. (A service that
is considered down by s6-rc will have a <tt>./down</tt> file in its live service
directory; a service that is considered up by s6-rc will not.) The
transition is considered successful as soon as the daemon dies (for
down transitions), or becomes up and
<a href="http://skarnet.org/software/s6/notifywhenup.html">ready</a>
(for up transitions). If a longrun service does not support
readiness notification, the
<a href="http://skarnet.org/software/s6/s6-svc.html">s6-svc</a>
command that is invoked by s6-rc will print a warning message, and
the transition will be considered successful as soon as the daemon
is up, i.e. as soon as the <tt>./run</tt> script is executed by
<a href="http://skarnet.org/software/s6/s6-supervise.html">s6-supervise</a>.
</p>
<p>
When a longrun service supports readiness notification, unless
a nonzero timeout has been declared in the <tt>timeout-up</tt> file
in the service definition directory,
s6-rc will wait forever on an "up" transition for the notification
to arrive. The transition will fail if a timeout occurs.
</p>
<p>
Transitions are supposed to be idempotent, but it is a general
rule of supervision that <tt>run</tt> and <tt>finish</tt> scripts
must be idempotent, so a properly designed service directory
should work with s6-rc with no additional effort.
</p>
<h4> Oneshot transitions </h4>
<p>
Transitions for oneshot services amount to running the <em>up</em>
or <em>down</em> script for the service; those scripts are stored
in the compiled service database that is linked from the live state.
The transition is considered successful if the script exits zero,
and unsuccessful otherwise.
</p>
<p>
s6-rc performs some black magic so that <em>up</em> and <em>down</em>
scripts are always run in a reproducible way, no matter when or how
<tt>s6-rc change</tt> is invoked. That black magic involves a special
longrun service, <tt>s6rc-oneshot-runner</tt>, that every oneshot
service automatically depends on, and that is actually used to fork
the <em>up</em> or <em>down</em> scripts as scions of the s6 supervision
tree, instead of children of the s6-rc process.
</p>
<p>
Transitions should be ideally transactional, or at the very least
idempotent. If a
transition fails, it should leave the machine in the same state as
before the transition was attempted; at the very least, it should not
prevent a subsequent run of the same transition from completing
successfully. If an <tt>s6-rc change</tt> invocation fails because
some transition experienced a temporary failure, it should be possible
to run the exact same <tt>s6-rc change</tt> invocation later, and be
met with success.
</p>
<p>
This is important: it means that oneshot scripts should be treated
as atoms, and that some care should be taken when writing them.
</p>
<h4> Dry runs </h4>
<p>
For any manual change, is it recommended to perform a dry run before
the state change itself: add the <tt>-n <em>dryrunthrottle</em></tt>
option to the s6-rc command line. s6-rc will then simulate all the
transitions, but not actually perform them or change the real live
state.
The command lines that s6-rc would have run will be printed to stdout
instead, and
each simulated transition will take <em>dryrunthrottle</em>
milliseconds to complete successfully.
</p>
<h2> Usage examples </h2>
<pre> s6-rc change <em>myservicebundle</em> </pre>
<p>
Brings up all the services represented by <em>myservicebundle</em>,
bringing up all its dependencies first (recursively).
</p>
<pre> s6-rc -ad change </pre>
<p>
Brings down all the currently running services in an orderly manner.
This is typically run at shutdown time.
</p>
<pre> s6-rc -l /zork -ua listall <em>myservicebundle</em> </pre>
<p>
Prints the names of all atomic services represented by
<em>myservicebundle</em> plus the current live services, as well as
everything they depend on, recursively. Assumes the live state is
stored in the <tt>/zork</tt> directory instead of <tt>/run/s6-rc</tt>.
</p>
<pre> s6-rc -d listall <em>myservicebundle</em> </pre>
<p>
Prints the names of all atomic services represented by
<em>myservicebundle</em>, as well as everything that depends on them.
</p>
<pre> s6-rc -pun0 change <em>myservicebundle</em> </pre>
<p>
Prints what s6-rc would do to bring the state to <em>exactly</em>
the contents of <em>myservicebundle</em> as well as its recursive
dependencies, and pruning all the rest. Does not wait any extra time
between simulated transitions.
</p>
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