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<title>s6-linux-init: the s6-linux-init program</title>
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<p>
<a href="index.html">s6-linux-init</a><br />
<a href="//skarnet.org/software/">Software</a><br />
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</p>
<h1> The <tt>s6-linux-init</tt> program </h1>
<p>
<tt>s6-linux-init</tt> is a program that is meant to run as pid 1,
as a <em>stage 1 init</em>: it performs the necessary early system preparation
and execs into <a href="//skarnet.org/software/s6/s6-svscan.html">s6-svscan</a>.
</p>
<h2> Interface </h2>
<pre>
s6-linux-init [ -c <em>basedir</em> ] [ -p <em>initial_path</em> ] [ -s <em>env_store</em> ] [ -m <em>umask</em> ] [ -d <em>slashdev</em> ] [ -D <em>initdefault</em> ] [ -n | -N ] [ -C ] [ -B ] [ <em>args...</em> ]
</pre>
<ul>
<li> If <tt>s6-linux-init</tt> isn't pid 1, it execs into
<a href="s6-linux-init-telinit.html">s6-linux-init-telinit</a> with the
same arguments. </li>
<li> Else, it performs some early preparation, spawns a process that
will run the <em>rc.init</em> script, then execs into
<a href="//skarnet.org/software/s6/s6-svscan.html">s6-svscan</a>. </li>
</ul>
<h2> Options </h2>
<p>
These options should exactly mirror the options with the same name that
have been given to <a href="s6-linux-init-maker.html">s6-linux-init-maker</a>.
If there is a discrepancy, the system might not boot.
</p>
<ul>
<li> <tt>-c</tt> <em>basedir</em> : read all its initialization
data from <em>basedir</em>. If the data has not indeed been copied to
<em>basedir</em>, <strong>the system will not boot</strong>. </li>
<li> <tt>-p</tt> <em>initial_path</em> : the initial value for
the PATH environment variable. </li>
<li> <tt>-s</tt> <em>env_store</em> : the place where to dump
kernel environment variables. </li>
<li> <tt>-m</tt> <em>initial_umask</em> : the initial file umask. </li>
<li> <tt>-d</tt> <em>slashdev</em> : mount a devtmpfs on
<em>slashdev</em>. By default, no such mount is performed - it is assumed
that a devtmpfs is automounted on <tt>/dev</tt> at boot time by the kernel. </li>
<li> <tt>-D</tt> <em>initdefault</em> : the initial runlevel
to boot to, if it isn't overridden by the kernel command line.
This is only given as a first argument to <em>rc.init</em>.
Default is <tt>default</tt>. </li>
<li> <tt>-n</tt> : instead of unmounting <tt>/run</tt> and mounting
a tmpfs on it, just remount <tt>/run</tt>. </li>
<li> <tt>-N</tt> : do not touch <tt>/run</tt> at all. </li>
<li> <tt>-C</tt> : run in a container. This option modifies a few
of the operations described below, to accommodate running in a container
instead of on real hardware. For instance: it does not scan the command
line for a specific runlevel, it does not trap ctrl-alt-del, and before
anything else it waits for its descriptor 3, if present, to close.
(Docker uses this fd 3 mechanism as synchronization between the Docker
daemon and the container's <tt>init</tt>.) </li>
<li> <tt>-B</tt> : do not run the catch-all logger. This option
removes the catch-all-logger-related operations from the list below;
<tt>s6-linux-init</tt> will not redirect output descriptors, and will
use a different synchronization mechanism to ensure <tt>rc.init</tt>
only runs when <a href="//skarnet.org/software/s6/s6-svscan.html">s6-svscan</a>
is ready. </li>
</ul>
<h2> Early preparation </h2>
<p>
When booting a system, <tt>s6-linux-init</tt> performs the following
operations:
</p>
<ul>
<li> It prints a banner to <tt>/dev/console</tt>. </li>
<li> It chdirs to <tt>/</tt>. </li>
<li> It sets the umask to <em>initial_umask</em>. </li>
<li> It becomes a session leader. </li>
<li> It mounts a devtmpfs on <em>slashdev</em>, if requested. </li>
<li> It uses <tt>/dev/null</tt> as its stdin (instead of <tt>/dev/console</tt>).
<tt>/dev/console</tt> is still used, for now, as stdout and stderr. </li>
<li> It unmounts <tt>/run</tt> (or the directory you have given to the
<tt>--tmpfsdir</tt> configure option at package build time), just in case;
then it creates a tmpfs on it. Alternatively, it remounts <tt>/run</tt>,
or does not touch it at all. </li>
<li> It copies the whole <tt><em>basedir</em>/run-image</tt> hierarchy to
<tt>/run</tt> (or your chosen tmpfsdir). </li>
<li> It reads the initial environment from <tt><em>basedir</em>/env</tt>. </li>
<li> If required, it stores the kernel environment into <em>env_store</em>. </li>
<li> It performs "the fifo trick", i.e. it redirects its stdout to the
catch-all logger's fifo, without blocking, before the catch-all
logger is even up (because it's a service that will be spawned a bit
later, when <a href="//skarnet.org/software/s6/s6-svscan.html">s6-svscan</a>
is executed). </li>
<li> It forks a child.
<ul>
<li> The child scans the kernel command line to find a suitable runlevel
(<tt>default</tt>, <tt>2</tt>, <tt>3</tt>, <tt>4</tt>, or <tt>5</tt>). If
it doesn't find any kernel command line argument that defines a runlevel,
it uses <em>initdefault</em>. </li>
<li> The child becomes a session leader. </li>
<li> The child blocks until the catch-all logger runs. </li>
</ul> </li>
<li> It also makes the catch-all logger's fifo its stderr. </li>
<li> It traps the ctrl-alt-del keyboard combination. </li>
<li> It execs into <a href="//skarnet.org/software/s6/s6-svscan.html">s6-svscan</a>
with <tt>/run/service</tt> as its scandir (or <em>tmpfsdir</em>/service). </li>
<ul>
<li> <a href="//skarnet.org/software/s6/s6-svscan.html">s6-svscan</a>
spawns the early services that are defined in
<tt><em>basedir</em>/run-image/service</tt>, and have been copied into
<tt>/run/service</tt> (or <em>tmpfsdir</em>/service). </li>
<li> One of those early services is <tt>s6-svscan-log</tt>, which is
the catch-all logger. When this service is up, <tt>s6-linux-init</tt>'s
child unblocks. </li>
<li> The child execs into <tt><em>basedir</em>/scripts/rc.init</tt>.
The first argument to <em>rc.init</em> is the chosen runlevel. The kernel
command line, as given by the kernel to <tt>s6-linux-init</tt> (i.e. without
the <tt>key=value</tt> arguments, which were passed into <tt>s6-linux-init</tt>'s
environment and were stored into <em>env_store</em>), makes for the rest of
the arguments given to <em>rc.init</em>. </li>
</ul>
</ul>
<p>
By the time <em>rc.init</em> runs,
<a href="//skarnet.org/software/s6/s6-svscan.html">s6-svscan</a> is running
as pid 1 and has spawned its early services - at least the catch-all logger,
and the other services, including the early getty if it has been defined, are
started in parallel and will be ready instantly. <em>rc.init</em> can then
perform <em>stage 2</em> of the initialization process, i.e. the handoff to
the service manager.
</p>
<h2> Exit codes </h2>
<p>
<tt>s6-linux-init</tt> never exits. It spawns the <em>rc.init</em> script
and execs into <a href="//skarnet.org/software/s6/s6-svscan.html">s6-svscan</a>,
which runs forever until the machine stops or reboots.
</p>
<h2> Notes </h2>
<ul>
<li> The <tt>s6-linux-init</tt>
binary is not meant to be called directly, or be linked to <tt>/sbin/init</tt>
directly, because it takes command-line options.
Instead, after a
<a href="s6-linux-init-maker.html">s6-linux-init-maker</a> invocation,
the <tt>bin/</tt> subdirectory of the target will contain a script called
<tt>init</tt>, which execs into <tt>s6-linux-init</tt> with the appropriate
command-line options, and <em>is</em> suitable as a <tt>/sbin/init</tt> program.
The <tt>bin/</tt> subdirectory
should be copied by the administrator into <tt>/sbin</tt> for full
interface compatibility with sysvinit. </li>
</ul>
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