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<title>s6-linux-init: why?</title>
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<a href="//skarnet.org/software/">Software</a><br />
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<h1> Why s6-linux-init ? </h1>
<h2> The s6 software stack </h2>
<p>
The s6 ecosystem is made of several parts, which are mainly the following:
</p>
<ul>
<li> <a href="//skarnet.org/software/skalibs/">skalibs</a>: a C system
programming library that is used in all skarnet.org software. </li>
<li> <a href="//skarnet.org/software/execline/">execline</a>: a small
scripting language that is mainly used in various parts of the s6
ecosystem because:
<ul>
<li> It is very quick to launch, and efficient with small scripts, so it
is a good choice for s6 run scripts. </li>
<li> It is much easier to programmatically generate execline scripts than
shell scripts. execline allows programs such as
<a href="s6-linux-init-maker.html">s6-linux-init-maker</a> to generate scripts
quite easily, whereas using the shell syntax would require them to understand
the full subleties of shell quoting. </li>
</ul>
<li> <a href="//skarnet.org/software/s6/">s6</a>, the main dish: a process
supervision suite. </li>
<li> <a href="//skarnet.org/software/s6-rc/">s6-rc</a>: a service manager
for s6. </li>
<li> and <a href="//skarnet.org/software/s6-linux-init/">s6-linux-init</a>: this
package. </li>
</ul>
<h2> Providing a complete init system </h2>
<p>
As explained in
<a href="https://archive.fosdem.org/2017/schedule/event/s6_supervision/">this
presentation</a>, an init system is made of four parts:
</p>
<ol>
<li> <tt>/sbin/init</tt>: the first userspace program that is run by the
kernel at boot time (not counting an initramfs). </li>
<li> <em>pid 1</em>: the program that will run as process 1 for most of
the lifetime of the machine. This is not necessarily the same executable
as <tt>/sbin/init</tt>, because <tt>/sbin/init</tt> can exec into something
else. </li>
<li> a <em>process supervisor</em>. </li>
<li> a <em>service manager</em>. </li>
</ol>
<p>
The <a href="//skarnet.org/software/s6/">s6</a> package obviously provides
part 3. It also provides part 2, because
<a href="//skarnet.org/software/s6/s6-svscan.html">s6-svscan</a> is suitable
as being pid 1 after some small setup is performed.
</p>
<p>
Part 4, service management, can be provided in a variety of ways. The
<a href="//skarnet.org/software/s6-rc/">s6-rc</a> service manager is the
natural complement to the s6 process supervisor, but it is not the only
possibility. The
<a href="https://jjacky.com/anopa/">anopa</a> package also provides a
service manager designed to work with s6. And, at the expense of
tight integration with the supervisor, it is possible to run a "traditional"
service manager, such as sysv-rc or OpenRC, with an s6-based init system.
This flexibility is possible because service management is one layer above
the mechanisms of init and process supervision.
</p>
<p>
Remains part 1. And that's where s6-linux-init enters the picture.
</p>
<h3> Portability </h3>
<p>
Part 1 of an init system, the <tt>/sbin/init</tt> program, has been purposefully
omitted from the main s6 package, for a simple reason: s6 aims to be portable
to any flavor of Unix, and <em>it is impossible to implement <tt>/sbin/init</tt>
in a portable way</em>.
</p>
<p>
For instance, to do its job,
<a href="//skarnet.org/software/s6/s6-svscan.html">s6-svscan</a> needs
a writable directory. Such a directory may not be available at boot time,
before mounting filesystems, because the root filesystem may be read-only.
So, at least one writable filesystem (typically a RAM-backed one) must be
mounted before s6-svscan can be executed and be pid 1. And mounting a
filesystem is a non-portable operation.
</p>
<h3> Complexity </h3>
<p>
Moreover, the sequence of operations that a <tt>/sbin/init</tt> program
needs to perform before executing into <tt>s6-svscan</tt> is a bit
tricky. It can be scripted, but it's not easy, and since it's so early
in the lifetime of the machine, there's no safety net at all (the
supervision tree itself, and the early getty, are supposed to be the
safety net, and they're not there yet). So it's better to automate
these operations.
</p>
<h2> Conclusion </h2>
<p>
<tt>s6-linux-init</tt> aims to provide a fully functional <tt>/sbin/init</tt>
program that executes into an s6 supervision tree with all the necessary
support services already in place, as well as the corresponding shutdown
commands. It also aims to be flexible enough to accommodate various needs
and be compatible with any user-chosen service manager.
</p>
<p>
As usual, it is about <em>mechanism</em>, not <em>policy</em>.
</p>
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