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<title>s6-frontend: the s6-frontend program</title>
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<p>
<a href="index.html">s6-frontend</a><br />
<a href="//skarnet.org/software/">Software</a><br />
<a href="//skarnet.org/">skarnet.org</a>
</p>
<h1> The <tt>s6-frontend</tt> internal program </h1>
<p>
<tt>s6-frontend</tt> is the program that does the real command dispatching behind
the <a href="s6.html">s6</a> command line. It is not meant to be called directly;
users should call <a href="s6.html">s6</a> instead.
</p>
<h2 id="interface"> Interface </h2>
<pre>
s6-frontend [ <em>global_options...</em> ] <em>command</em> <em>subcommand</em> [ <em>subcommand_options...</em> ] [ <em>args...</em> ]
</pre>
<h2 id="details"> Detailed operation </h2>
<p>
What really happens when you call <code>s6 <em>args...</em></code> is the following:
</p>
<ul>
<li> <a href="s6.html">s6</a> gets the name of its configuration file from the
<tt>S6_FRONTEND_CONF</tt> environment variable, or from the compiled-in default if
this variable is unset. Normally this default is <tt>/etc/s6-frontend.conf</tt>. </li>
<li> This configuration file (let's call it <em>conf</em>) contains
<a href="s6-frontend.conf.html">variable definitions</a> that set the default for
all the parts of the s6 system. </li>
<li> <a href="s6.html">s6</a> rewrites itself into
<code>envfile -I <em>conf</em> -- s6-frontend <em>args...</em></code> </li>
<li> The <a href="//skarnet.org/software/execline/envfile.html">envfile</a> program
reads and parses <em>conf</em>, and then runs <a href="s6-frontend.html">s6-frontend</a>
with the additional environment variables defined in <em>conf</em>.
<li> s6-frontend sets its defaults according to these environment variables, then
performs the work described in the documents linked from the <a href="s6.html">s6</a> page. </li>
</ul>
<p>
The separation between <a href="s6.html">s6</a> and s6-frontend is really just a
trick to avoid writing an additional parser for the <a href="s6-frontend.conf.html">s6-frontend.conf</a>
configuration file. The <a href="//skarnet.org/software/execline/">execline</a>
set of tools is a dependency of s6-frontend anyway, because s6-frontend uses
some if these tools to build complex command lines calling various parts of the
s6 ecosystem; so using the
<a href="//skarnet.org/software/execline/envfile.html">envfile</a> tool as well
to read the configuration was a natural fit.
</p>
<p>
That is why <em>usage</em> messages given by the various <code>s6 foobar help</code>
commands print <code>Usage: s6 foobar subcommand...</code> but error messages, if
they happen, print <code>s6-frontend: fatal: error condition</code>, because
the program where the error occurred is s6-frontend, not s6. </p>
<p>
Note that most error messages will print the name of <em>another command</em>,
somewhere in the s6 ecosystem. That is a command invoked by s6-frontend to perform
the task required by the user; and if it fails, its own error message will be more
informative than whatever wrapping s6-frontend could do. This might be a little
confusing at first, but a design decision was made that it was a better trade-off
to provide accurate error messages at the expense of a little predictability in the
name of the program that prepends the error messages.
</p>
<p>
In summary:
</p>
<ul>
<li> <a href="s6.html">s6</a> is what the user types </li>
<li> s6-frontend is what interprets the user's command into orders to give to
various parts of the s6 ecosystem </li>
<li> Various programs from the <a href="//skarnet.org/software/s6/">s6 supervision
suite</a> or the <a href="//skarnet.org/software/s6-rc/">s6-rc service manager</a>,
invoked by s6-frontend, perform the work. When they succeed, it is transparent;
when they fail, they prepend the error message with their name. </li>
</ul>
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