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<title>s6: the s6-envuidgid program</title>
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<h1> The s6-envuidgid program </h1>
<p>
s6-envuidgid potentially sets the UID, GID and GIDLIST environment
variables according to the options and arguments it is given; then it
executes into another program.
</p>
<h2> Interface </h2>
<pre>
s6-envuidgid [ -u | -g | -B ] [ -n ] [ -i | -D <em>uid</em>:<em>gid</em>:<em>gidlist</em> ] <em>account</em> <em>prog...</em>
</pre>
<ul>
<li> s6-envuidgid looks <em>account</em> up by name in the account database. </li>
<li> It sets the UID environment variable to <em>account</em>'s uid, and the GID
environment variable to <em>account</em>'s gid. </li>
<li> It also sets the GIDLIST environment variable to a comma-separated list of
supplementary group ids <em>account</em> is a member of according to the
group database. (If <em>account</em> doesn't belong to any other group than its
primary group, GIDLIST is still set, but empty.) </li>
<li> Then it executes into <em>prog...</em>. </li>
</ul>
<h2> Options </h2>
<ul>
<li> <tt>-u</tt> : user. <em>account</em> will be interpreted as a
user name; the UID environment variable will be set to its numerical
value, and the GID and GIDLIST variables will not be touched. </li>
<li> <tt>-g</tt> : group. <em>account</em> will be interpreted as a
group name instead of a user name; the GID environment variable will be set
to its numerical value, and the UID and GIDLIST variables will not be touched. </li>
<li> <tt>-B</tt> : both user and group. <em>account</em> will be
interpreted as <em>user<tt>:</tt>group</em>. The GIDLIST variable will not be
touched. If <em>user</em> does not exist, the UID variable will be set to 0 unless
a better default is provided with the <tt>-D</tt> option.
If <em>group</em> does not exist, the GID variable will be set to 0 unless a
better default is provided with the <tt>-D</tt> option. </li>
<li> <tt>-n</tt> : numerical fallback. If <em>account</em> cannot be
found in the user or group database, try to interpret the given values literally.
For instance, <tt>s6-envuidgid -B root:42</tt> will fail if there's no group
named <tt>42</tt> in the group database, but <tt>s6-envuidgid -nB root:42</tt>
will set UID to 0 and GID to 42. </li>
<li> <tt>-i</tt> : insist. If <em>account</em> is unknown, exit 1 with
an error message. This is the default. </li>
<li> <tt>-D <em>uid</em>:<em>gid</em>:<em>gidlist</em></tt> : if <em>account</em>
is unknown, use <em>uid</em>, <em>gid</em> and <em>gidlist</em> as the values for
UID, GID and GIDLIST. </li>
</ul>
<h2> Notes </h2>
<ul>
<li> s6-envuidgid without options behaves like
<a href="https://cr.yp.to/daemontools/envuidgid.html">envuidgid</a>, except that
the exit code is 1 if <em>account</em> doesn't exist, and it also exports
supplementary groups. </li>
<li> s6-envuidgid is useful when running a program that must start as root but can
drop its privileges later. Such a program can read its new uid/gid/groups info
from the UID, GID and GIDLIST environment variables. Super-servers such as
<a href="//skarnet.org/software/s6-networking/s6-tcpserver4.html">s6-tcpserver4</a>
make use of this. </li>
</ul>
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