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<h1> The <tt>wait</tt> program </h1>
<p>
<tt>wait</tt> waits for a set of children, then executes a program.
</p>
<h2> Interface </h2>
<p>
In an <a href="execlineb.html">execlineb</a> script:
</p>
<pre>
wait [ -a | -o ] [ -u ] [ -t <em>timeout</em> ] { [ <em>pids...</em> ] } <em>prog...</em>
</pre>
<ul>
<li> <tt>wait</tt> reads a list of <em>pids</em> in a
(possibly empty) <a href="el_semicolon.html">block</a>,
and unquotes it. </li>
<li> <tt>wait</tt> waits for every child whose pid is
listed in <em>pids...</em>. If <em>pids...</em> is an
empty list, it waits for every child process it has. </li>
<li> <tt>wait</tt> then execs into <em>prog...</em>. </li>
</ul>
<h2> Options </h2>
<ul>
<li> <tt>-t</tt> <em>timeout</em> : wait for a maximum
of <em>timeout</em> milliseconds. If there still are living processes
among <em>pids...</em> (or among <tt>wait</tt>'s children if
<em>pids...</em> is an empty list), after <em>timeout</em>
milliseconds, they will not be waited for. In particular, <tt>-t0</tt>
tells <tt>wait</tt> to perform a single reaping pass on current zombies
and immediately exec into <em>prog...</em>. </li>
<li> <tt>-o</tt> : wait for <em>one</em> of the listed
<em>pids</em> — exec into <em>prog</em> as soon as one of the
listed children dies. (If no pid is listed, wait for one child to die.)
<li> <tt>-a</tt> : wait for <em>all</em> of the listed <em>pids</em>.
This is the default. </li>
<li> <tt>-u</tt> : do not export the result in the environment,
even if <tt>wait</tt> succeeds. </li>
</ul>
<h2> Environment variables </h2>
<p>
If the <tt>-u</tt> option has been given, <tt>wait</tt> execs into <em>prog...</em>
with the <tt>!</tt> and <tt>?</tt> environment variables unset. Same thing if an
error occurred before <tt>wait</tt> could successfully reap a zombie.
</p>
<p>
Otherwise, the <tt>!</tt> environment variable is set to the pid of a reapt zombie,
and the <tt>?</tt> environment variable is set to its exit code. Here is how <tt>wait</tt>
decides what pid to report:
</p>
<ul>
<li> If the <tt>-o</tt> option has been given, <tt>!</tt> is set to the pid of <em>the<em>
one zombie that has just been reapt. </li>
<li> Otherwise, if <tt>wait</tt> has been given an empty list of pids (meaning it should
wait for all of its children), then <tt>!</tt> is set to the pid of the last child it has
waited for. If there were no children, <tt>!</tt> and <tt>?</tt> are unset. </li>
<li> Otherwise, <tt>!</tt> is set to the <em>last</em> pid in the list provided to <tt>wait</tt>. </li>
</ul>
<h2> Notes </h2>
<ul>
<li> For <a href="https://pubs.opengroup.org/onlinepubs/9799919799/utilities/wait.html">POSIX
compatibility</a>, <tt>wait</tt> also works when it cannot find a block.
In that case, all the options are still supported and have the same
effect, but the rest of the command line is interpreted as <em>pids...</em>
arguments and <tt>wait</tt> does not execute into a program; instead, it exits
with a conforming exit code. </li>
</ul>
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