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<title>execline: the pipeline command</title>
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<p>
<a href="index.html">execline</a><br />
<a href="//skarnet.org/software/">Software</a><br />
<a href="//skarnet.org/">skarnet.org</a>
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<h1> The <tt>pipeline</tt> program </h1>
<p>
<tt>pipeline</tt> runs two commands with a pipe between them.
</p>
<h2> Interface </h2>
<p>
In an <a href="execlineb.html">execlineb</a> script:
</p>
<pre>
pipeline [ -d ] [ -r | -w ] { <em>prog1...</em> } <em>prog2...</em>
</pre>
<ul>
<li> <tt>pipeline</tt> reads <em>prog1...</em> in a
<a href="el_semicolon.html">block</a> and unquotes it. </li>
<li> It runs <em>prog1...</em> as a child process and execs into
<em>prog2...</em>, with a pipe between <em>prog1</em>'s stdout and
<em>prog2</em>'s stdin. </li>
<li> <em>prog1</em>'s pid is available in <em>prog2</em> as the !
environment variable. </li>
</ul>
<h2> Options </h2>
<ul>
<li> <tt>-d</tt> : run <em>prog1...</em>
as a grandchild of <tt>pipeline</tt>. This is meant to prevent a zombie
from hanging around if <em>prog2...</em> fails to wait for its children.</li>
<li> <tt>-r</tt> : make <em>prog1...</em> the writer and
<em>prog2...</em> the reader. This is the default. </li>
<li> <tt>-w</tt> : make <em>prog1...</em> the reader and
<em>prog2...</em> the writer. </li>
</ul>
<h2> Notes </h2>
<ul>
<li> You can easily create a chain of pipes: <tt>pipeline a "" pipeline b "" c</tt>
is roughly equivalent to
<tt>sh -c 'exec a | b | c'</tt>, except that shells usually run <tt>c</tt>
as a child process like <tt>a</tt> and <tt>b</tt>, and <tt>exec</tt> has no
effect. </li>
</ul>
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