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<title>execline: the backtick command</title>
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<p>
<a href="index.html">execline</a><br />
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<h1> The <tt>backtick</tt> program </h1>
<p>
<tt>backtick</tt> runs a program and stores its output in an
environment variable, then executes another program.
</p>
<h2> Interface </h2>
<p>
In an <a href="execlineb.html">execlineb</a> script:
</p>
<pre>
backtick [ -i | -I | -x | -D <em>default</em> ] [ -N | -n ] [ -E | -e ] <em>variable</em> { <em>prog1...</em> } <em>prog2...</em>
</pre>
<ul>
<li> <tt>backtick</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 saves its
output in memory. This output must not contain a null character. </li>
<li><tt>backtick</tt> execs into <em>prog2...</em>, with
<em>variable</em> added to the environment with <em>prog1...</em>'s
output as a value. </li>
</ul>
<h2> Options </h2>
<ul>
<li> <tt>-N</tt> : store <em>prog1...</em>'s output as is, including the last newline, if any. </li>
<li> <tt>-n</tt> : chomp an ending newline off <em>prog1...</em>'s output. This is the default. </li>
<li> <tt>-e</tt> : no autoimport. This is the default. </li>
<li> <tt>-E</tt> : autoimport. Before exec'ing into
<em>prog2...</em>, perform a
<a href="https://skarnet.org/software/execline/el_substitute.html">substitution</a>
so that <tt>$?</tt> is replaced with <em>prog1...</em>'s exitcode (if
applicable, see below), and
<tt>${<em>variable</em>}</tt> is replaced with <em>prog1...</em>'s output,
instead of keeping the results in the environment. The variables will not
appear in the environment; if they preexisted in <tt>backtick</tt>'s
environment, they will be removed. </li>
</ul>
<p>
The other options tell backtick what to do if <em>prog1...</em>'s output
is not suitable as the contents of an environment variable (i.e. it
contains a null character) or if <em>prog1...</em> crashes or exits
nonzero:
</p>
<ul>
<li> <tt>-i</tt> : backtick exits with an
<a href="exitcodes.html">approximation</a> of <em>prog1</em>'s exit code,
or 124 if <em>prog1</em> wrote a null character. This is the default. </li>
<li> <tt>-I</tt>: the value of <em>variable</em> is set to whatever
the start of <em>prog1...</em>'s output is, up to the first null character,
or to whatever <em>prog1...</em> wrote before crashing; chomping is applied
if required; then execution proceeds. </li>
<li> <tt>-x</tt> : <em>variable</em> is <strong>removed</strong> from
the environment, and execution proceeds. </li>
<li> <tt>-D <em>default</em></tt> : the value of <em>variable</em>
is set to <em>default</em>, and execution proceeds. </li>
</ul>
<p>
If the command line can keep running after <em>prog1</em> crashes or exits
nonzero, i.e. if one of the <tt>-I</tt>, <tt>-x</tt> -or <tt>-D</tt> has been
given, then <tt>backtick</tt> also adds the <tt>?</tt> variable to the
environment. If <em>prog1</em> exited normally, the variable contains its exit
code; otherwise, 256 plus the number of the signal that killed it.
</p>
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