doc/if.html


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<h1> The <tt>if</tt> program </h1>

<p>
<tt>if</tt> performs conditional execution.
</p>

<h2> Interface </h2>

<p>
 In an <a href="execlineb.html">execlineb</a> script:
</p>

<pre>
     if [ -X ] [ -n ] [ -t | -x <em>exitcode</em> ] { <em>prog1...</em> } <em>prog2...</em>
</pre>

<ul>
 <li> <tt>if</tt> reads <em>prog1...</em> in a
<a href="el_semicolon.html">block</a>. It forks and executes it,
then waits for it to complete. </li>
 <li> If <em>prog1</em> crashes, <tt>if</tt> prints an error message
then exits 128 plus the number of the signal that killed <em>prog1</em>. </li>
 <li> If <em>prog1</em> exits with a non-zero code, <tt>if</tt> exits with an
<a href="exitcodes.html">approximation</a> of that code. </li>
 <li> Else <tt>if</tt> execs into <em>prog2</em>. </li>
</ul>

<h2> Options </h2>

<ul>
 <li> <tt>-X</tt>&nbsp;: treat a crash of <em>prog1</em> as a non-zero ("false") exit. This
is mostly useful in combination with <tt>-n</tt>. </li>
 <li> <tt>-n</tt>&nbsp;: negate the test. If <em>prog1</em> exits true, then <tt>if</tt> will
exit 1; else it will exec into <em>prog2</em>. </li>
 <li> <tt>-x</tt>&nbsp;<em>exitcode</em>&nbsp;: if <tt>if</tt> needs to exit, it will exit <em>exitcode</em>
instead of whatever else it would have exited. </li>
 <li> <tt>-t</tt>&nbsp;: if <tt>if</tt> needs to exit, it will exit 0. This is equivalent
to <tt>-x 0</tt>. </li>
</ul>

<h2> Notes </h2>

<ul>
 <li> <tt>if</tt> will exit if <em>prog1...</em> exits false. To use it in
an execline script that must run <em>prog3...</em> no matter the result of
the test, use a <tt>foreground</tt> wrapper:
<pre> foreground { if { <em>prog1...</em> } <em>prog2...</em> } <em>prog3...</em> </pre>
(in <a href="execlineb.html">execlineb</a> syntax) </li>
 <li> <tt>if <em>prog1...</em> "" <em>prog2...</em></tt> is
equivalent to <tt>sh -c '<em>prog1...</em> && exec <em>prog2...</em>'</tt>. </li>
</ul>

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