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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>&lt;</tt> program </h1>

<p>
<tt>&lt;</tt> redirects a given file descriptor to a file, then
executes a program.
</p>

<h2> Interface </h2>

<pre>
     &lt; [ -r | -w | -u | -a | -c | -x ] [ -n | -b ] <em>fd</em> <em>file</em> <em>prog...</em>
</pre>

<p>
<tt>&lt;</tt> redirects the file descriptor number <em>fd</em>
to <em>file</em>, then execs into <em>prog...</em>.
</p>

<h2> Options </h2>

<p>
 One and only one of the -r, -w, -u, -a, -c, or -x options must be given;
the -n and -b options may be added in any case.
</p>

<ul>
 <li> <tt>-r</tt>&nbsp;: open <em>file</em> for reading. </li>
 <li> <tt>-w</tt>&nbsp;: open <em>file</em> for writing, truncating it if it already exists. </li>
 <li> <tt>-u</tt>&nbsp;: open <em>file</em> for reading and writing. </li>
 <li> <tt>-a</tt>&nbsp;: open <em>file</em> for appending, creating it if it doesn't exist. </li>
 <li> <tt>-c</tt>&nbsp;: open <em>file</em> for appending. Do not create it if it doesn't exist. </li>
 <li> <tt>-x</tt>&nbsp;: open <em>file</em> for writing, creating it, failing if it already exists. </li>
 <li> <tt>-n</tt>&nbsp;: open <em>file</em> in non-blocking mode. </li>
 <li> <tt>-b</tt>&nbsp;: change mode of <em>file</em> after opening it:
to non-blocking mode if the <tt>-n</tt> option was not given,
to blocking mode if it was. </li>
</ul>

<h2> Notes </h2>

<ul>
 <li> <tt>&lt; -r <em>n</em> <em>file</em> prog...</tt> is roughly equivalent to
<tt>sh -c 'exec prog... <em>n</em>&lt;<em>file</em>'</tt></li>
 <li> <tt>&lt; -w <em>n</em> <em>file</em> prog...</tt> is roughly equivalent to
<tt>sh -c 'exec prog... <em>n</em>&gt;<em>file</em>'</tt></li>
 <li> <tt>&lt; -u <em>n</em> <em>file</em> prog...</tt> is roughly equivalent to
<tt>sh -c 'exec prog... <em>n</em>&lt;&gt;<em>file</em>'</tt></li>
 <li> <tt>&lt; -a <em>n</em> <em>file</em> prog...</tt> is roughly equivalent to
<tt>sh -c 'exec prog... <em>n</em>&gt;&gt;<em>file</em>'</tt></li>
 <li> <tt>&lt; -c <em>n</em> <em>file</em> prog...</tt> has no portable
shell equivalent. Some shells provide the <em>noclobber</em> option for
a similar feature. </li>
 <li> <tt>&lt; -x <em>n</em> <em>file</em> prog...</tt> has no portable
shell equivalent.</tt> </li>
</ul>

<h2> Special fifo handling </h2>

<p>
 The <tt>-n</tt> and <tt>-b</tt> options are especially useful with
named pipes. 
</p>

<ul>
 <li> Opening a fifo for reading, blocking if there is no writer:
<tt>&lt; -r <em>n</em> <em>fifo</em> prog...</tt></li>
 <li> Opening a fifo for reading, with instant success even if
there is no writer, and blocking at the first attempt to read from it:
<tt>&lt; -r -nb <em>n</em> <em>fifo</em> prog...</tt></li>
 <li> Opening a fifo for writing, blocking if there is no reader:
<tt>&lt; -w <em>n</em> <em>fifo</em> prog...</tt></li>
 <li> Opening a fifo for writing, with instant success even if
there is no reader:
<tt>&lt; -w -nb <em>n</em> <em>fifo</em> prog...</tt>. Warning:
the first attempt to write to the fifo will raise a SIGPIPE if there is
still no reader at that time. The named pipe semantics normally do not
allow a fifo to be open for writing without a reading end, and you
should know what you are doing if you're using <tt>&lt;</tt>
this way. </li>
</ul>

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