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<title>execline: the redirfd command</title>
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<h1> The <tt>redirfd</tt> program </h1>
<p>
<tt>redirfd</tt> redirects a given file descriptor to a file, then
executes a program.
</p>
<h2> Interface </h2>
<pre>
redirfd [ -r | -w | -u | -a | -c | -x ] [ -n | -b ] <em>fd</em> <em>file</em> <em>prog...</em>
</pre>
<p>
<tt>redirfd</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> : open <em>file</em> for reading. </li>
<li> <tt>-w</tt> : open <em>file</em> for writing, truncating it if it already exists. </li>
<li> <tt>-u</tt> : open <em>file</em> for reading and writing. </li>
<li> <tt>-a</tt> : open <em>file</em> for appending, creating it if it doesn't exist. </li>
<li> <tt>-c</tt> : open <em>file</em> for appending. Do not create it if it doesn't exist. </li>
<li> <tt>-x</tt> : open <em>file</em> for writing, creating it, failing if it already exists. </li>
<li> <tt>-n</tt> : open <em>file</em> in non-blocking mode. </li>
<li> <tt>-b</tt> : 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>redirfd -r <em>n</em> <em>file</em> prog...</tt> is roughly equivalent to
<tt>sh -c 'exec prog... <em>n</em><<em>file</em>'</tt></li>
<li> <tt>redirfd -w <em>n</em> <em>file</em> prog...</tt> is roughly equivalent to
<tt>sh -c 'exec prog... <em>n</em>><em>file</em>'</tt></li>
<li> <tt>redirfd -u <em>n</em> <em>file</em> prog...</tt> is roughly equivalent to
<tt>sh -c 'exec prog... <em>n</em><><em>file</em>'</tt></li>
<li> <tt>redirfd -a <em>n</em> <em>file</em> prog...</tt> is roughly equivalent to
<tt>sh -c 'exec prog... <em>n</em>>><em>file</em>'</tt></li>
<li> <tt>redirfd -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>redirfd -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>redirfd -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>redirfd -r -nb <em>n</em> <em>fifo</em> prog...</tt></li>
<li> Opening a fifo for writing, blocking if there is no reader:
<tt>redirfd -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>redirfd -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>redirfd</tt>
this way. </li>
</ul>
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