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    <p>The following works in bash 4, using <A HREF="http://www.gnu.org/software/bash/manual/html_node/Coprocesses.html" rel="nofollow">coprocesses</A>:</p> <pre><code>#!/bin/bash fd_re='^[0-9]+$' cleanup_and_wait() { if [[ ${COPROC[1]} =~ $fd_re ]] ; then eval "exec ${COPROC[1]}&lt;&amp;-" echo "waiting for $filename to finish" &gt;&amp;2 wait $COPROC_PID fi } while IFS= read -r line; do case $line in @*) cleanup_and_wait filename=${line:1} echo "starting $filename" &gt;&amp;2 coproc { sort &gt;"$filename"; echo "Finished with $filename" &gt;&amp;2; } ;; *) printf '%s\n' "$line" &gt;&amp;${COPROC[1]} ;; esac done cleanup_and_wait </code></pre> <p>For prior versions of bash, a named pipe can be used instead:</p> <pre><code>cleanup_and_wait() { if [[ $child_pid ]] ; then exec 4&lt;&amp;- echo "waiting for $filename to finish" &gt;&amp;2 wait $child_pid fi } # this is a bit racy; without a force option to mkfifo, # however, the race is unavoidable fifo_name=$(mktemp -u -t fifo.XXXXXX) if ! mkfifo "$fifo_name" ; then echo "Someone else may have created our temporary FIFO before we did!" &gt;&amp;2 echo "This can indicate an attempt to exploit a race condition as a" &gt;&amp;2 echo "security vulnarability and should always be tested for." &gt;&amp;2 exit 1 fi # ensure that we clean up even on unexpected exits trap 'rm -f "$fifo_name"' EXIT while IFS= read -r line; do case $line in @*) cleanup_and_wait filename=${line:1} echo "starting $filename" &gt;&amp;2 { sort &gt;"$filename"; echo "finished with $filename" &gt;&amp;2; } &lt;"$fifo_name" &amp; child_pid=$! exec 4&gt;"$fifo_name" ;; *) printf '%s\n' "$line" &gt;&amp;4 ;; esac done cleanup_and_wait </code></pre> <p>A few notes:</p> <ul> <li>It's safer to use <code>printf '%s\n' "$line"</code> than <code>echo "$line"</code>; if a line contains only <code>-e</code>, for instance, some versions of <code>echo</code> will do nothing with it.</li> <li>Using an EXIT trap for cleanup ensures that an unexpected SIGTERM or other error won't leave the stale fifo sitting around.</li> <li>If your platform provides a way to create a FIFO with an unknown name in a single, atomic operation, use it; this would avoid the condition that requires us to always test whether the mkfifo is successful.</li> </ul>
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    1. PObash: How do I ensure termination of process substitution used with exec?
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    1. USCharles Duffy
    1. USCharles Duffy
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    1. PObash: How do I ensure termination of process substitution used with exec?
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    1. CODon't know why but if you use directly > fifo, without the use of file descriptor, 3 for example, the sub-process will stop at the first command printf. And the aim in my answer was to avoid to change the code at the maximum.
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      1. USNahuel Fouilleul
    2. CO@NahuelFouilleul Good catch, thank you -- the way I had it written was trying to open the input side of the FIFO multiple times. Tested the coprocess version thoroughly; I should have done the same with the FIFO version.
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      1. USCharles Duffy
    3. COalso you need to close the file descriptor 4>&-
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      1. USNahuel Fouilleul
 

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