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POConnecting input _and_output between of two commands in shell/bash
 

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    <p>I have two (UNIX) programs A and B that read and write from stdin/stdout.</p> <p>My first problem is how to connect the stdout of A to stdin of B <em>and</em> the stdout of B to the stdin of A. I.e., something like A | B but a bidirectional pipe. I suspect I could solve this by <a href="http://tldp.org/LDP/abs/html/x16834.html" rel="noreferrer">using exec to redirect</a> but I could not get it to work. The programs are interactive so a temporary file would not work.</p> <p>The second problem is that I would like to duplicate each direction and pipe a duplicate via a logging program to stdout so that I can see the (text-line based) traffic that pass between the programs. Here I may get away with tee >(...) if I can solve the first problem.</p> <p>Both these problems seems like they should have well known solutions but I have not be able to find anything.</p> <p>I would prefer a POSIX shell solution, or at least something that works in bash on cygwin.</p> <p>Thanks to your answers I came up with the following solution. The A/B commands uses nc to listen to two ports. The logging program uses sed (with -u for unbuffered processing).</p> <pre><code>bash-3.2$ fifodir=$(mktemp -d) bash-3.2$ mkfifo "$fifodir/echoAtoB" bash-3.2$ mkfifo "$fifodir/echoBtoA" bash-3.2$ sed -u 's/^/A-&gt;B: /' "$fifodir/echoAtoB" &amp; bash-3.2$ sed -u 's/^/B-&gt;A: /' "$fifodir/echoBtoA" &amp; bash-3.2$ mkfifo "$fifodir/loopback" bash-3.2$ nc -l -p 47002 &lt; "$fifodir/loopback" \ | tee "$fifodir/echoAtoB" \ | nc -l -p 47001 \ | tee "$fifodir/echoBtoA" &gt; "$fifodir/loopback" </code></pre> <p>This listens for connection to port 47001 and 47002 and echos all traffic to standard output.</p> <p>In shell 2 do:</p> <pre><code>bash-3.2$ nc localhost 47001 </code></pre> <p>In shell 3 do:</p> <pre><code>bash-3.2$ nc localhost 47002 </code></pre> <p>Now lines entered in shell 2 will be written to shell 3 and vice versa and the traffic logged to shell 1, something like:</p> <pre><code>B-&gt;A: input to port 47001 A-&gt;B: input to port 47002 </code></pre> <p>The above has been tested on Cygwin</p> <p>Update: The script above stopped working after a few days(!). Apparently it can deadlock. Some of the suggestions in the answers may be more reliable.</p>
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