I have two (UNIX) programs A and B that read and write from stdin/stdout.
My first problem is how to connect the stdout of A to stdin of B and 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 using exec to redirect but I could not get it to work. The programs are interactive so a temporary file would not work.
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.
Both these problems seems like they should have well known solutions but I have not be able to find anything.
I would prefer a POSIX shell solution, or at least something that works in bash on cygwin.
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).
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->B: /' '$fifodir/echoAtoB' & bash-3.2$ sed -u 's/^/B->A: /' '$fifodir/echoBtoA' & bash-3.2$ mkfifo '$fifodir/loopback' bash-3.2$ nc -l -p 47002 < '$fifodir/loopback' \ | tee '$fifodir/echoAtoB' \ | nc -l -p 47001 \ | tee '$fifodir/echoBtoA' > '$fifodir/loopback' This listens for connection to port 47001 and 47002 and echos all traffic to standard output.
In shell 2 do:
bash-3.2$ nc localhost 47001 In shell 3 do:
bash-3.2$ nc localhost 47002 Now lines entered in shell 2 will be written to shell 3 and vice versa and the traffic logged to shell 1, something like:
B->A: input to port 47001 A->B: input to port 47002 The above has been tested on Cygwin
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.
http://bisqwit.iki.fi/source/twinpipe.html