Connecting input _and_output between of two commands in shell/bash

Question

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.

Answer 1

How about a named pipe?

# mkfifo foo
# A < foo | B > foo
# rm foo

For your second part I believe tee is the correct answer. So it becomes:

# A < foo | tee logfile | B > foo

Answer 2

http://bisqwit.iki.fi/source/twinpipe.html