Feedback stdin and stdout of two processes

Question

I have two processes that interact with each other via stdin and stdout.

Suppose I have process A and process B. The default value of B should be passed to stdin A, and stdout A should be connected to the file of process B.

Is there an easy way to express this relationship in a simple command, or is there a basic shell script that can enable this?

Thanks in advance.

Answer 1

See named pipes . Create one channel for AB and one channel for B in A. Then start A, and its stdout is redirected to the first, and its stdin is redirected to the second. Then run B with the opposite.

It will look something like this:

mkfifo --mode=0666 /tmp/AtoB mkfifo --mode=0666 /tmp/BtoA A < BtoA > AtoB B < AtoB > BtoA 

add: Of course, they will need some way to admit that both sides are present. Something like the simple "I'm here, right?" that both get an answer.

Important: As noted in the comments below, this process will be inhibited with the blocking of both programs when reading. Some form of coordination will be necessary to prevent this from happening.

Answer 2

Bash 4 introduces coproc :

 declare -a FDS coproc FDS { process_A; } process_B <&${FDS[0]} >&${FDS[1]} 

Answer 3

(I would comment on Keith's answer, but there is still not enough reputation.)

Testing this on OpenBSD, I could not run the scripts by doing:

 ./a < btoa > atob & ./b < atob > btoa 

( atob and btoa are FIFO, and the scripts a and b duplicate stdin)

However, after I also based on the second one, as soon as I ran > btoa in my shell (the null command, however, opening btoa for writing), they started. (Beware of the infinite loop!) I guess that means you need a third process.

I am not sure if the FIFO behavior is standardized in such cases (for example, opening multiple processes for writing).