What Is a Coprocess?

A coprocess is a process connected to the shell by pipes in both directions. The shell can write to the process's stdin and read from the process's stdout.

In zsh:

coproc cat
print -r -- hello >&p
read -r line <&p
print -r -- "$line"

The special p redirection target is the key limitation. zsh can start another coprocess, and the operating system can handle many pipes, but zsh exposes only one current p target at a time.

co-proc uses native zsh behavior to create the process, then duplicates the current p descriptors into ordinary numbered file descriptors:

exec {outfd}<&p
exec {infd}>&p

Those ordinary descriptors remain usable after another coprocess retargets p.