How can I capture output of background process

Question

What is the best way of running process in background and receiving its output only when needed?

Intended usage: make prompt-outputting script with heavy initialization be initialized once per session and not on each prompt run. Note: two-way communication is needed: shell needs to tell when new prompt is needed, what is the last command status.

Known solutions:

• some explicitly created files on filesystem (FIFO files, UNIX sockets): it would be better to avoid this as this means I need to choose file name, be sure it is garbage-collected on exit and add something to clean no longer used files in case of a crash.
• zsh/zpty module: it is a bit like overkill for this job and does not work in bash.
• coprocesses: does not work in bash and AFAIK only one coprocess per session is allowed.

Answer 1

Bash supports coprocesses sinces 4.0, but multiple coprocesses is still experimental.

I would have gone with some explicitly created files, naming them ~/.myThing-$HOSTNAME/fifo if they're per user and host. You can use flock to relatively easily determine if the command is still running and optionally start it:

(
  flock -n 123 || exit 1
  rm/mkfifo ..
  exec yourServer < .. > ..
) 123> ~/".myThing-$HOSTNAME/lockfile"

If the command or server dies, the lock is automatically released and you only have a few zero length files lying around. The next time the server starts, it deletes and sets them up again.

Querying the server would be similar, but exiting if the lock is not in use (and optionally using a wait lock to avoid contention).