How to get the PID of a process that is piped to another process in Bash?

user_102432 0
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I am trying to implement a simple log server in Bash. It should take a file as a parameter and serve it on a port with netcat.

( tail -f $1 & ) | nc -l -p 9977

But the problem is that when the netcat terminates, tail is left behind running. (Clarification: If I don't fork the tail process it will continue to run forever even the netcat terminates.)

If I somehow know the PID of the tail then I could kill it afterwards.
Obviously, using $! will return the PID of netcat.

How can I get the PID of the tail process?

pid pipe bash
14 Answers

You could use the coproc command twice.

The given example translates to:

coproc TAIL { tail -f $1; }; exec {TAIL[1]}<&-
coproc NC { nc -v -l -p 9977; } <&"${TAIL[0]}" >&1
wait $NC_PID; echo "nc exit code: $!"
kill $TAIL_PID; echo "done"

(I've thrown a -v and a couple echo in there for troubleshooting.)

Using coproc feels a lot like using Popen() in various other scripting languages.

user_106476800

bobbogo answer works but requires a intermediary pid file.

You can leverage the process substitution feature >() which works like| but without waiting for tail to finish

tail -f $1 > >(nc -l -p 9977) & pid=!
user_19296710
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