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bashpipepid

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

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?

Solution

  • Write tail's PID to file descriptor 3, and then capture it from there.

    ( tail -f $1 & echo $! >&3 ) 3>pid | nc -l -p 9977
kill $(<pid)