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

Question

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 -         


        

Answer 1

You may store the pid of the tail command in a variable using Bash I/O redirections only (see How to get the PID of a process in a pipeline).

# terminal window 1
# using nc on Mac OS X (FreeBSD nc)
: > /tmp/foo
PID=$( { { tail -f /tmp/foo 0<&4 & echo $! >&3 ; } 4<&0 | { nc -l 9977 ;} & } 3>&1 | head -1 )
kill $PID

# terminal window 2
nc localhost 9977

# terminal window 3
echo line > /tmp/foo