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

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=!

Answer 2

This works for me (SLES Linux):

tail -F xxxx | tee -a yyyy &
export TAIL_PID=`jobs -p`
# export TEE_PID="$!"

The ps|grep|kill trick mentioned in this thread would not work if a user can run the script for two "instances" on the same machine.

jobs -x echo %1 did not work for me (man page not having the -x flag) but gave me the idea to try jobs -p.

Answer 3

Maybe you could use a fifo, so that you can capture the pid of the first process, e.g.:

FIFO=my_fifo

rm -f $FIFO
mkfifo $FIFO

tail -f $1 > $FIFO &
TAIL_PID=$!

cat $FIFO | nc -l -p 9977

kill $TAIL_PID

rm -f $FIFO

Answer 4

Finally, I have managed to find the tail process using ps. Thanks to the idea from ennuikiller.

I have used the ps to grep tail from the args and kill it. It is kind of a hack but it worked. :)

If you can find a better way please share.

Here is the complete script:
(Latest version can be found here: http://docs.karamatli.com/dotfiles/bin/logserver)

if [ -z "$1" ]; then
    echo Usage: $0 LOGFILE [PORT]
    exit -1
fi
if [ -n "$2" ]; then
    PORT=$2
else
    PORT=9977
fi

TAIL_CMD="tail -f $1"

function kill_tail {
    # find and kill the tail process that is detached from the current process
    TAIL_PID=$(/bin/ps -eo pid,args | grep "$TAIL_CMD" | grep -v grep | awk '{ print $1 }')
    kill $TAIL_PID
}
trap "kill_tail; exit 0" SIGINT SIGTERM

while true; do
    ( $TAIL_CMD & ) | nc -l -p $PORT -vvv
    kill_tail
done

Answer 5

Not an ideal answer, but I found a workaround for a logger daemon I worked on:

#!/bin/sh
tail -f /etc/service/rt4/log/main/current --pid=$$ | grep error

from $info tail:

--pid=PID
          with -f, terminate after process ID, PID dies

Answer 6

how about this:

jobs -x echo %1

%1 is for first job in chain, %2 for second, etc. jobs -x replaces job specifier with PID.