finding the process group id created through setsid

Question

In a shell script, I see that using setsid, we could create a new process group. I am not able to find a reliable way to get the group id after the creation. My requirement is simple, launch a process, and after it is done, clean up any descendant (if any). I dont want to kill the main process, hence I have to wait for the main process to end. After which, I can kill the leftover child processes if I had somehow got the group id. which can be done with kill -- -pgid. The missing piece is how do I get the group id ?

This might help: https://stackoverflow.com/questions/9677306/linux-command-setsid — tk421, Apr 12 '19 at 17:29

score 2 · Answer 1 · answered Apr 14 '19 at 01:33

2

This script is what I came up with finally. Hope this helps someone. $! will give the pid, and a ps has to be used to find its gid. there was an extra space in front while using ps,the next line of variable expansion removes the leading space. Finally after waiting for the main process,it kills the group.

#!/bin/sh -x
setsid "$@"  &
pid=$!
gidspace=$(ps -o pgid= $pid)
gid="${gidspace## }"
echo "gid $gid"
echo "waiting"
wait $pid
ps -s $gid -o pid,ppid,pgid,command
kill -- -$gid

answered Apr 14 '19 at 01:33

melchi

627
6
10

Nice... There is a possibility though that the launched command terminates before you can get the gid. You should make sure `$gid` is not null, and if it is you won't be able to kill the pgid. – Thomas Guyot-Sionnest Mar 13 '21 at 16:08

score 0 · Answer 2 · answered Mar 13 '21 at 18:54

I managed to do it with a coproc, and a sleep to ensure we have enough time to read back the pid. This is bash-specific of course, and the only way to avoid using a hackish sleep inside a coproc is to write to a temp file and wait for the command to terminate (no need for coproc then).

Using a coproc

Note that I open filehandle 3 to write the pgid back to the parent shell and close it before executing the command.

#!/bin/bash -x
coproc setsid bash -c 'ps -o pgid= $BASHPID >&3; exec 3>&-; exec "$@" & sleep 1' -- "$@" 3>&1
read -u ${COPROC[0]} gid
echo "gid $gid"
ps -s $gid -o pid,ppid,pgid,command
kill -- -$gid

Using a temp file

To avoid having to pass the temp file to the subshell (and the risk the parent dies and removes it before child writes to it) I again open fh 3 so the children can write its pgid to it.

#!/bin/bash -x
t=$(mktemp)
trap 'rm -f "$t"' EXIT
exec {fh}>"$t"
setsid bash -c 'ps -o pgid= $BASHPID >&3; exec 3>&-; exec "$@" &' -- "$@" 3>&${fh}
read gid <$t
echo "gid $gid"
ps -s $gid -o pid,ppid,pgid,command
kill -- -$gid

finding the process group id created through setsid

2 Answers2

Using a coproc

Using a temp file