On 01/03/2026 01:38, Martin D Kealey wrote:
On Sat, 28 Feb 2026, 17:30 Ben Ashton, <[email protected]> wrote:
…
We now have sleep running as a child of the subshell as a process
group leader.
However in certain, somewhat contrived, circumstances, bash's
builtin kill
will send the signal to the WRONG process group.
> Sending SIGTERM to process group: 5710 (kill -TERM -5710)
[pid 5709] kill(-5709, SIGTERM) = 0
As you can see we ran `kill -TERM -5710`, but bash actually sent
it to process group 5709 (the subshell itself). If you don't use the
builtin kill then it works as expected
…
Here is the script to reproduce the issue:
setsid sleep 10 &
pid=$!
echo "> Started new process group $pid"
…
echo "> Sending SIGTERM to process group: $pid (kill -TERM
-$pid)"
kill -TERM -$pid
This is the core error: $! records a PID, NOT a PGID or a SID. On some
systems (e.g. Linux) a PGID or SID is always numerically equal to the
PID of the leader process, on others (e.g. BSD) they are unequal.
So what would you expect to happen if you go «kill -INT -$pid» on a
systems where pgrps and their respective leader pids are not identical?
Would you want it to translate the PID to its pgrps, or would you
prefer it to fail?
If it's going to do translations on BSD, why should it NOT do so on
Linux?
-Martin
I think you misunderstand the issue. It doesn't matter how I obtain the
PGID. The issue is that I'm running "kill -TERM -SOME_PGID" and the
syscall bash is making is "kill(-DIFFERENT_PGID, SIGTERM)". That's
objectively wrong. I don't expect any translation, I just expect it to
kill the process group that I tell it to.
It's unrelated, but I'm curious what systems don't make the PGID the
same as the PID of the leader? I thought that was a POSIX thing. The
FreeBSD syscall manual for "setsid" states: "the setsid() system call
returns the value of the process group ID of the new process group,
which is the same as the process ID of the calling process."
https://man.freebsd.org/cgi/man.cgi?query=setsid&apropos=0&sektion=2&manpath=FreeBSD+16.0-CURRENT&format=html
