Re: Kill sends signal to wrong process group with job control enabled

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