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

On 01/03/2026 22:18, Oğuz wrote:
> On Monday, March 2, 2026, Ben Ashton <[email protected] <mailto:[email protected]>> 
> wrote:
> 
>     As you can see, bash is the only one that translates $! to the PGID
>     of the subshell.
> 
> 
>   I don't think that is what's going on here. It looks more like bash 
> turns off job control in the subshell and includes `setsid sleep 5 &' in 
> the subshell's process group, which makes sense.
> 
> 
> -- 
> Oğuz
>

That IS what's going on. Setsid makes sleep the leader of a new process 
group.

Runing this script:

   #!/usr/bin/env bash
   set -m
   (
     setsid sleep 40 &
     pid=$!
     echo "Sleep pid: $pid"
     sleep 20
     kill -TERM -$pid
     echo "Here"
   )

With "strace -f -e 'trace=kill' ./script.sh", gives the following output 
(edited for brevity + comments)

   strace: Process 17584 attached <- Subshell
   strace: Process 17585 attached <- Sleep 40 (after setsid + execvp)
   Sleep pid: 17585 <- sleep PID is the same as its PGID (see below)
   strace: Process 17586 attached <- Sleep 20 (delay before kill)
   [pid 17586] +++ exited with 0 +++ <- Sleep 20 exits
   [pid 17584] --- SIGCHLD {si_pid=17586} --- <- Subshell receives CHLD
   [pid 17584] kill(-17584, SIGTERM)       = 0 <- Kill translates -17585 
to -17584
   [pid 17584] --- SIGTERM {si_pid=17584} --- <- Subshell receives TERM 
from itself
   [pid 17584] +++ killed by SIGTERM +++
   Terminated

Before the kill command is run, here is the process tree:

   PID     PGID
   17581   17581  |  \_ strace -f -e trace=kill ./script.sh
   17583   17581  |      \_ bash ./script.sh
   17584   17584  |          \_ bash ./script.sh
   17585   17585  |              \_ sleep 40
   17586   17584  |              \_ sleep 20