Bash – SIGINT not being propagated to background child process

When a process is forked, it inherits its PGID from its parent. The PGID changes when a process becomes a process group leader, then its PGID is copied from its PID. From then on, the new child processes it spawns, and their descendants, inherit that PGID (unless they start new process groups of their own).

In a shell with job control, such as most interactive shells, each job is put in its own process group. If you run a shell script, the shell process running the script will be the group leader, and the PGID will equal its PID.

In a shell without job control, such as most shells used to run scripts, commands are run in the shell's process group.

The syntax kill -- -N kills all the processes in the group with PGID = N. You can't use it with an arbitrary PID, only the PID of a process group leader, since that's the PGID. This is essentially how the shell's

kill %jobid

syntax works -- it internally translates %jobid to the PGID of the job and sends the signal to that PGID.

There's no simple way to run a script in its own process group from another shell script. See How to set process group of a shell script for some suggestions, though.

There are several ways you can cut off the effect of Ctrl+C:

• Change the terminal setting so that it doesn't generate a signal.
• Block the signal so that it is saved for later delivery, when the signal becomes unblocked.
• Ignore the signal, or set a handler for it.
• Run subprocesses in a background process group.

Since you want to detect that Ctrl+C has been pressed, ignoring the signal is out. You could change the terminal settings, but then you would need to write custom key processing code. Shells don't provide access to signal blocking.

You can however isolate subprocesses from receiving the signal automatically by running them in a separate process group. Interactive shells run background commands in a separate process group by default, but non-interactive shells run them in the same process group, and all processes in the foreground process group receive a signal from terminal events. To tell the shell to run background jobs in a separate process group, run set -m. Running setsid ping … is another way of forcing ping to run in a separate process group.

set -m
interrupted=
trap 'echo Interrupted, but ping may still be running' INT
set -m
ping … &
while wait; [ $? -ge 128 ]; do echo "Waiting for background jobs"; done
echo ping has finished

If you want Ctrl+Z to suspend a background process group, you'll need to propagate the signal from the shell.

Controlling signals finely is a bit of a stretch for a shell script, and shells other than ATT ksh tend to be a little buggy when you reach the corner cases, so consider a language that gives you more control such as Perl, Python or Ruby.