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From: vallor <vallor@cultnix.org>
Newsgroups: comp.unix.shell
Subject: Re: Cleaning up background processes
Date: Mon, 6 May 2024 00:19:32 -0000 (UTC)
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On Sun, 5 May 2024 19:06:22 -0000 (UTC), Christian Weisgerber
<naddy@mips.inka.de> wrote in <slrnv3fm5e.jrj.naddy@lorvorc.mips.inka.de>:

> Is there a standard POSIX shell idiom to clean up background processes?
> 
> You have a shell script that starts some background process with &. Now
> you want to make sure that the background process terminates when the
> shell script terminates.  In particular, when it terminates due to
> special circumstances.
> 
> A noninteractive shell doesn't use job control, so the background
> process shares the same process group.  That's great!  When somebody
> hits ^C, SIGINT is sent to the whole process group.  The same for ^\ and
> SIGQUIT, and for SIGHUP when the modem hangs up^W^W^Wxterm is closed. 
> So the background process will be terminated by default.
> 
> Except... bash seems to block SIGINT for background processes.  As does
> FreeBSD's sh with both SIGINT and SIGQUIT.  What now?
> 
> Also, the shell script is typically invoked from some implementation of
> make(1), which seems to add more complications.
> 
> This seems like a sufficiently common problem that there must be a
> standard solution.

I have scripts that need to kill off processes started with
"&", and there's a couple ways to do it.

One way is "kill -1 0" -- sending the HUP signal with "0"
as the process number will send the signal to the process group. 

Another way is to remember the process, such as:

sleep 1000000 & # backgrounded process
BACKGROUND=$!

And then set a trap:

trap "kill -1 $BACKGROUND" 0

You can get even fancier with something like:

hang_up_the_phone()
{
HPID=$1;
kill -0 $HPID > /dev/null 2>&1 && kill -1 $HPID;
}

trap "hang_up_the_phone $BACKGROUND" 0

I have a script that uses both, if you'd like me
to post it -- it runs xdaliclock in timer mode, as well
as a sleep for the length of the timer.  For that, the
trap looks like this:

trap "hang_up_the_phone $BACK1 ; \
 hang_up_the_phone $BACK2 ; " 0 CHLD;

-- 
-v