Re: [gentoo-user] [OT] kill a child and suicide

[Hans-Werner Hilse]

Hi,

On Tue, 2 May 2006 17:42:26 +0100 (WEST)
Jorge Almeida <jalmeida@××××××××××××.pt> wrote:

> On Tue, 2 May 2006, Zac Slade wrote:
>
> > You can find the PID of the last backgrouned process using the bash variable
> > $!
> >
> The child is not backgrounded!
> > So something like:
> > subprocess &
> > $pid=$!
> >
> > Using trap along with maybe setting alarms should get you what you want.
> >
> Based on the suggestions of Uwe and Vladimir, I tried
>  	trap 'pkill -TERM -P $$; kill -s TERM $$' TERM
>  	<do something>
>  	. /path/to/child.sh
>  	<do something else>
> Doesn't work, yet. Note that child.sh is a shell script that may execute
> some other command (like rsync), so the "." by itself may not be enough.

This can't work because of this (man bash):
--snip
If bash is waiting for a command to complete and receives a signal for
which a trap has been set, the trap will  not  be  executed until the
command completes.
--snip

What instead works (just tested):
--snip
#!/bin/sh
COMMAND="sleep 120"

# First we background:
$COMMAND &
# Save the PID
CHILDPID=$!
# Trap the signal:
trap "kill -TERM $CHILDPID" TERM
# And wait for the Child to finish:
wait $CHILDPID
# reset signal handling:
trap - TERM
--snip

Note that the code could hit a racing condition and should therefore
not carelessly run by root on a machine with untrusted users. This is:
The process may have finished before setting the signal handler.
Other processes *might* reuse the PID afterwards and might get
sig-TERM-ed until resetting the signal handler again. Probably a minor,
depending on the script's usage.


-hwh
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