PID

Posted Oct 8, 2011 0:22 UTC (Sat) by Richard_J_Neill (subscriber, #23093)
In reply to: PID by sytoka
Parent article: A Plumber's Wish List for Linux

What about increasing the size of the pid maximum number? Otherwise, there is a real danger that the following code:

#!/bin/bash
process2 &
PID=$!
#do stuff for several hours.
kill $PID
exit

could end up killing the wrong process (if process2 exits, and the PID gets reused).

At least sequentially assigned PIDs make this not too probable: if the PIDs were randomly allocated, without increasing the possible range, it is riskier.

PID

Posted Oct 8, 2011 11:53 UTC (Sat) by abacus (guest, #49001) [Link]

There's no need to increase PID size in order to solve the race in your program. Just use the following:

#!/bin/bash
process2 &
jobspec="%1"
#do stuff for several hours.
kill $jobspec
exit

Safe killing of a child process

Posted Oct 8, 2011 12:10 UTC (Sat) by pjm (guest, #2080) [Link]

Increasing the pid maximum number won't remove the danger, it would only make the bug rarer.

What you want is kill %process2, or have a look at bash's job control documentation.

If job control stuff won't do what you're looking for, then you can add a trap on SIGCHLD and keep track of liveness yourself. The jobs and wait builtins might be useful in some cases.

All of the above should work on any Free Unix kernel, including Linux-2.6.xx and earlier; it doesn't require adding any Linux-3.x-specific features.

PID

Posted Oct 8, 2011 12:35 UTC (Sat) by nix (subscriber, #2304) [Link]

You can't eliminate the race here, but you can narrow it radically by looking at the ppid:

#!/bin/bash
process2 &
PID=$!
# do stuff for several hours.
[[ "$(ps -o ppid= -p $PID)" == $BASHPID ]] && kill $PID
exit

Now this will only lose if $PID dies and a new process starts in that narrow window, or if this bash has many children and one of them started after $PID dies and has the same PID (which you can solve, if you really need to, by remembering PPID->PID mappings in an associative array).