Shell Programming and Scripting

I've been bitten by using a lock or pid file to prevent multiple instances of a script. A user typed kill -9, and the pid file didn't go away. You can't trap -9. So when he tried to restart, it said "already running", and I got trouble report. Argh.

So here's what we came up with:

Code:

# Stop if a previous version of this script is still running
ps | grep $0 | grep -v grep > /var/tmp/$0.pid
pids=$(cat /var/tmp/$0.pid | cut -d ' ' -f 1)
for pid in $pids
do
   if [ $pid -ne $$ ]; then
      logprint "$0 is already running. Exiting"
      exit 7
   fi
done
rm -f /var/tmp/$0.pid

It does use a temp file, but leaves no potential artifact to mess us up later.

If you try this without a temp file, you'll end up with a false positive because
pids=$(ps | grep $0 | grep -v grep | cut -d ' ' -f 1)
seems to leave it's own extra pid.

Different idea: save the PID of the script in the lock file. If the script is started again, check if that PID is still existing, and in an instance of the script, and stop or run accordingly.