asked on

How to create a bash lock file

I need a script snippet that will allow me to create a PID file and then check to see if its running and then delete it on exit.

The purpose of which is to make sure that the script can't be run if its already running. My bash skills are not that great but I have done this in PHP and other langs.

Thanks

ASKER CERTIFIED SOLUTION

farzanj

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woolmilkporc

Like this?

scriptname=$(basename $0)
pidfile=${scriptname}.pid

if [[ -e $pidfile ]]; then
echo "Script is already running"
exit
else
echo "Starting ..."
touch $pidfile
fi
echo " .... processing ..."

echo "Ending script ..."
rm $pidfile
exit

Maybe you should consider adding some signal handler, to be able to remove the pidfile even in case the script gets killed.

Here is an old EE case of mine where I described such an approach:

https://www.experts-exchange.com/questions/24411982/UNIX-determine-if-a-script-is-alreadfy-running.html

(Comment #24395687)

Good luck

wmp

Michael Sole

ASKER

Giving it to farzani cause he was first, thanks

Tintin

Note that farzani's solution would always exit as the script is creating the lock file before checking if it exists.

wmp's solution is better, but it doesn't handle stale locks.

Here's my suggestion

LOCKFILE=/path/to/$(basename $0).pid

function lock {
   if [ -f $LOCKFILE ]
   then
      pid=$(cat $LOCKFILE)
      if kill -0 $pid >/dev/null 2>&1
      then
         exit 1
      else
         echo "Stale lock.  Removing"
      fi
   fi

   echo $$ >$LOCKFILE
}

function unlock {
   rm -f $LOCKFILE
}

lock

# Your main script goes here

unlock

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Michael Sole

ASKER

I was able to figure it out and re-order the elements in the script to the right places. Thanks again!