How to check in a bash script if something is running and exit if it is

Question

I have a script that runs every 15 minutes but sometimes if the box is busy it hangs and the next process will start before the first one is finished creating a snowball effect.

Answer 1

I had recently the same question and found from above that kill -0 is best for my case:

echo "Starting process..."
run-process > $OUTPUT &
pid=$!
echo "Process started pid=$pid"
while true; do
    kill -0 $pid 2> /dev/null || { echo "Process exit detected"; break; }
    sleep 1
done
echo "Done."

Answer 2

To expand on what @bgy says, the safe atomic way to create a lock file if it doesn't exist yet, and fail if it doesn't, is to create a temp file, then hard link it to the standard lock file. This protects against another process creating the file in between you testing for it and you creating it.

Here is the lock file code from my hourly backup script:

echo $$ > /tmp/lock.$$
if ! ln /tmp/lock.$$ /tmp/lock ; then 
        echo "previous backup in process"
        rm /tmp/lock.$$
        exit
fi

Don't forget to delete both the lock file and the temp file when you're done, even if you exit early through an error.

Answer 3

For a method that does not suffer from parsing bugs and race conditions, check out:

• BashFAQ/045 - How can I ensure that only one instance of a script is running at a time (mutual exclusion)?

Answer 4

You can use pidof -x if you know the process name, or kill -0 if you know the PID.

Example:

if pidof -x vim > /dev/null
then
    echo "Vim already running"
    exit 1
fi

Answer 5

pgrep -f yourscript >/dev/null && exit

Answer 6

This is how I do it in one of my cron jobs

lockfile=~/myproc.lock
minutes=60
if [ -f "$lockfile" ]
then
  filestr=`find $lockfile -mmin +$minutes -print`
  if [ "$filestr" = "" ]; then
    echo "Lockfile is not older than $minutes minutes! Another $0 running. Exiting ..."
    exit 1
  else
    echo "Lockfile is older than $minutes minutes, ignoring it!"
    rm $lockfile
  fi
fi

echo "Creating lockfile $lockfile"
touch $lockfile

and delete the lock file at the end of the script

echo "Removing lock $lockfile ..."
rm $lockfile