Rsync cronjob that will only run if rsync isn't already running

The problem with creating a "lock" file as suggested in a previous solution, is that the lock file might already exist if the script responsible to removing it terminates abnormally. This could for example happen if the user terminates the rsync process, or due to a power outage. Instead one should use flock, which does not suffer from this problem.

As it happens flock is also easy to use, so the solution would simply look like this:

flock -n lock_file -c "rsync ..."

The command after the -c option is only executed if there is no other process locking on the lock_file. If the locking process for any reason terminates, the lock will be released on the lock_file. The -n options says that flock should be non-blocking, so if there is another processes locking the file nothing will happen.

Via the script you can create a "lock" file. If the file exists, the cronjob should skip the run ; else it should proceed. Once the script completes, it should delete the lock file.

if [ -e /home/myhomedir/rsyncjob.lock ]
then
  echo "Rsync job already running...exiting"
  exit
fi

touch /home/myhomedir/rsyncjob.lock

#your code in here

#delete lock file at end of your job

rm /home/myhomedir/rsyncjob.lock

To use the lock file example given by @User above, a trap should be used to verify that the lock file is removed when the script is exited for any reason.

if [ -e /home/myhomedir/rsyncjob.lock ]
then
  echo "Rsync job already running...exiting"
  exit
fi

touch /home/myhomedir/rsyncjob.lock

#delete lock file at end of your job

trap 'rm /home/myhomedir/rsyncjob.lock' EXIT

#your code in here

This way the lock file will be removed even if the script exits before the end of the script.