using flock with cron

The lock file you specify as the option to /usr/bin/flock; /var/cron.lock remains locked with the flock(2) system call for the duration of your script /usr/bin/myscript. Once your script completes that lock is again released by /usr/bin/flock.

When the /usr/bin/flock command can't achieve a lock, , i.e. because /var/cron.lock is already locked because your script is still running (or any other error condition), /usr/bin/flock won't execute your script. Your script doesn't need to be modified and does not need to do any checking of the lock, that's all done by /usr/bin/flock.

The lock file itself will be created as an empty file by the /usr/bin/flock command if it does not yet exist, but after it has been created the lock file doesn't change, only an flock(2) will be applied and removed. The file won't be removed after your script completes.