Only one same job at a time ?

My solution to deal with this is to bind an IPv6 localhost ::1 socket to a given port. Only one process can do this, and thus it’s a very effective mutex. No lock files to cause havoc, no dealing with the dark and buggy corners of advisory file locking.

For shell scripts, simply replace the #!/bin/sh with #!/somewhere/bin/lock 2048 where 2048 is the port number you will use to enforce the lock (greater than 1024 if you do not want to deal with the hassles of privileged ports).