Making sure script only runs once instance at a time.

[MaR]

A very brutal but simple and effective method is to bind() to a socket
on localhost eg (127.0.0.1, 4711), listen() but never accept().
Any other process trying to to bind() on the same port will fail..
When the process dies, the port is released automatically, pending som
timedelay..

But this assumes you have an execution environment where this is
acceptable. A sysadmin may have objections ;o)