It just checks for a pid file. If the pid file does not exist, or the pid no longer exists, it'll start the process (program), and write the new process' pid to the pid file.

It's at:
https://stromberg.dnsalias.org/svn/just-one/trunk/

...and usage looks like:

$ ./just-one -h
below cmd output started 2021 Sun Apr 18 04:44:03 PM PDT
Usage: ./just-one --command command --string string

The "string" part needs to be a unique identifier for each process you want only one of. The command, naturally, is a shell command.

It does not use locking: advisory or mandatory. Just a lock file containing a pid.

EG:

$ ./just-one --string 'unique ls' --command ls
below cmd output started 2021 Sun Apr 18 04:53:18 PM PDT
about to mkdir
about to get pid
about to start command
Makefile  just-one  test-just-one  this-pylint

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