Hi
redapplesonly!
You could use a pid file for that. This is, as far as I know, the mechanism commonly used by server software.
At start up, Your script checks if there is already a file with a given name, usually yourscript.pid in a directory, usually /var/run.
If there is one it may read the contents which would be the process id of the already running instance and print an error and exit.
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If there is none, Your script would create yourscript.pid and write its pid to it. Before the script exits, it deletes the file.