The easiest is to check for the existence of a special file on a start-up if the file not exists create a temporary file and delete this file after the script has finished.
That's not easy at all. In fact, if you do it as you state, you have a solution with a race condition, as you aren't doing the "check if a file exists; create it otherwise" atomically. Furthermore, you have a problem if the program dies and doesn't get the chance to clean up the special file. Or if something else removes the file.

In reply to Re^2: how to exit if tool is already running by JavaFan
in thread how to exit if tool is already running by srikrishnan

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