in reply to Re: Re: Determine if script is already running
in thread Determine if script is already running

Nitrox,
As I indicated in my first post. Using a lock file by itself, regardless of type, will not guarantee only a single copy of a script is running in Unix. This is because it is possible to delete a file that is locked. Using the /tmp directory most likely increases the odds of deletion by its nature. The subsequent instance of the script is able to create and lock the new file - and now you have two copies running. You really need to have multiple methods for validation and checking the process table is a good place to start.

Cheers - L~R

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Re: Re: Re: Re: Determine if script is already running
by Nitrox (Chaplain) on Mar 04, 2003 at 21:44 UTC
    This bugs me, why does *nix allow deletion of an open and locked file? On Win32 a file can't be deleted until the ref count == 0, is there a way to explicitly set this in *nix?

    -Nitrox

[reply]
      Nitrox,
      Not that I know of. There are things you can do to reduce the probability that the file will be accidentally deleted - security through obscurity. These things include:

    • Changing the directory where the file resides permissions to 000.
    • Use non-typical characters in the file name such as a space, asterick, or worse.
    • Use alarm for the original script to check if the file has been deleted and re-create. * This requires a lot of overhead to try and compete with the race condition.
    • Use hard links (not symbolic) so that more than one file combined equal your lock file

      Even with these suggestions, there is not the 100% guarantee. Sometimes you need decide that the probability is an acceptable risk and move on. If you are totally against checking the process table - semaphores may be the way to go.

      Cheers - L~R

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