Allowing only one instance of a script?

[Grant Edwards]

On 2005-06-23, Thomas Guettler <guettli at thomas-guettler.de> wrote:

> Create a file which contains the PID (process ID) of
> the current process in a directory. If the file
> already exists, the file is running.

That's how it's usually done.

> If your script dies without removing the pid-file, you need to
> look during the start if the PID which is in the file is sill
> alive.


> There is a small race condition between os.path.exists()
> and writing the file.

That's why it's pointless to call os.path.exists().

> If you want to be 100% sure you need to use file locking.

I've never seen it done that way.

The standard method is to use open() with flags O_CREAT|O_EXCL.
If the open() is sucessful, then you have the lock.  If it
fails, somebody else already has the lock.

Another method is to create a temp file containing the PID and
then call link() to rename it. 

Both open() and link() are atomic operations, so there's no
race condition.

-- 
Grant Edwards                   grante             Yow!  I don't know WHY I
                                  at               said that... I think it
                               visi.com            came from the FILLINGS inmy
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