Allowing only one instance of a script?

[Ali]

Hi,

I have a script which I double-click to run. If i double-click it
again, it will launch another instance of the script.

Is there a way to allow only one instance of a script, so that if
another instance of the script is launched, it will just return with an
error.

Thanks

Regards,
Ali

 

[Thomas Guettler]

Am Wed, 22 Jun 2005 23:49:21 -0700 schrieb Ali:

Hi,

I have a script which I double-click to run. If i double-click it
again, it will launch another instance of the script.

Is there a way to allow only one instance of a script, so that if
another instance of the script is launched, it will just return with an
error.

Hi,

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.

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

HTH,
Thomas

 

[Grant Edwards]

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.

 

[Aahz]

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

....unless you're running under NFS.

 

[Grant Edwards]

...unless you're running under NFS.

I think read somewhere than NFS 3 handles the open with
CREAT+EXCL in an atomic manner (older versions didn't). I don't
know about link.