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avoid script running twice

Tim Williams tim at tdw.net
Mon Jun 18 15:48:32 EDT 2007 
On 18/06/07, Nick Craig-Wood <nick at craig-wood.com> wrote:
> Tim Williams <tim at tdw.net> wrote:
> >  You can also do this by holding a file open in write mode until the
> >  script has finished.
> >
> >        try:
> >             open('lock.txt','w')
> >             my_script()
> >       except:
> >            #print script is already running
>
> That only works under windows
>
>  >>> f=open('lock.txt','w')
>  >>> g=open('lock.txt','w')
>  >>> f.write('hi')
>  >>> g.write('ho')
>  >>> f.close()
>  >>> g.close()
>  >>> open('lock.txt').read()
>  'ho'
>  >>>
>
> The best cross platform way to create a lock is creating a directory.
> It is atomic on both windows and linux anyway.
>
>  try:
>    os.mkdir("lock")
>  except OSError:
>    print "locked!"
>  else:
>    try:
>      do_stuff()
>    finally:
>      os.rmdir("lock")
>
> (untested)
>

Being a windows-only person, I didn't know that :)

Actually I think I did,  this thread has happened before - a few months ago :)

I would be worried with the directory-exists option for the same
reason I don't use the file-exists method currently.  It is possible
for the directory to exist when the script isn't running, thus
preventing the script from running again until someone notices.

On Windows the open-a-file-for-writing method works well, but as *nix
doesn't work the same way then maybe the socket solution is the best
cross-platform option.  The socket can't exist when the script isn't
running, and if you try and create a duplicate socket you catch the
exception and exit.

IMHO of course.

:)
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