Making sure script only runs once instance at a time.

[Fredrik Lundh]

Hari Sekhon wrote:

> How exactly do you check that the pid is still active in python? Is 
> there a library or something that will allow me to manipulate system 
> processes and listings etc the way everybody does in unix shells....

by passing zero to the os.kill primitive:

	os.kill(pid, 0)

if this raises an OSError, there's no active process with the given pid.

> I'm a huge fan of shell so I've done my own thing which leans on shell 
> as follows:
> 
> import sys,commands,os
> 
> scriptpath = sys.argv[0]
> scriptname = os.path.basename(scriptpath)
> 
> number_procs=commands.getstatusoutput('ps -ef|grep %s|grep -v grep|wc 
> -l' % scriptpath)
> 
> if number_procs > 1:
>     print "There appears to be another %s process running." % scriptname

what if you have commands with overlapping names (e.g. "bar.py" and 
"foobar.py"), or some other user on the machine happens to run a
command that, on purpose or by accident, contains your script's name 
(e.g. "emacs mybar.py") ?

</F>