Making sure script only runs once instance at a time.

[Hari Sekhon]

Fredrik Lundh wrote:
> Hari Sekhon wrote:
>
>   
>> I'm not sure if that is a very old way of doing it, which is why I was 
>> reluctant to do it. My way actually uses the process list of the os 
>> (linux) and counts the number of instances. If it is more than 0 then 
>> another process is running and the script exits gracefully.
>>     
>
> the code that reliably identifies instances of a given program would be 
> interesting to see.
>
>   
>> Also, apart from the fact the using lockfiles feels a bit 1970s, I have
>>     
>  > found that in real usage of other programs within the company that use
>  > lockfiles, it sometimes causes a bit of troubleshooting time when
>  > it stops working due to a stale lockfile.
>
> to minimize that risk, store the pid in the lockfile (and preferrably 
> also the host name), and make sure that the program checks that the pid 
> is still active before it "stops working".
>
> </F>
>
>   

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....

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
    print "Please do not run more than one instance of this program"
    print "Quitting for safety..."
    sys.exit(200)

This works nicely for me.

You might also want to add a bit of discrimination to the script 
(something along the lines of "get a decent os"...since this won't work 
on windows)

import platform
if platform.system() != 'Linux':
    print "Sorry but this program can only be run on Linux"
    sys.exit(201)
    #todo: should do a bit extra for our bsd cousins here....


Let me know if you think you have a better way. Please provide a code 
snippet if so.

-h
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-list/attachments/20061002/6fa6b8ba/attachment.html>