Making sure script only runs once instance at a time.

Hari Sekhon hpsekhon at googlemail.com
Mon Oct 2 12:18:30 CEST 2006


AMENDMENT:

The line

number_procs=commands.getstatusoutput('ps -ef|grep %s|grep -v grep|wc 
-l' % scriptpath)

was supposed to be

number_procs=int(commands.getstatusoutput('ps -ef|grep %s|grep -v 
grep|wc -l' % scriptpath)[1])

-h

Hari Sekhon



Hari Sekhon wrote:
> 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
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