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ps -ef failing sometimes


 
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# 1  
Old 05-08-2011
ps -ef failing sometimes

Hi Everyone,

we have a shell script "DLP_recv.sh" that has below command which is supposed to return the number of active instances of itself, which means of there is no other instance then commad would return 1 (for the current instance). The problem is that it sometimes it returns 0 which is unlikely because it should count its own instance. This happens rarely, does anyone have any idea, thanks in advance.
Code:
job_pattern="$0"
ps -ef|grep "${job_pattern}"|grep -v "grep ${job_pattern}" > /tmp/DLP_recv.$$

Regards.

Last edited by Franklin52; 05-09-2011 at 06:00 AM.. Reason: Please use code tags, thank you
# 2  
Old 05-08-2011
Most man pages for ps such as this one say
Quote:
Things can change while ps is running. The snapshot it gives is true only for a split-second, and it might not be accurate by the time you see it.
ps is scanning a table that can change very fast during the scan. It's especially bad on a multiple cpu system. It can never be 100% correct.
# 3  
Old 05-08-2011
ok, but the ps is executed within the program when that instance is still active, there is futher processing in the program after the ps is issued so it should return 1.

Thanks.
Rahul.
# 4  
Old 05-09-2011
As posted the script will never return a result because the name of the script does not normally appear in "ps -ef".
It might work if the script was invoked with "sh scriptname" such that the name of the script appears in memory.

This sort of application is best handled with flag files.


The output from "ps" is not reliable. It will sometimes miss processes on a busy system. I've seen this many times with monitoring scripts.

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