Hi,
I have a unix script that basically has a while loop inside which it checks Oracle database for certain records. If it finds the records, it does some processing and then goes back to the while loop. If it doesnot find any matching records, then it sleeps for 30 seconds and then goes back to the while loop to again check for existence of a new records.
We are supposed to have only one instance of this script running. The script is kicked off at the beginning of the week with the command sh scriptname.ksh &
It doesnot need to be started again until after server reboot the following week.
To ensure that not more than one instance of the script runs, we have a monitoring program that alerts us whenever multiple instances of the script are running due to either multiple users mistakenly starting a new instance of the script or some other reaosn. After seeing occassional alerts from this monitoring system that multiple instances of this script were running, I decided to see why the monitoring system was alerting us on multiple instances. So I wrote a temporary script that runs in an infinite loop and runs the command ps -ef|grep scriptname |grep -v grep every 2 secs. After analyzing the output of this test daemon that I wrote, I found that once in a while (eveyr 15 or 20 minutes), ps -ef returns two instances of the script.
(1) One instance has parent pid equal to 1
(2) The 2nd instance of this script has pid equal to the parent pid of the 1st process
Within a couple of seconds after that, my daemon shows that the 2nd instance has died and again there is only one instance.
So why does this 2nd instance get triggered by the parent process momentarily thereby misleading our monitoring system to believe that there are multiple instances of the script running.
Here is an output of the ps -ef run by my test daemon that runs every 2 seconds. As you can see from the output below, at 5:44:04 pm Aug 5, you can see 2 instances of the script running (in bold). 1st instance is parent script which has pid 12369. 2nd instance is Child script which has pid 5713 and whose parent pid is the pid of the parent script 12369. I have no idea how the child script with pid 5713 got triggered . myscriptname.ksh doesnot call itself. As mentioned before, it just runs in a while loop waiting for database changes and does some processing if there are database changes but it never calls itself anywhere. AS you can see below, within 2 secs at 5:44:06 PM, the child instance of the script with pid 5713 suddenly dies and we are back to just 1 instance again (pid 12369). This happens occassionally (maybe once every 15 minutes) but I donot know why. If you can explain the reason for this behavior of the parent process momentarily invoking a child process which calls the same script as the parent script and which dies within 2 secs, your help would be highly appreciated.
The script is running on SUSE Linux v10.3

Code:

Sun Aug  5 17:44:00 EDT 2012
ownerid     12359     1  0 04:10 ?        00:00:08 sh myscriptname.ksh
Sun Aug  5 17:44:02 EDT 2012
ownerid     12359     1  0 04:10 ?        00:00:08 sh myscriptname.ksh
Sun Aug  5 17:44:04 EDT 2012
ownerid      5713 12359  0 17:44 ?        00:00:00 sh myscriptname.ksh
ownerid     12359     1  0 04:10 ?        00:00:08 sh myscriptname.ksh
Sun Aug  5 17:44:06 EDT 2012
ownerid     12359     1  0 04:10 ?        00:00:08 sh myscriptname.ksh
Sun Aug  5 17:44:08 EDT 2012
ownerid     12359     1  0 04:10 ?        00:00:08 sh myscriptname.ksh

thanks

Moderator's Comments:

edit by bakunin: Please use code tags next time for your code and data.