Multiple invocations of cron-job AT THE SAME MINUTE on Raspbian

[Ajay Garg]

Hi All.

I have a raspbian-pi-3, and following is the first-level information ::

########################################
uname -a
Linux raspberrypi 4.4.11-v7+ #888 SMP Mon May 23 20:10:33 BST 2016
armv7l GNU/Linux
########################################



Following is the script that is run by the every-minute cron ::

########################################
PID=`ps -aux | grep instamsg | grep -v grep | grep -v tail`

if [ -z "${PID}" ]
then
    echo "Binary not running"
    cd /home/sensegrow
    chmod 777 instamsg

    sleep 3
    ./instamsg &
else
    echo "Binary running fine"
fi
########################################



Now, things are mostly fine, but very spuriously, I see that multiple
instances of "instamsg" are spawned, sometimes upto 5, ALL IN THE SAME
MINUTE.
The occurrence of such phenomena is like once in a day.

I know there are multiple posts about this on the internet, but all of
those point to faulty-script(s) that are run as part of cron.
However, I guess the script I am using is pretty simple, and can never
be "long-running, longer than a minute".

Please note that I may solve this by using the lock-file mechanism,
but I wish to know the root-cause, as the same script runs without any
issues for days the on Maestro-Wireless routers, and Lanner-systems
running Ubuntu.

In particular, could this be an issue with the cron-daemon code on
Raspbian-kernel, which could (in spurious cases) spawn multiple
instances of the same cron-job in the same minute?


Thanks and Regards,
Ajay