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Mon, 9 Jul 2007

Preventing “Overlapping” cron Processes

— SjG @ 9:43 am

I have a number of very time consuming processes that get run by cron on various machines. Some of these processes would cause problems if they “overlapped” — e.g., a new one gets started before the old one is done.

Now, there are a lot of ways to make sure you’re unique if you’re a process, but often I don’t want to modify the source of the process to add that (for many packages, I’d rather not patch and merge and recompile every time a new version comes out). So I write a simple shell script to run the process; cron calls my shell script, and prevents the overlap.

This uses the magic of “pgrep” — unfortunately, different versions of pgrep have different flags, so the code I originally wrote (which used the “-c” flag, which counts the matching processes) didn’t port to most systems. It’s easy enough to pipe the output through a “wc -l”.

I did have to move the pgrep exec out of my if statement, though, since the comparison was going against the return code, not the output. Doh!

#!/bin/bash

RUNNING_PROCS = `pgrep -f longRunningProcess | wc -l`
if [ "$RUNNING_PROCS" -gt "0" ]
then
        echo `date` longRunningProcess still running. I\'ll let it finish. 
else
        echo `date` Starting longRunningProcess.
        /path/to/longRunningProcess -flags 
fi
echo "----------------------------"

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