Preventing to Bash script from running in parallel or overlap using cron

Question

If i have the following entries in my cron table:

00 03 * * * /java_prog1.sh 
00 5 * * * /java_prog2.sh

The first job usually takes around 30 m

Answer 1

Just putting some flesh on top of the suggestions already there in the comments.

Put in the beginning of java_prog1.sh:

[ -f /var/run/java_prog1.sh.pid ] && exit
echo "$$" > /var/run/java_prog1.sh.pid

... everything else our script does ...

rm -f /var/run/java_prog1.sh.pid

Then in the beginning of java_prog2.sh:

[ -f /var/run/java_prog1.sh.pid ] && exit

... the rest of your java_prog2.sh ...

Or:

if [ -f /var/run/java_prog1.sh.pid ]; then
   kill -0 `cat /var/run/java_prog1.sh.pid` > /dev/null 2>&1 && exit
fi


... the rest of your java_prog2.sh ...

The first one will just exit immediately, if the a file (possibly and probably containing a PID from the first script) exists. The second one will exit only if the file exists and a process with a PID in in that file is in the process table (kill -0 will return 0 in case it does).

As far as the actual scripting goes, you probably have to experiment a bit. This is just to give you a rought idea how it might go.