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.

Answer 2

Is there a way I can configure the two jobs so that the second job does not start if the first one is still running?

Since I can't comment, I will give you another example of what you could consider but of course @sami-laine solution is more elegant.

You can simply edit a config file like that:

At startup:

echo 0 > file.config

In your /java_prog1.sh:

echo 1 > file.config    # at the beginning of the script

do something ...      

echo 0 > file.config    # at the end of script 
                        #let the other script know that the job was finished

In your /java_prog2.sh:

while [ `cat file.config` -eq 1 ] ; do sleep 5; done   # Wait whatever want

Answer 3

The creation of directories is atomic, so you could add this in both scripts:

lock_dir=/tmp/name_of_lock_directory
if ! mkdir "$lock_dir"; then
    echo "Cannot run $0: lock directory $lock_dir exists" >&2
    exit 1
fi
cleanup () { rm -r "$lock_dir"; }
trap cleanup EXIT
# if you want, you can add the running pid to a file in the lock directory
echo $$ > "$lock_dir/pid"