Cron Jobs - Quick Tips

Following are some quick tips related to Cron Jobs:

Format of crontab file

A cron tab entry has 5 fields. These are: minute, hour, day of month, month and day of week. For example:

*/30 * * * 1,3

The above entry will run every 30 minutes on Mondays and Wednesdays

Running Cron Jobs at random intervals

To run a Cron Job at random intervals we need to prefix the Cron Job command with the sleep bash command. For example to run a Cron Job every 5 to 7 hours at random intervals, we can use the following:

0 */5 * * * sleep $[RANDOM\%120]m; path-to-script

In the above code, the ’%’ has to be escaped. The code: $[RANDOM%120] generates a random number between 0 and 120. This is the number of minutes that script will pause. The script will run at the times: 05:00+(rand(0, 120)), 10:00+(rand(0, 120)), 15:00+(rand(0, 120)), 20:00+(rand(0, 120)) and 00:00+(rand(0, 120))

Running cron jobs on first weekday of month

To run a cron job on the first weekday of every month for example first Wednesday of each month, the following cron job entry can be used:

0 0 * * Wed [ `date +\%d` -le 7 ] && command-to-run

The above cron job will be run on first Wednesday of each month