A Cron Job is primarily used to schedule a command or job in Linux. It permits automating certain commands or scripts in the server to complete the repetitive tasks automatically. It can be set to run by the minute, hour, day of the week, day of the month, month, or the combination of the above mentioned. A Cron job can be set to delete the temporary log every week to avoid the disk space being used by logs which are not necessary. A cron job can also be set up to perform certain functions in Drupal.

Main components of cron job:

  1. The script to be executed.
  2. The command to execute the script.
  3. The action or output of the script, which depends on the action of the script called.

Commonly used crontab commands:

crontab -e

Edit or create a crontab fileone.

crontab -l

Display your crontab file.

crontab -r

Remove your crontab file.

crontab -v

Display the last time you edited your crontab file.

crontab -u user

This option allows you to modify or view the crontab file of user.

Crontab Fields and Allowed Ranges

Scheduling a Job For a Specific Time

The example below illustrates the working of a basic cron job.

Please note that the time field uses 24 hours format. So, for 10 AM use 10, and for 10 PM use 22.

30 10 11 07 * /home/user/backup.sh

This will execute the backup.sh script on 11th July 10:30 AM.

30 – 30th Minute
10 – 10 AM
11 – 11th Day
07 – 7th Month (July)
* – Every day of the week

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