Cron jobs

UNIX systems use cron daemon to schedule and run tasks at a specific dates and times. Crontab contains instructions for the cron daemon to execute. You can use <code>crontab</code> command to specify the tasks that need to be executed repeatedly (hourly, weekly, etc.)

Crontab Format

The format contains 6 fields, 5 fields are used to specify the time and the 6th is used for the command that needs be executed. Following is a general representation of a cron job:

* * * * * {Command to be executed}

Fields:

1. Minute 0-59
2. Hours 0-23
3. Day of month 1-31
4. Month 1-12 (1 for Jan, 2 for Feb...)
5. Day of week 0-7 (0 for Sunday, 1 for Monday and so on.)

Multiple field values

• The asterisk * operator is a wild card and applies for all possible values for the field. e.g. every minute, every hour etc.
• The hyphen - operator is used to specify a range of values for a field. e.g. * 2-4 * * * would run on hours 2, 3 and 4 every day.
• The comma , operator is used to specify a list of values for a field. e.g. * 3,5,6 * * * would run on hours 3, 5, and 6 every day
• The forward slash / operator is used to specify a step value. e.g. */5 * * * * would run every 5 minutes.

Examples

Consider you have a shell file named send-email.sh in your home directory that sends emails.

To run the file on the 12th hour of the 1st day of every month:

0 12 1 * * ~/send-email.sh

To send email every day at 5 am:

0 5 * * * ~/send-email.sh

To execute the script every 5 minutes:

*/5 * * * * ~/send-email.sh