Job Automation under Linux/Unix machines

Cron jobs are used to schedule commands to be executed periodically i.e. to setup commands which will repeatedly run at a set time, you can use the cron jobs.

crontab is the command used to install, deinstall or list the tables used to drive the cron daemon in Vixie Cron. Each user can have their own crontab, and though these are files in /var/spool/cron/crontabs, they are not intended to be edited directly. You need to use crontab command for editing or setting up your own cron jobs.

To edit your crontab file, type the following command:

$ crontab -e

Syntax of crontab

Your cron job looks like as follows:
1 2 3 4 5 /path/to/command arg1 arg2

Where,

* 1: Minute (0-59)
* 2: Hours (0-23)
* 3: Day (0-31)
* 4: Month (0-12 [12 == December])
* 5: Day of the week(0-7 [7 or 0 == sunday])
* /path/to/command - Script or command name to schedule

Same above five fields structure can be easily remembered with following diagram:

* * * * * command to be executed
- - - - -
| | | | |
| | | | ----- Day of week (0 - 7) (Sunday=0 or 7)
| | | ------- Month (1 - 12)
| | --------- Day of month (1 - 31)
| ----------- Hour (0 - 23)
------------- Minute (0 - 59)

Example(s)
If you wished to have a script named /root/backup.sh run every day at 3am, my crontab entry would look like as follows:

(a) Install your cronjob:
# crontab -e

(b)Append following entry:
0 3 * * * /root/backup.sh

Run five minutes after midnight, every day:
5 0 * * * /path/to/command

Run at 2:15pm on the first of every month:
15 14 1 * * /path/to/command

Run at 10 pm on weekdays:
0 22 * * 1-5 /path/to/command

Run 23 minutes after midnigbt, 2am, 4am ..., everyday
:23 0-23/2 * * * /path/to/command

Run at 5 after 4 every sunday
:5 4 * * sun /path/to/command