CRON - Scheduled jobs in unix/linux

What is CRON?
Cron is a unix/linux utlity that is used to schedule tasks that run at specific intervals. The tasks that are scheduled are called cron jobs.


What is crontab? 
This is file which contains a table of scheduled jobs that drives the cron.
Multiple cron jobs can be schedule in the crontab. Ideally, each line in the file contains a cronjob information.


Crontab Commands
 

Command	Description
crontab -e	edit or create a crontab file
crontab -i	list the crontab contents
crontab -r	remove the crontab settings
crontab -v	Last time the crontab was changed

Crontab Syntax

Examples of crontab

Min	Hours	Day	Month	Day of week	Command	Description
30	13	*	*	*	echo 'Executes at 1:30 PM everyday'	Executes at 1:30 PM everyday
30	13	1	1	*	echo 'Executes on Jan 01 at 1:30 PM'	Executes on Jan 01 at 1:30 PM
30	13	1	*	*	echo 'Executes on 1st of every month at 1:30 PM'	Executes on 1st of every month at 1:30 PM
30	13	*	*	1-5	Executes on all weekdays(Monday-Friday) at 1:30 PM	Executes on 01 of every month at 1:30 PM
*/5	*	*	*	*	echo 'Executes every 5 mins'	Executes every 5 mins
*	*/5	*	*	*	echo 'Executes every 5 hours'	Executes every 5 hours

Restart cron in Linux 

In case the timezone is changed, the CRON can be restarted using the following command

/etc/init.d/crond restart