Monday, 22 October 2012

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

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

MinHoursDayMonthDay of weekCommandDescription
3013***echo 'Executes at 1:30 PM everyday'Executes at 1:30 PM everyday
301311*echo 'Executes on Jan 01 at 1:30 PM' Executes on Jan 01 at 1:30 PM
30131**echo 'Executes on 1st of every month at 1:30 PM' Executes on 1st of every month at 1:30 PM

Executes on all weekdays(Monday-Friday) at 1:30 PMExecutes on 01 of every month at 1:30 PM
*/5****echo 'Executes every 5 mins' Executes every 5 mins
**/5***echo 'Executes every 5 hoursExecutes 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

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