How to schedule tasks with Cron

Introduction

cron runs a command on a schedule. Output is mailed to the crontab owner unless you redirect it.

Adding Cron Jobs

Cronjobs can provide you a time accuracy of 5 points,

Minute of the hour 0 - 60

Hour of the day 0 - 23

Day of the month 1 - 31

Month of the year 1 - 12

Day of the week 0 - 7 

The syntax of a cron job is as shown below.

MIN HOUR DOM MON DOW CMD

A sample cronjob to run on every tuesday 2.30am every month will be like shown below.

30 2 * * 2 CMD

* is used to represent all for that corresponding field.

Service Options.

Options:

-u <user> define user

-e edit user's crontab

-l list user's crontab

-r delete user's crontab

-i prompt before deleting

Tips

List users cronjobs:

crontab -l -u <user_name>

Edit users cronjobs:

crontab -e -u <user_name>

List user cronjobs from the user’s shell:

crontab -l

Edit user cronjobs from the user’s shell:

contab -e

Clear all cronjobs for user:

crontab -r

Cronjob to run every 2 hour:

0 */2 * * *

Please use the / character to specify the repetition interval */5 for every 5, can be used in any fields like minute hour day etc

Cronjob to run 2,3,4,5,6,7 hours

0 2-7 * * *

Please use the - character to specify the range, 2-7 is same as 2,3,4,5,6,7, can be used in any fields like minute hour day etc

Cronjob to run every minute

* * * * *