Working with Cronjobs in Linux

Cronjob is one of the basic and most useful utility that you can find on almost all the Linux distributions.  A cronjob is basically a task scheduled to run at a specific time. For example you can setup a cronjob that will run everyday to take backup of your files. Following are the details on how to setup a cronjob, its basic commands and syntax.

Every Cronjob entry has to satisfy the following syntax:

Minute (0-59)  Hour(0-23)  Day_of_Month (1-31)  Month(1-12) Day_of_week (0-6 SUN-SAT)  Command_to_execute

You can use following commands to when working with Cronjobs:

To list all the Cronjobs that are already setup.

$ conjob -l

To edit / add / remove the Crontabs

$ crontab -e 

if you are sudo user you can use -u option to list the Cronjobs of other users same is true for -e option.

$ sudo crontab -l -u user_name  
$ sudo crontab -e -u user_name

If Cronjobs are being setup first time on the system it will ask for you to choose an editor which will be used for Cronjobs editing.

crontab-choose-editor
crontab-choose-editor

Once you have opened the Cronjobs in an editor it will look as follows.

editing-cronjobs
editing-cronjobs

Here in the editor you can add modify or delete the Cronjobs.

Lets have a look at different examples of Cronjobs

Cronjob to run every minute:

* * * command_to_execute

Every 15 minute Cronjob

*/15 * * * * command_to_execute

you can also give comma separated values, to run cronjob every 0, 5, 10 minute of every hour add:

0,5,10 * * * * command_to_execute

To run Cronjob every hour add:

0 * * * * command_to_execute

you can also run Cronjob  every 4 hour:

0 */4 * * * command_to_execute

To run Cronjob everyday, following will run everyday at 00:00

0 0 * * * command_to_execute

Run a Cronjob every Sunday:

0 0 * * SUN command_to_execute      OR        0 0 * * 0 command_to_execute

Run a Cronjob every Month:

0 0 1 * * command_to_execute

Run a Job every 6 months:

0 0 1 */6 * command_to_execute

Other than defining the timing you can also use predefined strings to setup crojnobs:

To run a Cronjob every time system reboot

@reboot  command_to_execute

Following will run cronjob every year

@yearly command_to_execute

Run a Cronjob Monthly

@monthly command_to_execute

Above will give you basic understanding of how to work with Cronjobs. Did it help you let us know in comments section.

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