Set a Cron Job in Linux

There are occasions when you want to create a schedule tasks on your server. Suppose you want to backup your hard drive once a week, or you want to run a script at 6 am every morning. All those things that you want to run at specific interval can be set with cron job. The cron daemon is a long running process that executes commands at user specified intervals. This howto provides a step-by-step tutorial on how you can schedule such a task using a program named crontab.

Setting up crontab is very easy. To edit your crontab file use the following command

[jasonleon]$ crontab -e

If default editor is not defined then you will see an error message:

/bin/sh: /bin/vi: No such file or directory

crontab: "/bin/vi" exited with status 127

[jasonleon]$ export EDITOR=vim

Now, vim will be used as the default editor. You may use any editor of your choice here(emacs, nano etc.).

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

Suppose you want to run a shell script every hour

0 * * * * /root/script.sh

Run a command every 10 minutes

*/10 * * * * command

Run a command at 5 am in Morning

0 5 * * * command

Run a command every weekend 2am

0 2 * * 5-7 command

run a command on saturday

0 2 * * sat command

Disable Email Output

By default crontab returns  emails the root with the output of the cron, you might want to avoid it.

Just pipe all the output to the null device, also known as the black hole. On Unix-like operating systems, /dev/null is a special file that discards all data written to it.

0 * * * * /root/script.sh > /dev/null 2>&1

You can define a MAILTO variable to output the mail to a particular email address.

MAILTO="test@example.com"

But this MAILTO will output the result of all the cronjobs. Suppose you want to get the result of 1 cronjob. You can do it using mailx. Make sure mailx is installed.

[jasonleon]$ yum install mailx                 #For Fedora

[jasonleon]$ sudo apt-get install mailx    #For Ubuntu

*/10 * * * * /root/script.sh 2>&1 | mail -s "Output From Cron Job" username@example.com

Erase all the Crontab jobs by using

[jasonleon]$ crontab -r

Cronjob provide eight special strings that can also be used to make the file look more readable

@reboot   Run at startup
@hourly    Run at "0  1 1 * *"
@daily       Run at "0 0 * * *" 
@midnight Run at "0 0 * * *"
@weekly    Run at "0 0 * * 0"
@monthy   Run at "0 0 1 * *"
@yearly     Run at "0 0 1 1 *"
@annually Run at "0 0 1 1 *"