How To Setup Crontab Under Linux Or Unix

How do I add a cronjob on Linux? Thats a question even some veterans ask all the time, something that is easily forgotten. At times I have forgotten the exact incantation so I wrote this down. I hope it is useful to someone else as it is for me. We use cron jobs to schedule commands to be executed periodically. Entries in the cron can be commands as you would execute them on the command line or they can be scripts. The cron service runs in the background and reads the /etc/crontab and the /etc/cron,* directories. It also checks entries in /var/spool/cron directory. Users also have their own cron.

Editing cron entries.

Although you can edit the files directly with your favourite editor, (vi , emacs, nano.. etc..) you should instead invoke your default system editor using the "crontab" command. This will open the file and load up the cron entries and also do other things such as validation on save. Each user can also have their own crontab file. These files are saved in /var/spool/cron/crontabs. Running "crontab" as a normal user will open the user's relevant crontab file.

The linux system crontab vs user crontab

  • /etc/crontab aka system crontab. This is owned by root and anything placed in here runs as root by default. This is usually for jobs that requires root privileges. However you can also put the user in the 6th field and it will run the command as the user.
  • The user crontabs: Normal users can not edit the system crontab. However they have their own crontab entries. All commands run as the user who setup the crontab. One could of course use sudo to get around this if they have the sudo rights.

How Do I Install or create or edit my own cron jobs.

crontab -e

The syntax of crontab is as follows:

1 2 3 4 5 /path/to/command arg1 arg2

OR

1 2 3 4 5 /root/backup.sh

Where,

1: Minute (0-59)
2: Hours (0-23)
3: Day (0-31)
4: Month (0-12 [12 == December])
5: Day of the week(0-7 [7 or 0 == sunday])
/path/to/command - Script or command name to schedule
Easy to remember format:

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

Your cron job looks as follows for system jobs:

1 2 3 4 5 USERNAME /path/to/command arg1 arg2

OR

1 2 3 4 5 USERNAME /path/to/script.sh

Special operator for time settings.

How do I use operators?

An operator allows you to specifying multiple values in a field. There are three operators:

  • The asterisk (*) : This operator specifies all possible values for a field. For example, an asterisk in the hour time field would be equivalent to every hour or an asterisk in the month field would be equivalent to every month.
  • The comma (,) : This operator specifies a list of values, for example: "1,5,10,15,20, 25".
  • The dash (-) : This operator specifies a range of values, for example: "5-15" days , which is equivalent to typing "5,6,7,8,9,....,13,14,15" using the comma operator.
  • The separator (/) : This operator specifies a step value, for example: "0-23/" can be used in the hours field to specify command execution every other hour. Steps are also permitted after an asterisk, so if you want to say every two hours, just use */2.

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