Linux Desktop Hacks

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Chapter 9, Administration and Automation

#70 Automate Your Life with cron

HACK

Day

This is the number of the day in the month when the job is to be run.

Month

This is the month number between 1 and 12.

Day of the week

This is the day of the week, between 0 and 6, where 0 is Sunday.

In this example, you indicated the time as 3:30 a.m., and the day as the first

day of the month. The two asterisks (

*) show you want to run the script for

each time increment in that column—i.e., every day or month. As per the

example, the

* in the month and day-of-the-week columns means the cron-

job will run every month and every day.

When you have finished editing the crontab, saved the file, and exited, the

changes are automatically enabled. You can view the entries in the crontab

file by running:

foo@bar:~$ crontab -l

You can also remove the entire crontab by using the -r option:

foo@bar:~$ crontab -r

More Advanced Crontabs

Although creating a crontab is fairly simple, if you need to have cronjobs

running at specific intervals during a day, month, or year, this can result in a

number of duplicated crontab entries with different times. To solve this

problem, cron has a number of special symbols you can use to configure

more elaborate times.

The first symbol is a comma (

,), and you can use this to add multiple entries

to a column. As an example, you can run the cronjob on the 5th, 10th, and

15th of each month with this line: ...