cron is a time-based job scheduler. This piece of software utility is used when you need to have a program run repeatedly at set time.
The daemon that runs in the background is named crond, while the file that contains the jobs themselves is named crontab.
You have three main directories:
- /etc/crontab is the main system crontab file.
- /var/spool/cron/ a directory for storing crontabs defined by users.
- /etc/cron.d/ a directory for storing system crontabs.
Creating a crontab-file for your user is easy. Usage depends on what you need to do.
This command will allow you to edit the current user’s crontab. If a crontab doesn’t already exist, it will create a new one once you have saved your changes.
Edit another user’s crontab
You can also edit another user’s crontab, but only if you have the correct permissions in place to do so.
..to list another user’s crontab.
These are the most common usages of the command crontab. For a couple of more options, see man crontab.
The syntax of a crontab-file looks like this:
* * * * * /path/to/program
Each asterisk represents a field. Starting from left to right, the fields are as follows:
- Minute (0 – 59)
- Hour (0 – 23)
- Day of month (1 – 31)
- Month (1 – 12)
- Day of week (0 – 7, where 0 and 7 is Sunday)
On the fields Month and Day of the week, you can also use names instead of numeric values.
There are various operators that you can use with cron. Here’s the list:
An asterisk (*) is used to indicate that every instance (i.e. every hour, every weekday, etc.) of the particular time period will be used. So if you use an asterisk in the field Hour, it means that it will run every hour.
With this operator you can specify a list of values. If you use 1,5 in the field Day of week, this would mean Monday and Friday.
While comma specifies a list of values, dash specifies a range. 1-5 in the field Day of week would mean Monday to Friday.
Forward slash (/)
Forward slash specifies a step value. If you’d like to execute a command every other hour, you could use 0-23/. You can also use steps after an asterisk, so if you want the command to run every two hours instead, use */2.
45 14 1 * * /home/jorge/backup.sh
In this example the script backup.sh runs at 14:45, the first day of the month, every month.
30 04 1,15 * 5 /home/jorge/list_files.sh
This line runs the command list_files.sh at 04:30 on the 1st and 15th of each month, plus every Friday.
00 08 * * 1-5 /home/jorge/wake_up.sh
The line above runs the command wake_up.sh at 08:00, Monday to Friday.
*/5 * * * * * /home/jorge/annoy_wife.sh
..and this line would run the program annoy_wife.sh every five minutes, every day.
As always, if you want to know more about cron, check out the command man cron.