Cron
Overview
Cron is a Unix-based program that enables you to schedule recurrent jobs. Here, we provide you a simple overview of how to get started using Cron on the daemon server. For a more in-depth explanation of Cron, please visit one of the pages in the resources section.
Configuration and backup
To schedule a task to run regularly with cron, you will need to edit the cron table or crontab. Please note that configuration is server-specific so you must be logged in to the server on which tasks will run.
All users wishing to make use of cron at Research Computing are highly encouraged to configure cron on the server daemon2. While we backup cron on daemon2, at this point, users are responsible for maintaining a backup of their cron configurations.
Editing crontab file
The default crontab editor is vi. For ease of use, we recommend using a different text editor such as nano. You can change editors with the EDITOR or VISUAL command. For example:
export EDITOR=nano
or
export VISUAL=nano
Once your preferred editor is set, create a new or open an existing crontab file by typing the following command:
crontab -e
Email notifications
To receive an email with the output of each command you run, you should add the MAILTO variable at the beginning of your file:
MAILTO=$USER@ufl.edu
where $USER@ufl.edu is substituted with your email address.
Environment Modules
If you plan to load any environment modules in a script run through cron make sure to first initialize the environment modules in the cron environment. By default the cron environment has nothing in it, so the following line is necessary before your load any modules.
source /etc/profile.d/modules.sh
Crontab fields
Next, in the same file, type one or more lines with instructions for cron. Each line has six fields. If you do not wish to specify a value for a field, place an asterisk (*) there. The crontab fields are listed in the table below.
| Crontab fields | ||||||
| minute | hour | day of month | month | day of week | command to be executed | |
| 0-59 | 0-23 | 1-31 | 1-12 (1 = January) |
0-6 (0 = Sunday) |
must be an absolute path where $USER is your UFRC username and my.process.sh is the command you wish to run
| |
| * | * | * | * | * | /home/$USER/bin/my.process.sh
| |
Some simple examples
Runs every minute:
* * * * * my.process.sh
Runs at 30 past the hour:
30 * * * * my.process.sh
Runs at 3:15 a.m. every day:
15 3 * * * my.process.sh
Runs at 3:15 p.m. every day:
15 15 * * * my.process.sh
Runs at 3:15 a.m. every Monday:
15 3 * * 1 my.process.sh
Runs at 3:15 a.m. on the first of every month:
15 3 1 * * my.process.sh
Resources
If you want to learn more about Cron and create more complex crontab files, we recommend the following resources:
- Wikipedia: http://en.wikipedia.org/wiki/Cron
- Intro to cron: http://www.unixgeeks.org/security/newbie/unix/cron-1.html
- How to set-up a crontab file: http://v1.corenominal.org/howto-setup-a-crontab-file/