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 daemon.

While we backup cron on the daemon server, at this point, users are responsible for maintaining a backup of their cron configurations.

Cron environment does not inherit your shell environment. So, unless you use absolute paths to all executables (binaries, scripts) you will have to set environment variables like PATH.

If you need to use environment modules you will have to run the following command in the script you run

source /etc/profile.d/modules.sh

before you can use 'module load'.

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/