How To Use Cron to Automate Tasks on CentOS 8

A previous version of this tutorial was written by Shaun Lewis.

Introduction

Cron is a time-based job scheduling daemon found in Unix-like operating systems, including Linux distributions. Cron runs in the background and tasks scheduled with cron, referred to as “cron jobs,” are executed automatically, making cron useful for automating maintenance-related tasks.

This guide provides an overview of how to schedule tasks using cron’s special syntax. It also goes over a few shortcuts one can use to make job schedules easier to write and understand.

Prerequisites

To complete this guide, you’ll need access to a computer running CentOS 8. This could be your local machine, a virtual machine, or a virtual private server.

Regardless of what kind of computer you use to follow this guide, it should have a non-root user with administrative privileges configured. To set this up, follow our Initial Server Setup guide for CentOS 8.

Installing Cron

Almost every Linux distribution has some form of cron installed by default. However, if you’re using a CentOS machine on which cron isn’t installed, you can install it using dnf.

Before installing cron on a CentOS machine, update the computer’s local package index:

sudo dnf update

Then install the cron daemon with the following command:

sudo dnf install crontabs

This command will prompt you to confirm that you want to install the crontabs package and its dependencies. Do so by pressing y then ENTER.

This will install cron on your system, but you’ll need to start the daemon manually. You’ll also need to make sure it’s set to run whenever the server boots. You can perform both of these actions with the systemctl command.

To start the cron daemon, run the following command:

sudo systemctl start crond.service

To set cron to run whenever the server starts up, type:

sudo systemctl enable crond.service

Following that, cron will be installed on your system and ready for you to start scheduling jobs.

Understanding How Cron Works

Cron jobs are recorded and managed in a special file known as a crontab. Each user profile on the system can have their own crontab where they can schedule jobs, which is stored under /var/spool/cron/.

To schedule a job, you just need to open up your crontab for editing and add a task written in the form of a cron expression. The syntax for cron expressions can be broken down into two elements: the schedule and the command to run.

The command can be virtually any command you would normally run on the command line. The schedule component of the syntax is broken down into 5 different fields, which are written in the following order:

Together, tasks scheduled in a crontab are structured like this:

minute hour day_of_month month day_of_week command_to_run

Here’s a functional example of a cron expression. This expression runs the command curl http://www.google.com every Tuesday at 5:30 PM:

30 17 * * 2 curl http://www.google.com

There are also a few special characters you can include in the schedule component of a cron expression to make scheduling easier:

• *: In cron expressions, an asterisk is a wildcard variable that represents “all.” Thus, a task scheduled with * * * * * ... will run every minute of every hour of every day of every month.
• ,: Commas break up scheduling values to form a list. If you want to have a task run at the beginning and middle of every hour, rather than writing out two separate tasks (e.g., 0 * * * * ... and 30 * * * * ...), you could achieve the same functionality with one (0,30 * * * * ...).
• -: A hyphen represents a range of values in the schedule field. Instead of having 30 separate scheduled tasks for a command you want to run for the first 30 minutes of every hour (as in 0 * * * * ..., 1 * * * * ..., 2 * * * * ..., and so on), you could just schedule it as 0-29 * * * * ....
• /: You can use a forward slash with an asterisk to express a step value. For example, instead of writing out eight separate separate cron tasks to run a command every three hours (as in, 0 0 * * * ..., 0 3 * * * ..., 0 6 * * * ..., and so on), you could schedule it to run like this: 0 */3 * * * ....

Here are some more examples of how to use cron’s scheduling component:

• * * * * * - Run the command every minute.
• 12 * * * * - Run the command 12 minutes after every hour.
• 0,15,30,45 * * * * - Run the command every 15 minutes.
• */15 * * * * - Run the command every 15 minutes.
• 0 4 * * * - Run the command every day at 4:00 AM.
• 0 4 * * 2-4 - Run the command every Tuesday, Wednesday, and Thursday at 4:00 AM.
• 20,40 */8 * 7-12 * - Run the command on the 20th and 40th minute of every 8th hour every day of the last 6 months of the year.

If you find any of this confusing or if you’d like help writing schedules for your own cron tasks, Cronitor provides a handy cron schedule expression editor named “Crontab Guru” which you can use to check whether your cron schedules are valid.

Managing Crontabs

Once you’ve settled on a schedule and you know the job you want to run, you’ll need to put it somewhere your daemon will be able to read it.

As mentioned previously, a crontab is a special file that holds the schedule of jobs cron will run. However, these are not intended to be edited directly. Instead, it’s recommended that you use the crontab command. This allows you to edit your user profile’s crontab without changing your privileges with sudo. The crontab command will also let you know if you have syntax errors in the crontab, while editing it directly will not.

You can edit your crontab with the following command:

crontab -e

This will open up your crontab in your user profile’s default text editor.

sudo dnf install nano

nano ~/.bash_profile

. . .
export VISUAL="nano"

. ~/.bash_profile

Once in the editor, you can input your schedule with each job on a new line. Otherwise, you can save and close the crontab for now. If you opened your crontab with vi, the default CentOS 8 text editor, you can do so by pressing ESC to make sure you’re in vi’s command mode, then type :x and press ENTER.

Please note that, on Linux systems, there is another crontab stored under the /etc/ directory. This is a system-wide crontab that has an additional field for which user profile each cron job should be run under. This tutorial focuses on user-specific crontabs, but if you wanted to edit the system-wide crontab, you could do so with the following command:

sudo nano /etc/crontab

If you’d like to view the contents of your crontab, but not edit it, you can use the following command:

crontab -l

You can erase your crontab with the following command:

crontab -r

This command will delete the user’s crontab immediately. However, you can include the -i flag to have the command prompt you to confirm that you actually want to delete the user’s crontab:

crontab -r -i

Output
crontab: really delete sammy's crontab?

When prompted, you must enter y to delete the crontab or n to cancel the deletion.

Managing Cron Job Output

Because cron jobs are executed in the background, it isn’t always apparent that they’ve run successfully. Now that you know how to use the crontab command and how to schedule a cron job, you can start experimenting with some different ways of redirecting the output of cron jobs to help you track that they’ve been executed successfully.

If you have a mail transfer agent — such as Sendmail — installed and properly configured on your server, you can send the output of cron tasks to the email address associated with your Linux user profile. You can also manually specify an email address by providing a MAILTO setting at the top of the crontab.

For example, you could add the following lines to a crontab. These include a MAILTO statement followed by an example email address, a SHELL directive that indicates the shell to run (bash in this example), a HOME directive pointing to the path in which to search for the cron binary, and a single cron task:

. . .

MAILTO="example@digitalocean.com"
SHELL=/bin/bash
HOME=/

* * * * * echo ‘Run this command every minute’

This particular job will return “Run this command every minute,” and that output will get emailed every minute to the email address specified after the MAILTO directive.

You can also redirect a cron task’s output into a log file or into an empty location to prevent getting an email with the output.

To append a scheduled command’s output to a log file, add >> to the end of the command followed by the name and location of a log file of your choosing, like this:

* * * * * echo ‘Run this command every minute’ >> /directory/path/file.log

Let’s say you want to use cron to run a script but keep it running in the background. To do so, you could redirect the script’s output to an empty location, like /dev/null which immediately deletes any data written to it. For example, the following cron job executes a PHP script and runs it in the background:

* * * * * /usr/bin/php /var/www/domain.com/backup.php > /dev/null 2>&1

This cron job also redirects standard error — represented by 2 — to standard output (>&1). Because standard output is already being redirected to /dev/null, this essentially allows the script to run silently. Even if the crontab contains a MAILTO statement, the command’s output won’t be sent to the specified email address.

Restricting Access

You can manage which users are allowed to use the crontab command with the cron.allow and cron.deny files, both of which are stored in the /etc/ directory. If the cron.deny file exists, any user listed in it will be barred from editing their crontab. If cron.allow exists, only users listed in it will be able to edit their crontabs. If both files exist and the same user is listed in each, the cron.allow file will override cron.deny and the user will be able to edit their crontab.

For example, to deny access to all users and then give access to the user ishmael, you could use the following command sequence:

sudo echo ALL >>/etc/cron.deny
sudo echo ishmael >>/etc/cron.allow

First, we lock out all users by appending ALL to the cron.deny file. Then, by appending the username to the cron.allow file, we give the ishmael user profile access to execute cron jobs.

Note that if a user has sudo privileges, they can edit another user’s crontab with the following command:

sudo crontab -u user -e

However, if cron.deny exists and user is listed in it and they aren’t listed in cron.allow, you’ll receive the following error after running the previous command:

Output
The user user cannot use this program (crontab)

By default, most cron daemons will assume all users have access to cron unless either cron.allow or cron.deny exists.

Special Syntax

There are also several shorthand commands you can use in your crontab file to help streamline job scheduling. They are essentially shortcuts for the equivalent numeric schedule specified:

Additionally, the @reboot shorthand will run whatever command follows it any time the server starts up:

@reboot echo "System start up"

Using these shortcuts whenever possible can help make it easier to interpret the schedule of tasks in your crontab.

Conclusion

Cron is a flexible and powerful utility that can reduce the burden of many tasks associated with system administration. When combined with shell scripts, you can automate tasks that are normally tedious or complicated.