About the role
If you've spent any time working on a *nix-based operating system, chances are you've heard the term cron job, but for many developers cron jobs remain a source of confusion and painful mistakes.
In this article, you will learn more about the basics of cron, exploring the syntax for creating a cron job, as well as how to manage your cron jobs using the crontab command. We'll also review common mistakes that developers make when configuring cron jobs.
What Is a Cron Job?
A Cron Job is a Linux program that allows users to schedule the execution of a piece of software, often in the form of a shell script or a compiled executable. Cron is typically used when you have a task that needs to be run on a fixed schedule, and/or to automate repetitive tasks like downloading files or sending emails.
At its most basic level, a cron job is an entry written into a table called the cron table, otherwise known as the crontab for short. This entry contains a schedule and a command to be executed. The cron daemon (crond) looks for entries in the crontab to determine what jobs it should run, and when it should run them according to the specified schedule.
Note: The examples below are run on the Ubuntu operating system. Different Linux OSs might use different package managers, but all other commands will work the same.
How Do Cron Jobs Work?
Most standard installations of cron consists of two commands:
cron or crond, which is the daemon that runs the scheduling utility
crontab, which is the command that allows you to edit the cron entries for your jobs
When you talk about a daemon from a Linux perspective, it's a program that is running in the background and is noninteractive. This means that the program does not accept any user input and doesn't display output to the user. The word daemon is historically used in a Unix/Linux context and is not a universal term across different operating systems.
The daemon will be running under the root user. You can run the following command to see if cron is running:
$ ps aux | grep cron
You should see an output like this:
root 617 0.0 0.0 9420 2800 ? Ss 17:00 0:00 /usr/sbin/cron -f
If you received no output from the command at all, either cron isn't running or is not installed.
On Ubuntu you can quickly install cron by running the following command:
$ sudo apt update && sudo apt install cron
If you're using something other than Ubuntu, you'll need to run the equivalent command for your package manager.
Once cron is installed, remember to make sure it is enabled and running using the systemctl command provided by systemd:
$ sudo systemctl enable cron
Cron should now be running, and you should be able to see it if you again run the ps command shown above.
