What is CRON and What You Can Use It For?

“What the heck is a cron?” I field this question at least once a month from non-developers. It’s a great question. I’m going to break it into two questions though.

“What is a CRON?”
“What is WP-CRON?”

What is a CRON?

At its heart, a cron is a “time-based scheduler”. It handles tasks that need to be done on a regular basis and at a specific time. As an example, if you want your WordPress blog to display the weather forecast in the header, then each morning you need to go get the weather forecast. Yes, you could hire someone to log in each morning, go get the forecast and paste it into a widget.

A better plan is to have a program that runs each morning and talks to an API to fetch the day’s forecast and update your database for you. The program that runs your weather fetching program is called a CRON. The name is derived from “chronological” which roughly translates into “in order of time”.

Most systems these days have some concept of a cron. Unix based systems (Unix, Linux, macOS, etc.) actually have a version of a traditional cron. While some might put a nice graphical interface on them, they all boil down to a program named cron and a file named crontab.

The program cron is always running in the background and every minute it looks at the crontab and figures out if something needs to be done. If not, it goes back to sleep.

The crontab file contains when a program should be run and which program should be run. Each line represents a different task. They look something like this.

1 0 * * * ~/fetchForcast.sh

While this may look cryptic, all it is telling cron is that at 12:01 AM every day, run a program called fetchForcast.sh. Note that time added in the Cron tool is in UTC by default. Here is an easy guide to reading a crontab.

# ┌───────────── minute (0 – 59)
# │ ┌───────────── hour (0 – 23)
# │ │ ┌───────────── day of the month (1 – 31)
# │ │ │ ┌───────────── month (1 – 12)
# │ │ │ │ ┌───────────── day of the week (0 – 6) (Sunday to Saturday;
# │ │ │ │ │ 7 is also Sunday on some systems)
# │ │ │ │ │
# │ │ │ │ │
# * * * * *
1 0 * * * ~/fetchForcast.sh

Now that you have the key, it’s pretty easy, huh?

That really is all there is to a traditional cron. Most hosts like SiteGround allow you access to the cron for your system. Sometimes you have to edit the crontab manually, but many hosts have a much better interface for you to use. Either way, you have the ability to run programs at a specific time and on a regular basis.

What is WP-CRON

Like most things, WordPress does things just a little differently. Because many plugin authors needed to be able to schedule things to happen regularly, and because many WordPress site owners don’t know where their crontab is, much less how to edit it, WordPress re-invented the cron.

At its core, WP-CRON acts like a traditional cron in that a developer can “schedule” a task to be done on a regular basis. However, unlike a traditional cron, WordPress does not have a program that is always running in the background of your server. So to make this world, WP-CRON is a process that is called every time a page is viewed.