Alright my friends, it’s about time we talk about Cronjobs, specifically Ubuntu 16.04 Cronjob and how to create them.

In yesterday’s tutorial, we were installing Certbot to auto renew our SSL certificates with a Cronjob, so I thought it might be a good idea to explain how Cronjobs work and how to set them up properly.

1. Ubuntu 16.04 Cronjob: What is a Cronjob?

A Cronjob is a task that gets done automatically on a specific time. Let’s take sudo apt-get update for example.

If you want to keep your apt repository up to date at all times, you can utilize a Cronjob to do that for you, once a day for example.

2. How does a Cronjob work?

Let’s login to our Ubuntu instance for that. After you are in, run:

sudo crontab -e

If you run this for the first time, you will get asked which editor you want to use per default. I recommend using Nano for beginners.

And this is how your crontab file looks like when you first open it:

Now, when you look at the bottom like, you can see m h dom mon dow command

This is what you need to understand, so let’s break it down:

m h dom mon dow Minute Hour Day of Month Month Day of Week

Alright, this should shed some light already.

Now we need to understand with which values we can work:

Minutes: 0 – 59

Hours: 0 – 23

DOM: 1 – 31

Month: 1 – 12

DOW: 0 is Sunday and 7 is Sunday for example: 0 – 6 => Sunday – Saturday or 1 – 7 => Monday – Sunday You usually dont use 7 tho, only 0-6

* Is a Wildcard, which means basically if you take the minute field and put a wildcard in it, it will run every Minute.

Let’s do a little test for yourself: Set a Cronjob that runs every Saturday at 14:30.

Answer:

Answer: 30 14 * * 6

Alright, so this should give you a pretty good idea of how to set the time accordingly. The whole command would then look something like this:

15 23 15 * * sudo apt-get update

This would run sudo apt-get update at 23:15, every month on the 15th, no matter which day.

In case you really can’t wrap your head around it, head over to https://crontab.guru/ and let it do the thinking 🙂

3. Wrapping up

And finally let’s actually set the Ubuntu 16.04 Cronjob:

sudo crontab -e

Scroll down all the way to the bottom of the file and enter your desired time and command:

30 14 * * * sudo apt-get update

Hit CTRL + O to write the file and CTRL + X to leave (Nano)

And this concludes today’s lesson.

Once you understand how to set the time, it’s a piece of cake.

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