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Cron is a service that lets you run scheduled tasks on a computer. These tasks are called cronjobs. If you have already followed the initial course you will have already used cron when you set up Certbot, but we'll explain how they work generally here.

What tasks would I want to schedule?

You can schedule anything! Some examples of what you might have done already include:

  • updatedb to update your locate database to let you quicking search for files
  • certbot to update renewing of your https certs

Some tasks that you might want to schedule may include:

  • Package updates - if you really just want to leave your server alone you can automated updating packages on your server
  • Backups - you may want to backup certain files every day and some every week, this is possible with cron

And many more, anything you can do can be turned into a cronjob.

Basic Cronjobs

This the preferred method for personal tasks and scripts; it's also the easiest to get started with. Run the command crontab -e to access your user's crontab

Once you have figured out the command you want to run you need to figure out how often you want to run it and when. I am going to schedule my system updates once a week on at 3:30 AM on Mondays.

We now have to convert this time (Every Monday at 3:30 AM) into a cron time. Cron uses a simple but effective way of scheduling when to run things.

Crontab expressions look like this * * * * * command-to-run The five elements before the command tell when the command is supposed to be run automatically.

So for our Monday at 3:30 AM job we would do the following:

 .---------------- minute (0 - 59)
 | .------------- hour (0 - 23)
 | | .---------- day of month (1 - 31)
 | | | .------- month (1 - 12)
 | | | | .---- day of week (0 - 6)
 | | | | |
 * * * * *
30 3 * * 1 apt -y update && apt -y upgrade

Some notes

  • On the day of the week option, Sunday is 0 and counting up from there, Saturday will be 6.
  • * designates "everything". Our command above has a * in the day of month and month columns. This means it will run regardless of the day of the month or month.
  • The hour option uses 24 hour time. 3 = 3AM, while use 15 for 3PM.

More examples

Let's add another job, our backup job (for the purposes of this our backup command is just called backup). We want to run backup every evening at 11PM. Once we work out the timings for this we can add the to the same file as the above by running crontab -e This would mean our full crontab would look like this:

0 23 * * * backup

Consecutive times

Suppose we want a command to run every weekday. We know we can put 1 (Monday), but we can also use 1-5 to signify from day 1 (Monday) to day 5 (Friday).

0 6 * * 1-5 echo "Wakey, wakey, wagie!" >> /home/wagie/alarm

The above echo command runs every Monday through Friday at 6:00AM.

Non-consecutive times

We can also randomly specify non-consecutive arguments with a comma. Suppose you have a script you want to run at the midday of the 1st, 15th, and 20th day of every month. You can specify that by putting 1,15,20 for the day of the month argument:

0 12 1,15,20 * * /usr/bin/pay_bills_script

"Every X minutes/days/months"

We can also easily run a command every several minutes or months, without specifying the specific times:

*/15 * * * * updatedb

This cronjob will run the updatedb command every 15 minutes.

Beware of this Rookie Mistake Though...

Suppose you want to run a script once every other month. You might be tempted write this:

* * * */2 *

That might feel right, but this script will be running once every minute during that every other month. You should specify the first two arguments, because with * it will be running every minute and hour!

0 0 1 */2 *

This makes the command run only at 0:00 (12:00AM) on the first day of every two months, which is what we really want.

Consult the website for an intuitive and interactive tester of cronjobs.

User vs. Root Cronjobs

It is important to note that user accounts all have different cronjobs. If you have a user account chad and edit his crontab with crontab -e, the commands you add will be run as the chad user, not root or anyone else.

Bear in mind that if you need root access to run a particular command, you will usually want to add it as root.

System-wide cron directories

crontab -e is the typical interface for adding cronjobs, but it's important to at least know that system-wide jobs are often stored in the file directory. Some programs which need cronjobs will automatically install them in the following way.

Run the command ls /etc/cron* you should see a list of directories and there contents. The directories should be something like the below:

  • /etc/cron.d This is a crontab like the ones that you create with crontab -e
  • /etc/cron.hourly
  • /etc/cron.daily
  • /etc/cron.weekly
  • /etc/cron.monthly

The directories cron.{hourly,daily,weekly,monthly} are where you can put scripts to run at those times. You don't put normal cron entries here. I prefer to use these directories for system wide jobs that don't relate to an individual user.

Last update: September 24, 2023