Cron is a time-based scheduler to execute scripts/commands on a time frame that you set. This is a neat little utility that is often over-looked for forgotten. I’ll be honest I didn’t have much experience for cron for a few years so basically forgot all there is to know!

Well here is the basics anyone needs to know… cron has the concept of tabs, a tab is each “thing to do” on the time scheduled. For example, my use case recently was to renew my Lets Encrypt certificates (check them out here, #securethenet), they expire every 3 months and I always am bad at tracking so I decided to use a crontab to run the certbot renew command for me.

crontab -l

Will list out all of the existing cron tabs that you have deployed, it might show or complain if you’re not root; maybe sudo the command see if that changes output.

Now to create a crontab you run the below, this will use your editor you have selected (or need to select) to open the file where you can define 1->X tabs.

crontab -e

You may be wondering “how the hell do I know the time formatting“… well a website called cronguru has got you covered!

An example of a crontab definition is below, I used the nicely annotated options which are not always understood/accepted by all *nix distro’s so heads up. Below this runs each week on a Sunday which executes the shell script that contains a few commands to renew certs (it by nature ignores if cert renewal isn’t due so don’t worry I am not being a tit and hammering Lets Encrypt).

@weekly /root/

I hear you ask now well how do I verify it’s running, well cron by nature will output into the syslog log file under /var/log/syslog (or maybe messages if you are on a different distro), one way I check that cron is running is using the following command…

# the -n 5 is interval time in seconds, -c is interpret ANSI colours 
watch -n 5 -c grep CRON /var/log/syslog

# example output
Every 5.0s: grep CRON /var/log/syslog crowzfx: Sun Jul 25 13:59:49 2021

Jul 25 12:29:01 crowzfx CRON[9216]: (root) CMD (/root/ )

Cron is that tool in your *nix toolbelt that you usually forget about but when you remember it can bring some powerfully automation and processing. Happy cron-ing!