Cron is a cmdline utility used for scheduling commands or shell scripts to run periodically at fixed intervals.

Cron uses a configuration file called a crontab (cron table) that specifies shell commands to run.

Begin

Start by listing out any running cronjobs with crontab -l

Create a cronjob by running crontab -e.

Select your editor of choice.

At the top of the crontab file, insert your first cronjob.

Syntax:

$ [cron expression] [command/shell script]
$ minute hour day(month) month day(week) command

You can use asterixes for the time fields to select every hour/minute/etc.

Cron examples:

1. This runs a shell script located at an absolute path every five minutes.

*/5 * * * * /home/orangeburrito/Documents/scripts/script.sh 

Inside the aforementioned shell script:

#!/bin/sh
date "+Date: %D%nTime: %r" >> /home/orangeburrito/Documents/output.txt

2. This runs a command that outputs the current date to a file located at an absolute path, once a day at 6 AM.

0 6 * * * date "+Date: %D%nTime: %r" >> /home/orangeburrito/Documents/output.txt

Save and edit the crontab file. Wait the alloted number of time specified in the cron expression and then check.

Learn more about cron schedule expressions here.

Note: Cron uses absolute paths.
The location of the script file should be determined using pwd

If running folder-specific commands inside your script file, navigate to that folder beforehand.
e.g. running a compose script:

#!/bin/sh
cd /path/to/folder
docker-compose up -d

logs

By default cron jobs get logged to /var/log/syslog

You can see cronjobs there by running grep CRON /var/log/syslog

Cronjobs can be forced to log to a specific file like so:
*\5 * * * * /path/to/script.sh > /path/to/output.txt

etc

You can remove all cronjobs with crontab -r

To delete a singular cronjob, run crontab -e and remove the line containing that cronjob.

You can insert environment variables into the cron config file at the top.

For example:

PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin
/bin/sh
*/2 * * * * /home/orangeburrito/Documents/scripts/script.sh > /home/orangeburrito/Documents/cronlog.log