What are cron jobs?

The act of manually going through your files and deleting them can be very tedious. What if there was a simpler way to automate this process? Cue in cron jobs.

Cron is a utility program that lets users input commands for scheduling tasks repeatedly at a specific time. Tasks scheduled in cron are called cron jobs. Users can determine what kind of task they want to automate and when it should be executed. A cron file is a simple text file that contains commands to run periodically at a specific time. The default system cron table or crontab configuration file is /etc/crontab, located within the crontab directory /etc/cron.*/.

Basic Cron operations

This is not a class on cron jobs but it would be unfair for me not to mention some basic cron commands. Some of them are;

To create or edit a crontab file:

crontab -e

If no crontab files are found in your system, the command will automatically create a new one. crontab -e allows you to add, edit, and delete cron jobs. You'll need a text editor like vi or nano to edit a crontab file. When entering crontab -e for the first time, you'll be asked to choose which text editor you want to edit the file with.

To see a list of active scheduled tasks:

crontab -l

If your system has multiple users, you can view their crontab file lists by entering the following command as a superuser:

crontab -u username -l

You can also easily edit other users' scheduled jobs by typing the following crontab command:

sudo su crontab -u username -e

Lastly, to delete all scheduled tasks:

crontab -r

Alternatively, the following command is the same as crontab -r, except it will prompt the user with a yes/no option before removing the crontab:

crontab -i

In addition to crontab, the root user can also add cron jobs to the etc/cron.d directory. It's most suitable for running scripts for automatic installations and updates.

Example:

In this part, we'll create a cron job that deletes files in a particular folder that are older than a certain number of days;

1. Create a bash script to locate the folder and delete the contents of the folder (todelete) that are older than 30 days:

find /var/log/todelete/ -type f -name '*.txt' -mtime +30 -exec rm {} \;

1. Save the script and make it executable by all users using the command:

   chmod +x file.sh
   

2. Create a cron job for the script and make it run every 1st of every month:

   crontab -e
   0 0 1 * * /home/file.sh
   

3. Save and exit.

Conclusion

Setting up automatically scheduled jobs is a very practical solution that prevents you from forgetting important tasks and cron jobs are an excellent way for system administrators to manage repetitive tasks. I hope this article has given you a basic understanding of what cron jobs are and how they can be used to automate tasks. Happy learning :)