A Guide to Crontabs: Scheduling Tasks

Cron is the standard way to schedule and automate tasks on Linux, from backups to cleanup scripts. In this guide, we’ll cover how to view, create, and remove scheduled tasks, and how to read the cron timing format so you can write your own schedules with confidence.

Viewing Your Cron Table

To see your existing scheduled tasks, list your cron table with:

crontab -l

Setting Your Default Editor

Cron opens your crontab in whatever editor is set as default. To use nano, which is the easier one to start with, set it first:

export EDITOR=/usr/bin/nano

Creating or Editing Your Cron Table

Open your crontab for editing with:

crontab -e

The Cron Format

Every cron entry follows the same five-field timing format, followed by the command to run:

* * * * * command_to_execute

Reading those five fields from left to right:

• * is a wildcard meaning “every”
• Minute (0 to 59)
• Hour (0 to 23)
• Day of the month (1 to 31)
• Month (1 to 12)
• Day of the week (0 to 7, where both 0 and 7 mean Sunday)

So to run a command at 01:32 on the 17th of January, regardless of the day of the week:

32 01 17 01 * command_to_execute

Or to run it at 20:14 every Monday in March:

14 20 * 03 1 command_to_execute

The day of the week can also be written as an abbreviation, which reads more clearly than a number:

14 20 * 03 mon command_to_execute

Running a Script

You can point a cron entry at a script rather than a single command:

32 01 17 01 * /path/to/your/script.sh

Just make sure the script is executable first, or cron won’t be able to run it:

chmod +x /path/to/your/script.sh

Removing a Crontab

To clear your crontab entirely, use:

crontab -r

Editing Another User’s Crontab

To edit a different user’s crontab, use sudo and specify the user with -u:

sudo crontab -u root -e

This is useful for scheduling jobs as a service account, such as www-data or apache, rather than your own user.

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