Understanding Linux Cron Expressions

If you’re a Linux user, you might have heard of cron expressions before. These expressions define when and how often a task or command should run. In this article, we’ll explore the basics of cron expressions and show you how to use them effectively.

What is a Linux Cron Expression?

A cron expression is a string that defines the schedule of a recurring task. It consists of five fields separated by spaces or tabs, each representing a different aspect of the task’s schedule.

The five fields are:

* * * * *
- - - - -
| | | | |
| | | | ----- Day of the week (0 - 7) (Sunday is 0 or 7)
| | | ------- Month (1 - 12)
| | --------- Day of the month (1 - 31)
| ----------- Hour (0 - 23)
------------- Minute (0 - 59)

Each field can be a single number (for example, “7”), a range of numbers (for example, “3-5”), a list of numbers (for example, “1,3,5”), or the wildcard symbol “*”, which means “any value.”

Anatomy of a Cron Expression

Let’s look at an example cron expression in-depth to understand each field.

0 0 * * *

This expression means “run the command at midnight every day.”

Breaking it down:

• The first field (0) specifies the minute (0-59).
• The second field (0) specifies the hour (0-23).
• The third field (*) specifies the day of the month (1-31).
• The fourth field (*) specifies the month (1-12).
• The fifth field (*) specifies the day of the week (0-7, where both 0 and 7 represent Sunday).

You can also use aliases instead of numeric values in the day-of-week field. For example, “MON-FRI” means “Monday through Friday,” “SAT,SUN” means “Saturday and Sunday,” and so on.

Special Characters

Linux cron expressions also support other special characters that allow for more complex schedules.

The Slash (/)

The slash allows you to define step values. For example, “*/15” in the minute field means “every 15 minutes,” and “0 */2” in the hour field means “every two hours on the hour.”

The Comma (,)

The comma allows you to specify multiple values for a field. For example, “1,3,5” in the hour field means “at 1, 3, and 5 o’clock.”

The Dash (-)

The dash allows you to specify a range of values for a field. For example, “2-5” in the day-of-week field means “Monday through Thursday.”

The Asterisk (*)

The asterisk represents “any value” for a field. For example, “* * * * *” means “every minute of every hour of every day.”

Examples

Here are some sample cron expressions and what they mean:

• “0 0 * * *” – Run the command at midnight every day.
• “0 2 * * *” – Run the command at 2 AM every day.
• “0 0 * * 7” – Run the command at midnight every Sunday.
• “0 */2 * * *” – Run the command every two hours.
• “0 8-18 * * *” – Run the command every hour between 8 AM and 6 PM.
• “0 0-12 * * *” – Run the command every hour between midnight and noon.
• “0 0,12 * * *” – Run the command at midnight and noon every day.
• “0 0 * * MON-FRI” – Run the command at midnight every weekday.

Conclusion

Cron expressions are an essential tool for Linux users who need to schedule recurring tasks or commands. Remember that each field of a cron expression represents a different aspect of the schedule, and that special characters like the slash, comma, and dash allow for more complex schedules. With this knowledge, you’ll be able to efficiently manage your recurring tasks and make the most out of your Linux system.