What Is Crontab : Understanding Crontab on Linux with Examples

In this article, you’ll learn about What Is Crontab, Crontab Format, Crontab Values, Creating & Editing Crontab Files, Crontab Commands and How to Use Crontab.

What Is Crontab

The Cron daemon is a built-in Linux utility that reads the crontab (cron table) file and executes commands and scripts at predefined times and intervals.

Users set up cron jobs in the crontab to streamline routine maintenance activities, such as updating software, creating backups, or clearing caches.

Crontab Format

A cron job is defined using a line in the crontab file. Each line consists of six fields, separated by spaces:

minute hour day_of_month month day_of_week command

Crontab Values

For the cron daemon to understand instructions correctly, the correct crontab syntax must be used. Crontab syntax consists of five fields. Each one can be filled with any of the values as shown in the following table:

Aside from possible crontab values, some special characters need to be memorized:

Crontab Commands

Only a few cron commands are used in the command-line, thus making it easy to create, delete, and manage cron entries:

• crontab -e – used to edit system crontabs. This command will create a new crontab if it has not been made yet.
• crontab -l – used to view crontab entries (cron jobs) and display system crontab file contents.
• crontab -r – will remove the current crontab file.
• crontab -i – will show a prompt before removing a user’s crontab. Highly recommended to use it together with the -r flag, making the flag -ri.

Creating and Editing Crontab Files

You can create or edit a crontab file using the following command:

crontab -e

This will open your default text editor (usually vi or nano) to edit the crontab file. Make sure to save the file and exit the editor.

How to Use Crontab: Examples of Crontab Syntax

First, use the crontab command to create your first crontab entry:

crontab -e

You will be asked to choose an editor. We recommend using nano, the first option in our example:

Afterward, you will be redirected to the crontab file. To add new entries, simply choose a new line and proceed with the cronjob.

1. To Schedule a Job for Every Minute using Cron:

• Ideally, you may not have a requirement to schedule a job every minute. But understanding this example will help you understand the other examples.

* * * * * command

• The * means all the possible units — i.e. every minute of every hour throughout the year. More than using this * directly, you will find it very useful in the following cases.
• When you specify */5 in the minute field means every 5 minutes. When you specify 0-10/2 in the minute field means every 2 minutes in the first 10 minutes. Thus the above convention can be used for all the other 4 fields.

2. To Schedule a Job For More Than One Time (e.g. Twice a Day):

• The following script takes an incremental backup twice a day every day. This example executes the specified incremental backup shell script (incremental-backup) at 11:00 and 16:00 every day.
• The comma-separated value in a field specifies that the command needs to be executed at all the mentioned times.

00 11, 16 * * * /home/test/bin/inc-backup.sh

• 00 – 0th Minute (Top of the hour) 11, 16 – 11 AM and 4 PM * – Every day * – Every month * – Every day of the week

3. Schedule a Background Job Every Day

To schedule a background job to run every day, you can use the @daily cron command:

@daily cat /home/helloworld.sh

Mind that the script will be executed at 12am every day.

4. Schedule a Job for a Certain Range of Time

It is possible to schedule a job for a specific range of time. For example, every weekday, including weekends, from 8am to 5pm. The end result would look like this:

00 08-17 * * * cat /home/helloworld.sh

Here’s another example of the same cron, but just on the weekends:

00 08-17 * * 6-0 cat /home/helloworld.sh

5. Schedule a Cron Job at the Beginning of Every Month

In order to schedule a job at the beginning of every month, you can use the @monthly operator:

@monthly cat /home/helloworld.sh

Keep in mind that this will execute the job at 12am on the first day of every month. Similarly, there is a @yearly operator who will run the job on the first day of every year.