Crontab in Linux

Introduction

Crontab is a boon for system admins who have to wake up all night to run some tasks. It is a savior for those who manually run the frequent jobs in the system. Crontab can automatically schedule tasks, and we don't have to stay up late. It will automatically perform the task at the scheduled time with the help of a cron daemon in Linux without any human effort.

For example, we can automate Linux system backup, schedule updates, synchronization of files, and many more using Cron daemon, which runs scheduled tasks from the command line.

Cron wakes up every minute and checks scheduled tasks in countable – Crontab (Cron Table) is a table where we can schedule such repetitive tasks.

“Also See, Internal and External Fragmentation, Multiprogramming vs Multitasking"

Basic Crontab Syntax

Linux crontab has six fields. The first five fields define the date and time of execution. The last field is used for the command or script to be executed. The Linux crontab syntax is as follows:

Syntax

[Minutes] [Hours] [Day_of_the_Month] [Month_of_the_Year] [Day_of_the_Week] [command]

• Minute: A minute value can be between 0-59
• Hour: A hour value can be between 0-23
• Day_of_the_month: This can be between 1-31. For the months having fewer days will ignore the remaining part
• Month_of_the_year: This can be between 1-12. You can also define this value with the first three alphabets of the month like Jan, Feb, Mar, Apr, etc.
• Day_of_the_Week: This can be between 0-7 where 0 and 7 for Sunday, 1 for Monday, 2 for Tuesday, etc. You can also use the first three alphabets of days like Sun, Mon, Tue, Wed, etc.

Now, the below statements will describe how to define multiple values or ranges. Read below and understand.

• Asterisk (*) – to match anything.
• Multiple values – Use comma (,) to write multiple values like 2,4,8 or Sun, Wed or Jan, Feb, Nov, etc.
• Define range – We can define range using the hyphen like 1-10 or 20-30 or Sun-Fri or Feb-apr.
• Define multiple ranges – You can define various ranges with commands separated like Jan-mar, and Jul-sep.

List Crontab Entries

We use the following command to see the crontab entries of current users.

crontab -l

We can use -u followed by the username to view the crontab entries of the specified user.

crontab -u username -l

Edit Crontab Entries

We use the following command to add or update jobs in crontab. This command will open a crontab file in the editor where a job can be added/updated.

crontab -e

It will edit crontab entries of currently logged-in users by default. To edit another user's crontab, use the command below.

Crontab -u username -e

Remove Crontab Entry

We use the command given below to remove complete scheduled jobs without confirmation from crontab. Use -i option before deleting the user's crontab.

crontab -r

Scheduling cron jobs

​​Here are some important examples of Crontab

You can also read about the Multilevel Queue Scheduling And Open Source Operating System.