Linux Crontab and At

Crontab

One limitation of the at command is that we can't use it to run recurring tasks. For example, if I want to run a task every Monday, we can't use at to accomplish this. For this, we use crontab. Before we see what crontab is, let us look at the cron daemon first.

Cron Daemon

The cron daemon is a background process that is always running on a Linux machine. It is used to schedule jobs at regular time intervals, at specific recurring intervals, or on a particular date and time.

Cron Tables - known as crontab for short, are where the commands to be run, and the times to run them are stored. 

Crontab

A crontab file is a simple plain text file in which each line represents a job. They are usually in the /etc folder (or its subdirectories), and each user has their own crontab file. Crontab files contain all of the scheduled jobs of a particular user. It is a text file that each user can edit and add their tasks to. The tasks can be scheduled to run once, or they can also be recurring (eg - running every week on Monday at 8 PM, yearly on January 1 at 12 AM, etc.)

In many cases, a systemwide crontab file also exists - that only the system administrator is allowed to edit. Each line in a crontab file represents a job that should be run on a particular schedule. There is a specific format in which we must define each job. 

Format for Crontab

Each line must have five fields, separated by a space - followed by the command or the script to execute. It looks like this:

Each * must be replaced by the value for the corresponding field. For example, if we want to execute our command on a Monday, we will replace the Day of Week field with 1. 

* values can be left when we want our command to occur for all values of that field. For e.g., if we want to execute our command every Monday, no matter what the Month is, then we can leave * in the month field. 

We can also specify values using commas, dashes, and / sign 
Commas - When commas are used, all comma-separated values are accepted. For e.g., to run a command on the 5th and 8th of a month, we can set the Day of Month as 

5,8 

Dashes - If we set a value as x-y, all values between x and y are accepted. For example, to execute a command in the first 15 days, we can write Day of Month as 

1-15

/ Symbol - This is used to specify intervals. If we want a job to be done every 10 minutes, we can set the minute field as 

*/10

Special keywords also exist that we can use instead of specifying all 5 values:

• @yearly - equivalent to 0 0 1 1 *
• @daily  - equivalent to 0 0 * * *
• @hourly - equivalent to 0 * * * * 
• @reboot - Runs the command whenever the system reboots. 
   

NOTE - If the day of the week and the date of the month both are specified, the command will run when the current time satisfies either one or both of those values. For e.g., if the date was set as the 5th and the Day was set as 1 (Monday), then the command will run on all 5ths (even if they are not Mondays) and all Mondays (even if the date is not 5th), and also on 5ths that are Mondays.

Example crontab formats

1. Every Monday at 12 AM.

0 0 * * 1

2. The 5th of every month at 5 PM.

0 17 5 * *

3. Hourly on every Monday.

0 * * * 1

4. Every hour during working hours, on weekdays.

0 9-17 * * 1-5

5. Every 10 minutes on Mondays and Wednesdays

*/10 * * * 1,3

Usage

Cron is installed by default, so we do not have to install it. However, we may have to create a crontab file for the current user. If it has not been created yet, there will be no crontab file by default. 

We can check this by running:

crontab -l 

This means there is no crontab file for the user named admin.

We will create one by typing the command:

crontab -e

You can choose any editor you like. We are choosing nano. You should see this screen.

At the end, write a sample cron command such as

*/1 * * * * echo "Hi Ninja";date; >> ~/crontest.txt

This will create a file called crontest.txt in our home directory, and add a line "Hi Ninja" to it every minute. The current date of the system will also be added to the file whenever the command runs. Exit the editor (Ctrl +X in nano, :wq in Vim). If prompted to save the file, choose Yes.

You should see this output. Verify it by running crontab -l again.

The file you just created should be visible.

In your home directory, check if the file has been created.

Let's see it and check.

We can see we have one entry, corresponding to 19:52 UTC. We will wait another minute and see it once more.

We can see exactly another minute later, another entry has been created. Thus, we can verify that our crontab works!

Frequently Asked Questions

What are the major differences between using at and using crontab?

The biggest difference is in functionality, which is because at cannot be used to do recurring tasks. At will only perform a task exactly once. If we wish to perform it again, we need to write another at command for it. Crontab can handle recurring tasks easily. Another difference is that crontab uses a file to store its commands, with at we use the command line and use it as a command.

How are different time zones handled?

Generally, the cron daemon simply uses the time zone of the local machine. However, if it is a large machine and people around the world are using it, it must cater to different time zones. For this, special implementations of cron exist that allow each user to set their own time zone by specifying a CRON_TZ value in the crontab.

Can one user edit another user's crontab?

No, this is not possible. Each user can only edit its own crontab. However, the root user is allowed to edit all crontab files. 

Conclusion

This blog has explored what crontab and at commands are in Linux. We have seen examples of both, and the different use cases of both of them. 

We hope you leave this article with a broader knowledge of Linux, processes, cron, and running commands over CLI. We recommend that you explore our different articles on these topics as well, such as:

Introduction to Linux

Linux Kernel

Linux Directories

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