How to schedule tasks using at command on Linux

Learning how schedule and manage tasks using the at program

Requirements

• Root permissions to start the atd daemon
• Having the at program installed

Difficulty

EASY

Conventions

• # - requires given linux commands to be executed with root privileges either directly as a root user or by use of sudo command
• $ - requires given linux commands to be executed as a regular non-privileged user

Introduction

During the administration of a system, being able to schedule a task for a later execution it's one crucial ability: to perform a backup of a database for example, or perhaps to run a maintenance script. Less known than cron or anacron, the at program let us do this in a pretty easy way: in this tutorial we will learn how to use it and how it is different from the programs mentioned above.

What is at?

Unlike cron, which let us run a task on a regular basis, at gives us the ability to execute a command or a script at a specified date and hour, or after a given interval of time. Minutes, hours, days or weeks can be used as units. It's even possible to use certain "keywords" as midnight or teatime (which corresponds to 4pm).

Installing at

If not installed by default, at should be available in almost all distributions' repositories.

To install it on Fedora, just run:

# dnf install at

On RHEL or CentOS yum is still the default package manager:

# yum install at

On Debian or Ubuntu:

# apt-get install at

On Archlinux:

# pacman -S at

Starting the daemon

Once the program it's installed, we must start the atd daemon and eventually enable it if we want it to be launched automatically at boot. I will here assume the use of systemd as the init system. The command must be executed with root privileges:

# systemctl enable --now atd.service

Scheduling a job from the at prompt

With everything in place, we can now use at. Let's suppose we want to run a command 1 minute from now. The correct syntax would be:

$ at now + 1 minute

To run the same command at 4pm, three days from now, instead, we would run:

$ at 4pm + 3 days

Once the above line is executed, the at prompt will appear, waiting for us to enter the command to be executed after the specified time interval:

$ at now + 1 minutes
at> echo "Hello world" > test.txt
at> 
job 4 at Tue Dec 19 11:29:00 2017

To exit the at prompt we should press the CTRL+d key combination. At this point we will presented with a summary of the scheduled task, which will show us the job id (4 in this case) and the date at which it will be executed.

Just as an example, we entered a trivial command to show how at works. A minute from now, the "Hello world" string will be written to the file test.txt, which will be automatically created if doesn't already exist.

Schedule the execution of a script

Instead of specifying the command to be executed, interactively, from the prompt, we can instruct at to execute an existing script or program simply by passing it as an argument to the -f flag or, alternatively, by using the < redirection operator. Therefore, assuming we want to run a script which is present in our current working directory, we would run:

# Using the dedicated -f flag
$ at now + 1 minute -f script.sh

# Using the < redirection operator $ at now + 1 minute < script.sh

Manage scheduled jobs

To queue, examine or delete jobs scheduled with at, we can either use dedicated commands like atrm and atq or run at with specific flags, the latter being just aliases for the former. For example, say we want to obtain a list of all pending jobs scheduled with at by our user:

 $ atq
4      Tue Dec 19 11:29:00 2017 a egdoc

The above command, if launched as root, will display the task scheduled by all users in the system.

To delete a queued job, we could use atrm or run at with the equivalent flags: -r or -d. The job to be deleted must be referenced by its number. In the case above, we would therefore run:

 $ atrm 4 

Conclusions

Although simpler than cron or anacron, the at program can be very useful in certain situations: to run a program with a specific delay or when you know exactly the time in which the task must be executed. Reference the manual for further information, and add this little tool to your toolbox, it will surely come in handy.