Need to run a job once at a specific time? Sounds like a cron job. But cron is for repeated periodic execution. To run a task once at a predefined time, use the "at" command. In this article, I'll teach you how to use the "at" command.

Installation

As is standard with Linux distributions, all applications are available as a ready-made package for that Linux distribution. Similarly, the "at" command is commonly available in your distribution's repository.

In the Mageia distribution, you can install the "at" program with the following command:

urpmi at

In the Debian distribution, you install the "at" program with the following command:

apt-get install at

The "at" command runs in the background, as a daemon, and runs jobs under the user who scheduled the job. Therefore, you need to start the daemon and enable the "at" daemon to start after the computer boots. To do this, use the command:

systemctl enable etc && systemctl start etc

Command at

The "at" command is used to schedule a command or script to run once at a specific time in future.

Basic Usage

Interactive mode

at 16:00

Then type the commands and exit by typing Ctrl+D.

Scheduling a job

echo "ls -l /home/user" | at 14:30

Executes the command "ls -l /home/user" at 14:30 on the same day.

Schedule the computer to shut down on a specific date

echo "reboot" | at 23:59 31.12.2025

Show scheduled tasks

If you want to check which user scheduled what for what time, use the following command, which will list the scheduled jobs with their start time:

atq

Alternatively, you can use

at -l

Cancel a Scheduled Job

You can decide that you want to cancel a task. You can cancel a task that is scheduled. To delete a scheduled task, use the command:

atrm <task number>

You can get the job number from the atq statement.

Special times

• at now + 1 hour → Runs the command in 1 hour.
• at now + 30 minutes → Executes the command in 30 minutes.
• at midnight → Runs the job at midnight.
• at teatime → Runs the job at 4:00 pm.

Video tutorial

Practical video demonstration of how to use the AT command:

Commands in file

You don't have to enter commands manually, but you can write what you need to execute to a file. And then pass that file to the "at" program. Especially if you have a complex command or know you will want to run it sometime in the future, it is a good idea to write it down and save time afterwards.

You then pass your script to the "at" program as follows:

at -f config.at 13:00 2024-04-05

Beware of the default shell

The "at" command allows you to run tasks at a specific time, but it is important to note that these tasks are by default run in a non-interactive shell (usually /bin/sh or another default system shell). This can cause problems if the user expects their preferred shell to be used (e.g. bash, zsh) or that certain environment variables will be available.

Why specifically this needs to be considered:

• A shell other than your interactive - For example, if you use bash but "at" triggers /bin/sh, then some constructs for example([ ... ]], declare, source may not work.
• Missing environment variables - A task run via at does not have the same environment as your interactive shell. For example, your custom aliases, functions, or modified $PATH.
• Need to explicitly specify a shell - If you want the at task to run in a specific shell, you must explicitly specify it.

Therefore, I recommend to always specify which shell you want to run your commands in, so that you don't have to wonder why your script behaves differently when you run it manually than when you run it with the "at" program.

Automate

The "at" program is one of many programs, that will make your job easier if you're using a powerful tool like the Linux operating system on your servers. Use tools that make your job easier and automate tasks that you would otherwise have to do manually. This will save you a lot of time and therefore money.