When you want commands or scripts to run at some particular time, you don’t have to sit with your fingers hovering over the keyboard waiting to press the enter key or even be at your desk at the right time. Instead, you can set your task to be run through the **at** command. In this post, we’ll look at how tasks are scheduled using **at**, how you can precisely select the time you want your process to run and how to view what’s been scheduled to run using **at**.
For those who’ve been scheduling tasks on Linux systems using **cron**, the **at** command is something like **cron** in that you can schedule tasks to run at a selected time, but **cron** is used for jobs that are run periodically – even if that means only once a year. Most **cron** jobs are set up to be run daily, weekly or monthly, though you control how often and when.
The **at** command, on the other hand, is used for tasks which are run only once. Want to reboot your system at midnight tonight? No problem, **at** can do that for you assuming you have the proper permissions. If you want the system rebooted every Saturday night at 2 a.m., use **cron** instead.
After typing “at” and the time the command should be run, **at** prompts you for the command to be run (in this case, the **date** command). Type **^D** to complete your request.
Assuming we set up this **at** command earlier than 5 p.m., the date and time will be added to the end of a file named “thisfile” later the same day. Otherwise, the command will run at 5 p.m. the following day.
You can enter more than one command when interacting with the **at** command. If you want more than one command to be run at the same time, simply specify more than one command line:
In the commands above, we’re using a regular user account and adding some simple text to a file in that user’s home directory. If it’s after 6:22 a.m. when this command is run, the command will run the following day because 6:22 is taken to mean 6:22 a.m. If you want it to run at 6:22 p.m., either use 6:22 PM or 18:22. “6:22 PM” also works.
You can use **at** to schedule commands to run on specific dates either by specifying the dates or specifying dates and times like “10:00AM April 15 2021” or “noon + 5 days” (run at noon five days from today). Here are some examples:
After you specify the command to run and press **^D**, you will notice that the **at** command has assigned a job number to each request. This number will show up in the **at** command's job queue.
You can look into the details of a scheduled task using the **at -c** command. Additional details (the active search path, etc.) are also available, but the bottom lines of the output will show you what command has been scheduled to run.
Notice that the command shown begins with testing whether the user’s directory can be entered with a **cd** command. The job will exit with an error if this is not the case. Otherwise the command specified when the **at** command was issued will be run. Read the command as "move into /home/shs OR exit with the error shown".
The **/etc/at.deny** file provides a way to disallow users from being able to make use of the **at** command. By default, it will probably include a list of disallowed accounts like **ftp** and **nobody**. An**/etc/at.allow** file could be used to do the opposite but, generally, only the **at.deny** file is configured.
The **at** command is very versatile and easy to use when you want to schedule a one-time task – even if you want it to run this afternoon or years in the future.
