How to Use the Command 'at' (with Examples)

The at command is a scheduling utility found in Unix and Unix-like operating systems that allows users to execute commands at a specified future time. Unlike cron jobs, which are used for recurring tasks, at is specifically designed for one-time tasks. This command proves indispensable for situations when a task needs to be scheduled just once or at non-regular intervals. When you schedule a command using at, its output is sent to the user’s mail, providing a convenient way to track the execution outcome. Below are various use cases illustrating how to leverage the at command effectively.

Use Case 1: Start the atd daemon

Code:

systemctl start atd

Motivation:

Before scheduling any tasks with the at command, the atd daemon needs to be running. The daemon atd is responsible for executing commands scheduled by the at command. Starting this daemon ensures that your scheduled tasks will be executed at the specified times.

Explanation:

• systemctl: This is a command used to examine and control the systemd system and service manager.
• start: This argument tells systemctl to start the specified service.
• atd: This refers to the daemon of the at command, responsible for carrying out scheduled tasks.

Example Output:

There might be no direct output on successful execution of this command. You can confirm that atd has started by checking the status:

systemctl status atd

Use Case 2: Create Commands Interactively and Execute Them in 5 Minutes

Code:

at now + 5 minutes

Motivation:

Scheduling a task to execute after a short delay can be useful for troubleshooting or experimenting with different configurations, allowing you just enough time to configure or set up prerequisites for the task’s execution. It also provides a way to perform tasks that should happen once without manual intervention.

Explanation:

• at: Starts the command scheduling interface.
• now: Indicates the current time, serving as the base point for scheduling the command.
• + 5 minutes: Defines a relative time, scheduling the command to execute five minutes from the current time.

Example Output:

The system waits for input. After pressing <Ctrl> + D, you’ll receive a job number indicating the task is scheduled:

job 1 at Fri Aug 19 16:55:00 2023

Use Case 3: Create Commands Interactively and Execute Them at a Specific Time

Code:

at 15:00

Motivation:

Scheduling tasks for specific times can be essential for maintenance tasks that should minimize disruption. For example, you may want specific data processing jobs to run after business hours to not impact server performance.

Explanation:

• at: Initiates the scheduler interface.
• 15:00: Specifies an exact time (in 24-hour format) for the command execution.

Example Output:

After providing input and completing it with <Ctrl> + D, you’ll receive:

job 2 at Fri Aug 19 15:00:00 2023

Use Case 4: Execute a Command from stdin at 10:00 AM Today

Code:

echo "command" | at 1000

Motivation:

This use case is beneficial for quick, one-liner commands that need to be executed at a certain time without opening an interactive session with at. It’s particularly useful for users familiar with scripting and who want to pass commands directly via pipes.

Explanation:

• echo "command": Prints the command you wish to schedule. Replace “command” with your actual command.
• |: Pipes the output of the echo command to another command.
• at 1000: Schedules the piped command to run at 10:00 AM today.

Example Output:

job 3 at Fri Aug 19 10:00:00 2023

Use Case 5: Execute Commands from a Given File Next Tuesday

Code:

at -f path/to/file 9:30 PM Tue

Motivation:

Executing multiple commands from a file on a specific day can be useful for batch processing tasks or deploying pre-written event scripts. This approach streamlines workflow by keeping all tasks encapsulated in a script file, ideal for environments where tasks are repeatedly needed.

Explanation:

• at: Instigates the scheduling feature.
• -f path/to/file: Specifies a file containing commands to execute. Replace “path/to/file” with your actual file path.
• 9:30 PM: Designates the time for the task on Tuesday.
• Tue: Specifies the day of the week to execute the command(s).

Example Output:

Once the command is entered, output similar to this appears:

job 4 at Tue Aug 23 21:30:00 2023

Conclusion:

The at command provides a straightforward method for scheduling one-time jobs on Unix-like systems. Its versatility allows for scheduling simple tasks or complex batch processes, adapting to various administrative tasks and user needs. By understanding different ways to implement at, users can enhance their automation capabilities and optimize their workflow scheduling.