Linux

How to Schedule a Command on Particular Time in Linux

Here’s how to use the at command in Linux to schedule a one-time task for execution at a specific time.

Alby Andersen
By Alby Andersen
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The at command in Linux is used to schedule a one-time task to be executed at a specific time or later. It’s useful for running commands or scripts in the future without needing to keep a terminal open. Tasks scheduled with at are executed once at the specified time.

Contents
1. Install at (if not already installed)2. Basic Syntax3. Schedule a Task4. Time Formats5. List Pending Jobs6. Remove a Scheduled Job7. Advanced Usage8. Verify the atd Service

1. Install at (if not already installed)

Most Linux systems have at preinstalled. If not, install it using your package manager:

sudo apt install at   # Debian/Ubuntu
sudo dnf install at   # Fedora/CentOS
sudo pacman -S at     # Arch/Manjaro

2. Basic Syntax

at [time]

After running this command, you’ll enter an interactive prompt where you can type the commands to execute.
Press Ctrl+D to save and exit.

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3. Schedule a Task

Example 1: Run a command at 10:00 PM

at 10:00 PM
at> echo "System update started" | mail -s "Update Alert" [email protected]
at> sudo apt update
at> Ctrl+D  # Save and exit

Output:

job 1 at Tue Oct 24 22:00:00 2023

Example 2: Run a script in 2 hours

at now + 2 hours
at> /path/to/backup_script.sh
at> Ctrl+D

4. Time Formats

The at command supports flexible time specifications:

  • Absolute time:
    10:00 PM, 23:45, 2023-12-31, midnight, noon.
  • Relative time:
    now + 30 minutes, tomorrow, next week, next monday.

5. List Pending Jobs

atq

Output:

1       Tue Oct 24 22:00:00 2023 a user
2       Wed Oct 25 14:30:00 2023 a user

6. Remove a Scheduled Job

atrm [job_number]

Example:

atrm 1  # Deletes job 1

7. Advanced Usage

Redirect output to a file:

at now + 1 hour
at> ls -l /var/log > log_list.txt
at> Ctrl+D

Specify a different shell:

at -t 202310241800 -b /bin/zsh
at> echo "Running in Zsh" >> ~/output.log
at> Ctrl+D

Run as root:

sudo at 11:00 PM
at> systemctl restart nginx
at> Ctrl+D

8. Verify the atd Service

Ensure the atd daemon is running to execute scheduled jobs:

systemctl status atd   # Check status
sudo systemctl start atd  # Start if inactive

Important Notes:

  • Permissions: Users must be listed in /etc/at.allow (or not in /etc/at.deny).
  • Logs: Check /var/mail/[user] for command output (if not redirected).
  • Limitations: Jobs are one-time (use cron for recurring tasks).

Notes:

  • at commands execute only once at the given time.
  • Make sure the atd daemon is running; otherwise, at will not work.
  • at is more suitable for tasks that need to be scheduled only once, whereas cron is more appropriate for recurring tasks.

Common Use Cases:

  • Schedule system updates during off-peak hours.
  • Send automated reminders.
  • Run backups at a specific time.
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