<span>at</span> is a command-line tool used to automatically execute various commands or scripts at a specified time and date. Tasks created with <span>at</span> are executed only once, making it an ideal tool for managing one-time tasks at precise time points.
This article will introduce how to use <span>at</span> along with its companion tools <span>batch</span>, <span>atq</span>, and <span>atrm</span> to view, delete, and create delayed execution tasks.
1. Installing at
Depending on the Linux distribution, <span>at</span> may not be pre-installed. If it is not installed, you can quickly install it using the package manager:
Installation on Ubuntu/Debian systems:
$ sudo apt update
$ sudo apt install at
Installation on CentOS/Fedora systems:
$ sudo yum install at
After installation, ensure that the scheduling daemon <span>atd</span> is running properly and set to start on boot:
$ sudo systemctl enable --now atd
2. Basic Usage of the at Command
<span>at</span> command has a simplified syntax as follows:
$ at [options...] execution_time
This command requires specifying the task execution time (<span>runtime</span>) in the parameters and entering the command to be executed via standard input.
For example, to create a task that executes at 9 AM:
$ at 09:00
After pressing Enter, you will enter the <span>at</span> interactive prompt (usually displayed as <span>at></span>), along with a prompt for execution environment information:
warning: commands will be executed using /bin/sh
at>
Input the command to be executed (multiple lines are allowed):
at> tar -xf /home/haopython/file.tar.gz
After finishing the input, press <span>Ctrl-D</span> to save the task, and the system will return the task number and execution time:
at> <EOT>
job 4 at Tue May 5 09:00:00 2025
3. Other Command Input Methods
1) Piping Parameters: Pass commands using <span>echo</span>:
$ echo "command_to_be_run" | at 09:00
2) Here Document:
$ at 09:00 <<END
command_to_be_run
END
3) Reading from a File: Use <span>-f</span> to specify the script path:
$ at 09:00 -f /home/haopython/script.sh
4) Email Notification Control
By default, the task output will be sent to the user via email. Use <span>-M</span> to disable notifications:
$ at 09:00 -M
Use <span>-m</span> to force sending an empty output email:
$ at 09:00 -m
4. Batch Command
<span>batch</span> (or alias <span>at -b</span>) will execute batch tasks when the system load is below a threshold. The default threshold is 1.5 (which can be adjusted via the <span>atd</span> daemon). If the system load exceeds the threshold, the tasks will be queued.
Example usage:
$ echo "command_to_be_run" | batch
5. Time Parameter Specifications
<span>at</span> supports flexible time formats, allowing the combination of the following elements:
1) Time: Supports <span>HH:MM</span> or <span>HHMM</span> formats, with 12-hour format allowing the addition of <span>am/pm</span> (e.g., <span>10:30am</span>). Keywords include: <span>now</span> (immediate), <span>midnight</span> (midnight), <span>noon</span> (noon), <span>teatime</span> (4 PM). If the specified time has passed, it will be postponed to the same time the next day.
2) Date:
- Month name + date (optional year), e.g.,
<span>Oct 21 2020</span> - Keywords:
<span>today</span>(today),<span>tomorrow</span>(tomorrow),<span>weekday</span>(weekday) - Formats:
<span>MMDD[CC]YY</span>,<span>MM/DD/[CC]YY</span>,<span>DD.MM.[CC]YY</span>or<span>[CC]YY-MM-DD</span>
3) Time Increment:<span>now + number time_unit</span> (units: <span>minutes</span>/<span>hours</span>/<span>days</span>/<span>weeks</span>)
Here are some combination examples:
- Execute 10 minutes after the current time (next Sunday):
$ at sunday +10 minutes
- Execute at 1 PM two days later:
$ at 1pm + 2 days
- Specify execution at 12:30 on October 21, 2025:
$ at 12:30 102125
- Execute one hour later:
$ at now +1 hours
- Use
<span>-t</span>to specify the<span>[[CC]YY]MMDDhhmm[.ss]</span>format:
$ at -t 202505111321.32
6. Task Queue Management
- The default queue:
<span>at</span>tasks use the<span>a</span>queue, while<span>batch</span>tasks use the<span>b</span>queue. - Queue priority: The lower the alphabetical order of the queue (e.g.,
<span>a</span>), the higher the priority (lower nice value). - Example of specifying a queue (using
<span>L</span>queue):
$ at monday +2 hours -q L
7. Viewing Pending Tasks
Execute <span>atq</span> or <span>at -l</span> to list the current user’s tasks:
$ atq
Output format: task number execution time queue letter username
9 Mon May 5 12:22:00 2025 a haopython
12 Tue Oct 21 12:30:00 2025 a haopython
...（略）...
When executed by an administrator, all user tasks will be displayed.
8. Deleting Pending Tasks
Use <span>atrm</span> or <span>at -r</span> + task number to delete:
$ atrm 9
9. User Permission Control
Limit user access to <span>at/batch</span><span> using the following files:</span>
<span>/etc/at.deny</span>: Blacklist users (default exists and is empty, allowing all users)<span>/etc/at.allow</span>: Whitelist users (if it exists, only users in the list are allowed)
When both files do not exist, only administrators can use <span>at</span>.
10. Typical Application Scenarios
# Automatically package logs after work (today at 17:30)
$ at 17:30 <<EOF
tar -czf /var/log/nginx/$(date +\%F).tar.gz /var/log/nginx/*.log
EOF
# Run data cleaning during low load (execute when the system is idle)
$ echo "python3 /scripts/data_clean.py" | batch
11. Summary
As one of the core tools for task scheduling in Linux, <span>at</span> focuses on the precise management of one-time delayed tasks, complementing the <span>crontab</span> for periodic tasks. The typical division of labor between the two is as follows:
| Tool | Applicable Scenarios | Execution Characteristics | Management Complexity |
|---|---|---|---|
<span>at</span> |
Scheduled shutdowns / Temporary backups / Late-night batch processing | Single execution | Low (no ongoing maintenance) |
<span>crontab</span> |
Log rotation / Regular synchronization / Daily reports | Cyclic execution | High (requires long-term maintenance) |
