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[ Linux Command Manual](#) * * * `at` is a powerful scheduling tool in Linux systems that allows users to execute one-time tasks at a specified time, unlike `cron` which handles periodic execution. Compared to `cron`, `at` is more suitable for the following scenarios: * Tasks that only need to run once * Tasks scheduled to run at a specific future time * Temporary system maintenance operations * * * ## Installation and Basic Usage ### Check Installation Most Linux distributions come with `at` pre-installed. Check using the following command: which at If not installed, use your package manager: ## Example # Debian/Ubuntu sudo apt-get install at # CentOS/RHEL sudo yum install at ### Start the at Service After installation, start the `atd` daemon: ## Example sudo systemctl start atd sudo systemctl enable atd # Enable auto-start on boot * * * ## Command Syntax and Parameters ### Basic Syntax at time ### Common Options | Option | Description | | --- | --- | | `-f file` | Read commands from a specified file instead of standard input | | `-m` | Send mail to the user after the job completes | | `-l` | List pending jobs (same as `atq`) | | `-d job ID` | Delete a specified job (same as `atrm`) | | `-v` | Show the time when the job will be executed | ### Time Formats `at` supports various time formats: 1. **Absolute Time** * `HH:MM` (e.g., 14:30) * `YYYY-MM-DD` (e.g., 2023-12-25) * Combined format `HH:MM YYYY-MM-DD` 2. **Relative Time** * `now + quantity unit` (e.g., `now + 2 hours`) * Units can be: minutes, hours, days, weeks 3. **Special Keywords** * `noon` (12:00 PM) * `midnight` (12:00 AM) * `teatime` (4:00 PM) * `tomorrow` (Same time tomorrow) * * * ## Usage Examples ### Example 1: Basic Usage ## Example $ at 15:30 warning: commands will be executed using /bin/sh at>echo"Hello at command"> ~/at_test.txt at># Press Ctrl+D to end input job 1 at Tue Jul 18 15:30:00 2023 ### Example 2: Reading Commands from a File Create a script file `myscript.sh`: ## Example #!/bin/bash echo"This is a test">>/tmp/at_log date>>/tmp/at_log Then use: at -f myscript.sh now + 5 minutes ### Example 3: Viewing and Deleting Jobs ## Example $ atq # or at -l 1 Tue Jul 18 15:30:00 2023 a username $ atrm 1# or at -d 1 * * * ## Advanced Usage ### Setting Environment Variables ## Example $ at 16:00 at>export MY_VAR="test" at>echo$MY_VAR> ~/env_test.txt at> ### Executing Complex Commands ## Example $ at 17:00 today at>cd/var/log &&grep"error" syslog > ~/error_log.txt at> ### Using Here Document ## Example at 18:00 << 'EOF' #!/bin/bash echo "Starting backup..." tar -czf ~/backup-$(date +%F).tar.gz ~/Documents EOF * * * ## Important Notes 1. **Permission Control** * Access is controlled by `/etc/at.allow` and `/etc/at.deny` files * If `at.allow` exists, only listed users can use `at` * If `at.allow` does not exist, check `at.deny`; listed users are denied access 2. **Output Handling** * By default, command output is mailed to the user * Use the `-m` option to force sending mail, even if there is no output 3. **Environment Differences** * The execution environment for `at` may differ from an interactive shell * Important environment variables should be explicitly set within the commands 4. **Error Handling** * Check if the `atd` service is running: `systemctl status atd` * Check system logs for error messages: `journalctl -u atd` * * * ## Practice Exercises 1. Create a job to append the current date and time to `~/time_log.txt` in 5 minutes 2. Write a script to back up the `/etc` directory and schedule it with `at` to run at 2 AM tomorrow 3. List all current pending `at` jobs, then delete one of them * * * ## Summary The `at` command is a powerful tool for managing one-time scheduled tasks in Linux systems. Through this article, you should have learned: * The basic concepts and installation methods for `at` * Various ways to specify time formats * Operations for creating, viewing, and deleting jobs * Advanced usage and important considerations For tasks requiring periodic execution, consider using `cron`, but for one-time tasks, `at` is undoubtedly the best choice. * * Linux Command Manual](#)