> For the complete documentation index, see [llms.txt](https://www.shellspells.net/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://www.shellspells.net/linux/system-ops/scheduled-jobs/modify.md). # Modify ## Cron Jobs ### Modifying Existing Tasks

Command	Description
`crontab -e`	Opens the current user's crontab for editing.
`crontab -e -u <username>`	Opens another user's crontab for editing (requires root privileges).

### Deleting Tasks

Command	Description
`crontab -l`	Lists all crontab entries.
`crontab -r`	Removes all entries from the current user's crontab.
`crontab -r -u <username>`	Removes all entries from another user's crontab (requires root privileges).

Edit the crontab file to comment out or remove specific lines. Examples:\ Run a script every hour: `0 * * * * /path/to/script.sh`\ Backup files daily at midnight: `0 0 * * * tar -czf /path/to/backup.tar.gz /path/to/files`\ Send an email weekly on Sundays: `0 0 * * 0 mail -s "Weekly Report" user@example.com < /path/to/report.txt`\ Run a backup script at 3 am every day: `0 3 * * * /path/to/backup_script.sh` ``` * * * * * command to execute - - - - - | | | | | | | | | +------------ Day of the week (0 - 7) (Sunday=0 or 7) | | | +------------ Month (1 - 12) | | +------------ Day of the month (1 - 31) | +------------ Hour (0 - 23) +------------ Minute (0 - 59) ``` ## Systemd Timers ### Example Workflow Create the **service** unit that specifies what command or script to run.\ Create the **timer** unit that specifies when the service should be triggered.\ Start and **enable** the timer to ensure it runs as scheduled.\ **Inspect** the timer as needed to verify its configuration and status. <== ### Creating a Service Unit Create a .service file in /etc/systemd/system/ for your task.\ For example, mytask.service: ``` [Unit] Description=Example systemd service. [Service] Type=simple ExecStart=/path/to/your/script.sh ``` ### Creating New Timer Unit

Command	Description
`systemctl edit <timer_name>.timer`	Opens the new timer unit file for editing in your default text editor.
`systemd-run --on-active=<time> [command]`	Creates a transient timer that executes a command after a specified time (e.g., systemd-run --on-active=30 /bin/touch /tmp/foo).
`systemd-analyze timers`	Analyzes and reports information about existing timers, helping you understand the system's current timer configuration.

Create a corresponding .timer file in the same directory. For example, mytask.timer: ``` [Unit] Description=Timer for mytask [Timer] OnBootSec=15min OnUnitActiveSec=24h Unit=mytask.service [Install] WantedBy=timers.target ``` ### Managing Systemd Timers #### Editing an existing timer:

Command	Description
`systemctl edit <timer_name>.timer`	Opens the existing timer unit file for editing in your default text editor.
`systemctl reload`	Reloads systemd after editing the timer unit file to apply changes.

#### Enabling/Disabling Timers:

Command	Description
`systemctl enable <timer_name>`	Enables a timer to start automatically at its scheduled time.
`systemctl disable <timer_name>`	Disables a timer from starting automatically.

#### Manually Start/Stop:

Command	Description
`systemctl start <timer_name>`	Starts a timer manually.
`systemctl stop <timer_name>`	Stops a timer manually.

### Inspecting Timers

Command	Description
`systemctl list-timers`	Lists all active timers along with their next scheduled execution time.
`systemctl status mytask.timer`	Shows the status of the specified timer, including whether it's active and when it last triggered the service.
`systemctl cat mytask.timer`	Displays the content of the timer unit file.
`systemctl cat mytask.service`	Displays the content of the service unit file.

## Anacron ### Modifying Anacrontab Jobs

Command	Description
`crontab -e`	Opens the user's Anacrontab for editing.
`ls -l /var/spool/anacron`	User anacron jobs are here.
`ls -l /etc/anacrontab`	ain anacron configuration file

Edit the entries like a standard crontab, but use specific Anacron time specifiers (e.g., "daily", "10d"). Example:\ `sudo vim /etc/anacrontab`\ add the following to create a new anacron job that runs a script daily, even if the system is not always on:\ `1 5 my_daily_script /path/to/script.sh`\ `[period] [delay] [job-identifier] [command]` | `period` | How often the job should run, in days. | | ---------------- | -------------------------------------------------------------- | | `delay` | Delay in minutes before starting the job after anacron starts. | | `job-identifier` | A unique name for the job's log file. | | `command` | The command or script to be executed. | ### Creating Anacron Jobs Example:\ Run a script every 3 hours with a random delay of 60 minutes:\ `360 * /path/to/script.sh` ### Forcing Task Execution:

Command	Description
`anacron -f`	Runs all scheduled tasks immediately, regardless of their scheduled times.
`sudo anacron -f -d <job_name>`	Force the execution of a specific Anacron job manually with debug output.
`sudo anacron -n`	Runs all jobs in the Anacrontab as if anacron had just been started, respecting the delay specified for each job.

## at

Command	Description
`atrm <job_id>`	Removes a specific job from the queue.
`atq`	View and manipulate tasks scheduled with the `at` command.

### Creating at Jobs

Command	Description
`at <time>`	Schedules a job to run at a specific time (e.g., at 10:30AM).
`at +<duration>`	Schedules a job to run after a specific duration (e.g., at +30 minutes).
`at now + 2 hours`	Schedules a one-time job to run at a specific time (e.g., 2 hours from now).

Examples:\ `at 10:30AM < /path/to/script.sh` ## Other Tools | Command | Description | | ------- | -------------------------------------------------------------- | | `batch` | Modifies jobs submitted with the batch command (if available). | --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://www.shellspells.net/linux/system-ops/scheduled-jobs/modify.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.