> 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). |