Knowledge/timer-as-service-gemini.md

How to Set Up a systemd Timer

This guide provides a quick walkthrough for creating, enabling, and managing systemd timer units to schedule tasks.

1. Create a Service Unit

First, create a .service file that defines the job you want to run. This file will be placed in /etc/systemd/system/.

Example: /etc/systemd/system/mytask.service

[Unit]
Description=A description for your task

[Service]
Type=oneshot
ExecStart=/usr/local/bin/mytask.sh

• Type=oneshot: Indicates that this service is a short-lived task.
• ExecStart: The command or script to be executed.

2. Create a Timer Unit

Next, create a corresponding .timer file. This file defines the schedule for when the service should run.

Example: /etc/systemd/system/mytask.timer

[Unit]
Description=Run mytask.service every day at 3 AM

[Timer]
OnCalendar=*-*-* 03:00:00
Persistent=true

[Install]
WantedBy=timers.target

• OnCalendar: Defines a calendar-based schedule. The format is DayOfWeek Year-Month-Day Hour:Minute:Second. An asterisk (*) matches any value.
• Persistent=true: If the system was down when the timer should have run, the job will execute as soon as the system boots up.

3. Enable and Start the Timer

After creating the files, reload the systemd daemon and start the timer:

# Reload systemd to recognize the new files
sudo systemctl daemon-reload

# Enable the timer to start on boot
sudo systemctl enable mytask.timer

# Start the timer immediately
sudo systemctl start mytask.timer

You can do this in one command with --now:

sudo systemctl enable --now mytask.timer

4. List Existing Timers

To see all active timers and when they are scheduled to run next, use the list-timers command:

systemctl list-timers

Output Example:

NEXT                        LEFT          LAST                        PASSED       UNIT          ACTIVATES
Wed 2025-08-27 03:00:00 CEST  12h left      Tue 2025-08-26 03:00:00 CEST  9h ago       mytask.timer  mytask.service

• NEXT: The next time the timer will trigger.
• LEFT: Time remaining until the next trigger.
• LAST: The last time the timer was triggered.

To include inactive timers, add the --all flag:

systemctl list-timers --all

5. Check Status and Logs

To check the status of a specific timer or service:

systemctl status mytask.timer
systemctl status mytask.service

To view the logs generated by your service:

journalctl -u mytask.service

To follow the logs in real-time, add the -f flag.