## 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`**
```ini
[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`**
```ini
[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:
```bash
# 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`:
```bash
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:

```bash
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:
```bash
systemctl list-timers --all
```

### 5. Check Status and Logs

To check the status of a specific timer or service:
```bash
systemctl status mytask.timer
systemctl status mytask.service
```

To view the logs generated by your service:
```bash
journalctl -u mytask.service
```
To follow the logs in real-time, add the `-f` flag.