# Debian services and systemd

`systemd` is the service manager. It starts, stops, and manages daemons (background processes). Services are defined as unit files.

## Basic commands

Start a service:

```bash
sudo systemctl start ssh
```

Stop:

```bash
sudo systemctl stop ssh
```

Restart:

```bash
sudo systemctl restart ssh
```

Check status:

```bash
sudo systemctl status ssh
```

Enable on boot:

```bash
sudo systemctl enable ssh
```

Disable from boot:

```bash
sudo systemctl disable ssh
```

## Listing services

List all loaded services:

```bash
systemctl list-units --type=service
systemctl list-units --type=service --state=running
systemctl list-units --type=service --state=failed
```

## Unit files

Located in:
- `/etc/systemd/system/`: local customizations
- `/usr/lib/systemd/system/`: package-provided units

Example unit file:

```ini
[Unit]
Description=My Service
After=network.target

[Service]
Type=simple
User=www-data
ExecStart=/usr/bin/myapp
Restart=on-failure

[Install]
WantedBy=multi-user.target
```

After editing:

```bash
sudo systemctl daemon-reload
sudo systemctl start myservice
```

## Common services

- `ssh`: remote access
- `apache2`/`nginx`: web servers
- `mysql`/`postgresql`: databases
- `docker`: container runtime
- `cron`: scheduled tasks
- `syslog`: system logging

## Troubleshooting

Check logs:

```bash
journalctl -u ssh
journalctl -u ssh -n 50     # last 50 lines
journalctl -u ssh -f        # follow in real-time
```

Reload config after editing service files:

```bash
sudo systemctl daemon-reload
```

Services set to start on boot if `[Install]` section has `WantedBy=multi-user.target`.