# Debian Configuration Files

Most system configuration lives in `/etc`. Understanding common formats and safety practices is essential.

## File Locations

- `/etc/` — system config
- `~/.config/` — per-user config (modern apps)
- `~/.bashrc`, `~/.profile` — shell config

## Common Formats

**Key=value:**

```ini
DEBUG=true
MAX_CONNECTIONS=100
```

**INI-style sections:**

```ini
[database]
host=localhost
port=5432

[logging]
level=info
```

**YAML (indentation matters):**

```yaml
database:
  host: localhost
  port: 5432
logging:
  level: info
```

**JSON:**

```json
{
  "database": {
    "host": "localhost",
    "port": 5432
  }
}
```

## Editing Safely

Always backup before editing:

```bash
sudo cp /etc/nginx/nginx.conf /etc/nginx/nginx.conf.bak
sudo nano /etc/nginx/nginx.conf
```

Test configuration (if tool supports it):

```bash
sudo nginx -t
sudo apachectl configtest
```

## Applying Changes

Many services auto-reload on file change. Always verify:

```bash
sudo systemctl restart nginx
```

Check status after restart:

```bash
sudo systemctl status nginx
```

## Viewing Changes

Compare original and backup:

```bash
diff /etc/file /etc/file.bak
```

Show what changed:

```bash
sudo git diff /etc/file
```

## Version Control

On production servers, version control `/etc`:

```bash
cd /etc
sudo git init
sudo git config user.name "Admin"
sudo git config user.email "admin@example.com"
sudo git add -A
sudo git commit -m "Initial config"
```

Now track all changes:

```bash
sudo git diff                # uncommitted changes
sudo git log --oneline      # history
```

## Common Files

- `/etc/hostname` — system name
- `/etc/hosts` — local DNS mappings
- `/etc/resolv.conf` — DNS servers
- `/etc/fstab` — mount points
- `/etc/sudoers` — sudo permissions (edit with `visudo`)
- `/etc/ssh/sshd_config` — SSH server config
- `/etc/cron.d/` — system cron jobs

Never edit by hand:
- `/etc/sudoers` — use `visudo`
- `/etc/shadow` — use `passwd`, `usermod`
- `/etc/group` — use `groupadd`, `groupmod`

## Comments

Comments start with `#`. Some formats support `;` instead. Remove old configs when possible rather than commenting them out.