## 🗂️ Directory Layout

```
mini-systemd/
├── services/
│   ├── web.service.json
│   └── db.service.json
└── systemd.js
```

---

### 📄 Example `web.service.json`

```json
{
  "name": "web",
  "description": "Simple web server",
  "execStart": "node -e \"require('http').createServer((_,res)=>res.end('ok')).listen(3000)\"",
  "after": ["db"]
}
```

### 📄 Example `db.service.json`

```json
{
  "name": "db",
  "description": "Mock DB",
  "execStart": "sleep 1000"
}
```

---

### 🧠 `systemd.js`: Lightweight Service Manager

```js
const fs = require('fs');
const { spawn } = require('child_process');
const path = require('path');

const SERVICES_DIR = path.join(__dirname, 'services');
let services = {};
let running = {};

function loadServices() {
    const files = fs.readdirSync(SERVICES_DIR).filter(f => f.endsWith('.json'));
    for (const file of files) {
        const fullPath = path.join(SERVICES_DIR, file);
        const config = JSON.parse(fs.readFileSync(fullPath, 'utf-8'));
        services[config.name] = config;
    }
}

async function startService(name) {
    if (running[name]) return;

    const service = services[name];
    if (!service) {
        console.error(`[!] Service "${name}" not found.`);
        return;
    }

    // Handle dependencies
    if (service.after && service.after.length > 0) {
        for (const dep of service.after) {
            await startService(dep);
        }
    }

    console.log(`[+] Starting service: ${name}`);
    const parts = service.execStart.split(' ');
    const proc = spawn(parts[0], parts.slice(1), { stdio: 'inherit', shell: true });

    running[name] = proc;

    proc.on('exit', (code) => {
        console.log(`[-] Service "${name}" exited with code ${code}`);
        delete running[name];
    });
}

async function stopService(name) {
    const proc = running[name];
    if (!proc) {
        console.log(`[i] Service "${name}" is not running.`);
        return;
    }
    proc.kill('SIGTERM');
    console.log(`[x] Stopped service: ${name}`);
    delete running[name];
}

async function main() {
    loadServices();
    for (const name in services) {
        await startService(name);
    }
}

main();
```

---

## 🧪 Run it

```bash
node systemd.js
```

It will:

* Load the services from JSON.
* Start them in order of dependencies.
* Run them in the foreground.
* Stop them if terminated.

---

## ✅ You Learn

* How services are defined and orchestrated.
* Process management via `spawn`.
* Dependency resolution (`after`).
* Basic logging and lifecycle tracking.