> ## Documentation Index
> Fetch the complete documentation index at: https://docs.americ.io.vn/llms.txt
> Use this file to discover all available pages before exploring further.

# systemctl

> `systemctl` Cheat Sheet — Ubuntu/Debian Developer Reference

## 1. Managing & Listing Services

```bash theme={null}
# Check status of a service
systemctl status nginx
systemctl status postgresql

# Start / Stop / Restart
sudo systemctl start nginx
sudo systemctl stop nginx
sudo systemctl restart nginx

# Reload config WITHOUT downtime (if the service supports it)
sudo systemctl reload nginx

# Restart only if the service is already running
sudo systemctl try-restart nginx
```

> 💡 Use `reload` over `restart` for nginx/postgres in production — it avoids dropping connections.

```bash theme={null}
# List ALL services (all states)
systemctl list-units --type=service

# List only RUNNING services
systemctl list-units --type=service --state=running

# List only FAILED services
systemctl list-units --type=service --state=failed

# List ENABLED services (start on boot)
systemctl list-unit-files --type=service --state=enabled

# Suppress the pager (useful for scripting / piping)
systemctl list-units --type=service --no-pager

# Filter by keyword (e.g. find anything docker-related)
systemctl list-units --type=service | grep docker

# Group filter using glob pattern (e.g. all snap services)
systemctl list-units 'snap.*' --type=service

# Show only the service name column (clean output)
systemctl list-units --type=service --no-legend | awk '{print $1}'
```

> 💡 **Reading the output columns:**
>
> | Column        | Meaning                                          |
> | ------------- | ------------------------------------------------ |
> | `UNIT`        | Service name (e.g. `nginx.service`)              |
> | `LOAD`        | Whether systemd parsed the unit file             |
> | `ACTIVE`      | High-level state: `active`, `inactive`, `failed` |
> | `SUB`         | Low-level state: `running`, `exited`, `dead`     |
> | `DESCRIPTION` | Human-readable label                             |

***

## 2. Debugging & Logs

```bash theme={null}
# View logs for a specific service
journalctl -u nginx

# Follow live logs (like tail -f)
journalctl -u nginx -f

# Show logs since last boot only
journalctl -u nginx -b

# Show last 50 lines
journalctl -u nginx -n 50

# Show logs within a time range
journalctl -u nginx --since "2024-01-01" --until "2024-01-02"

# Check why a service failed
systemctl status nginx
journalctl -xe   # Full system log with explanations
```

***

## 3. Boot & Startup Behavior

```bash theme={null}
# Enable a service to start automatically on boot
sudo systemctl enable nginx

# Disable autostart on boot
sudo systemctl disable nginx

# Check if a service is enabled
systemctl is-enabled nginx

# Check if a service is currently running
systemctl is-active nginx

# See ALL running services
systemctl list-units --type=service --state=running

# See FAILED services (great for debugging boot issues)
systemctl list-units --type=service --state=failed

# Reboot / shutdown via systemctl
sudo systemctl reboot
sudo systemctl poweroff
```

***

## 4. Writing a Custom `.service` File

**Step 1 — Create the file:**

```bash theme={null}
sudo nano /etc/systemd/system/myapp.service
```

**Step 2 — Paste this template:**

```ini theme={null}
[Unit]
Description=My Node.js App
After=network.target          # Start after network is ready

[Service]
Type=simple
User=ubuntu                   # Run as this user (not root!)
WorkingDirectory=/home/ubuntu/myapp
ExecStart=/usr/bin/node server.js
Restart=on-failure            # Auto-restart if it crashes
RestartSec=5                  # Wait 5s before restarting
Environment=NODE_ENV=production
StandardOutput=journal        # Logs go to journalctl
StandardError=journal

[Install]
WantedBy=multi-user.target    # Start at normal boot
```

**Step 3 — Activate it:**

```bash theme={null}
sudo systemctl daemon-reload   # Tell systemd about the new file
sudo systemctl enable myapp
sudo systemctl start myapp

# Verify
systemctl status myapp
journalctl -u myapp -f
```

***

## Quick Reference Summary

| Task                         | Command                                                 |
| ---------------------------- | ------------------------------------------------------- |
| Check service status         | `systemctl status <name>`                               |
| Start service                | `sudo systemctl start <name>`                           |
| Stop service                 | `sudo systemctl stop <name>`                            |
| Restart service              | `sudo systemctl restart <name>`                         |
| Enable on boot               | `sudo systemctl enable <name>`                          |
| Disable on boot              | `sudo systemctl disable <name>`                         |
| View logs live               | `journalctl -u <name> -f`                               |
| Reload after `.service` edit | `sudo systemctl daemon-reload`                          |
| List running services        | `systemctl list-units --type=service --state=running`   |
| List failed services         | `systemctl list-units --state=failed`                   |
| Filter services by name      | `systemctl list-units --type=service \| grep <keyword>` |