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Linux/LPIC1/runlevels.md

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+# Linux Runlevels Guide
+
+This document outlines the standard runlevels for **Red Hat** and **Debian**-based systems. Runlevels are used by `init` systems to define different states or modes of operation for the system.
+
+---
+
+## 📊 Runlevels Overview
+
+| Runlevel | Description                          | Red Hat      | Debian          |
+| -------- | ------------------------------------ | ------------ | --------------- |
+| 0        | Halt                                 | ✅ Supported  | ✅ Supported     |
+| 1        | Single User Mode                     | ✅ Supported  | ✅ Supported     |
+| 2        | Multi-user (No Network)              | ❌ (HaveNet)  | ✅ Supported     |
+| 3        | Multi-user (Network, No GUI)         | ✅ Supported  | ✅ Supported     |
+| 4        | Custom/User Defined                  | ✅ Supported  | ✅ Supported     |
+| 5        | Multi-user (GUI Mode) / Halt *(err)* | ❌ (GUI Mode) | ❌ (Halt/Error?) |
+| 6        | Reboot                               | ✅ Supported  | ✅ Supported     |
+
+> 💡 **Note**:
+>
+> * On **Red Hat**, runlevel 5 typically starts the system with a graphical user interface (GUI).
+> * On **Debian**, runlevel 5 is often unused or reserved for custom setups.
+> * Runlevel behavior can be customized depending on system configuration.
+
+---
+
+## 🔧 Commands
+
+### Check Current Runlevel
+
+```bash
+runlevel
+```
+
+### Change Runlevel
+
+```bash
+telinit <runlevel>
+```
+
+or
+
+```bash
+init <runlevel>
+```
+
+> ⚠️ Changing runlevels may stop or restart services. Use with caution, especially on production systems.
+
+---
+
+## 📚 Additional Tips
+
+* Modern Linux systems (especially systemd-based) may not rely on traditional runlevels. Instead, they use *targets*. Example:
+
+  ```bash
+  systemctl get-default
+  systemctl isolate graphical.target
+  ```
+