disk info doc

Linux/Basic Administration/01-runlevels.md

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+# 🐧 Linux Runlevels Guide
+
+This guide provides a concise overview of traditional **runlevels** in Linux systems, particularly for **Red Hat** and **Debian**-based distributions. Runlevels define specific states of system operation, historically managed by the `init` system.
+
+---
+
+## 🔄 System Boot Sequence
+
+```plaintext
+BIOS → Bootloader → Kernel → init
+```
+
+* **BIOS**: Performs hardware checks via **POST** (Power-On Self Test).
+* **Bootloader**: Loads the kernel.
+* **Kernel**: Initializes system and mounts the root filesystem.
+* **init**: Launches system processes based on the selected runlevel.
+
+---
+
+## 📊 Runlevels Comparison
+
+| Runlevel | Description                         | Red Hat          | Debian          |
+| -------- | ----------------------------------- | ---------------- | --------------- |
+| 0        | Halt / Shutdown                     | ✅ Supported      | ✅ Supported     |
+| 1        | Single-User Mode                    | ✅ Supported      | ✅ Supported     |
+| 2        | Multi-User (No Network)             | ❌ (Includes Net) | ✅ Supported     |
+| 3        | Multi-User (Network, No GUI)        | ✅ Supported      | ✅ Supported     |
+| 4        | User-Defined / Custom               | ✅ Supported      | ✅ Supported     |
+| 5        | GUI Mode / *(Halt on some systems)* | ✅ GUI Mode       | ⚠️ Custom/Halt? |
+| 6        | Reboot                              | ✅ Supported      | ✅ Supported     |
+
+> 💡 **Notes**:
+> • **Runlevel 5** on Red Hat typically launches a full graphical environment (GUI).
+> • On Debian, runlevels 2–5 are often configured identically and can be customized.
+> • Runlevel behavior is configurable via `/etc/inittab` (SysVinit systems).
+
+---
+
+## 🔧 Useful Commands
+
+### ✅ Check Current Runlevel
+
+```bash
+runlevel
+```
+
+### 🔁 Change Runlevel
+
+```bash
+telinit <runlevel>
+```
+
+or
+
+```bash
+init <runlevel>
+```
+
+> ⚠️ **Caution**: Switching runlevels may stop services or terminate user sessions. Use carefully on production systems.
+
+---
+
+## 🚀 Modern Systems: systemd Targets
+
+Most modern Linux distributions use **systemd**, which replaces runlevels with **targets**.
+
+| Runlevel | systemd Target      |
+| -------- | ------------------- |
+| 0        | `poweroff.target`   |
+| 1        | `rescue.target`     |
+| 3        | `multi-user.target` |
+| 5        | `graphical.target`  |
+| 6        | `reboot.target`     |
+
+### 📌 Common systemd Commands
+
+```bash
+# Show default target
+systemctl get-default
+
+# Change to graphical mode
+systemctl isolate graphical.target
+```
+