added apt doc
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Linux/LPIC1/1-runlevels.md
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Linux/LPIC1/1-runlevels.md
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# 🐧 Linux Runlevels Guide
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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.
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---
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## 🔄 System Boot Sequence
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```plaintext
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BIOS → Bootloader → Kernel → init
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```
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* **BIOS**: Performs hardware checks via **POST** (Power-On Self Test).
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* **Bootloader**: Loads the kernel.
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* **Kernel**: Initializes system and mounts the root filesystem.
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* **init**: Launches system processes based on the selected runlevel.
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---
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## 📊 Runlevels Comparison
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| Runlevel | Description | Red Hat | Debian |
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| -------- | ----------------------------------- | ---------------- | --------------- |
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| 0 | Halt / Shutdown | ✅ Supported | ✅ Supported |
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| 1 | Single-User Mode | ✅ Supported | ✅ Supported |
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| 2 | Multi-User (No Network) | ❌ (Includes Net) | ✅ Supported |
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| 3 | Multi-User (Network, No GUI) | ✅ Supported | ✅ Supported |
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| 4 | User-Defined / Custom | ✅ Supported | ✅ Supported |
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| 5 | GUI Mode / *(Halt on some systems)* | ✅ GUI Mode | ⚠️ Custom/Halt? |
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| 6 | Reboot | ✅ Supported | ✅ Supported |
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> 💡 **Notes**:
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> • **Runlevel 5** on Red Hat typically launches a full graphical environment (GUI).
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> • On Debian, runlevels 2–5 are often configured identically and can be customized.
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> • Runlevel behavior is configurable via `/etc/inittab` (SysVinit systems).
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---
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## 🔧 Useful Commands
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### ✅ Check Current Runlevel
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```bash
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runlevel
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```
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### 🔁 Change Runlevel
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```bash
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telinit <runlevel>
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```
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or
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```bash
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init <runlevel>
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```
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> ⚠️ **Caution**: Switching runlevels may stop services or terminate user sessions. Use carefully on production systems.
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---
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## 🚀 Modern Systems: systemd Targets
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Most modern Linux distributions use **systemd**, which replaces runlevels with **targets**.
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| Runlevel | systemd Target |
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| -------- | ------------------- |
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| 0 | `poweroff.target` |
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| 1 | `rescue.target` |
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| 3 | `multi-user.target` |
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| 5 | `graphical.target` |
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| 6 | `reboot.target` |
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### 📌 Common systemd Commands
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```bash
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# Show default target
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systemctl get-default
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# Change to graphical mode
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systemctl isolate graphical.target
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```
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Linux/LPIC1/3-package-managers.md
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Linux/LPIC1/3-package-managers.md
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# 📦 Linux Libraries & Package Management
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This guide provides a quick overview of **library types**, **package sources**, and **Debian-based package management** commands using `apt`.
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---
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## 📚 Library Types
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Linux supports two main types of libraries:
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1. **Static Libraries (`.a`)**
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- Linked at **compile-time**
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- Included in the final binary
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- Larger file size, faster execution
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2. **Shared Libraries (`.so`)**
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- Linked at **run-time**
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- Saved separately from the binary
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- Saves space and allows updates without recompiling
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---
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## 📦 Package Sources
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### ✅ Official Packages
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Provided and maintained by the distribution (e.g., Debian, Ubuntu, Red Hat).
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### 🌐 Third-Party Packages
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Created by external developers or organizations. Use with caution and verify trustworthiness.
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---
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## 🏬 Linux Package Managers
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### 📦 Debian-based Systems
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- **Package Manager**: `apt`
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- **Low-level Tool**: `dpkg`
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- **Package Format**: `.deb`
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### 📦 Red Hat-based Systems
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- **Package Manager**: `yum` or `dnf` (newer)
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- **Low-level Tool**: `rpm`
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- **Package Format**: `.rpm`
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---
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## 🌍 Package Mirrors
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Mirrors are alternative download sources for package repositories, often closer geographically for faster updates.
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To sync with package mirrors and get the latest updates:
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```bash
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sudo apt update
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```
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---
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## 🔧 Common `apt` Commands
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| Command | Description |
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|--------------------------|---------------------------------------|
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| `apt update` | Refresh package lists |
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| `apt upgrade` | Upgrade all installed packages |
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| `apt list` | List packages |
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| `apt show <package>` | Show details of a package |
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| `apt install <package>` | Install a new package |
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| `apt reinstall <package>`| Reinstall an existing package |
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| `apt remove <package>` | Remove a package (keep config files) |
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| `apt purge <package>` | Remove package **and** config files |
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| `apt-cache search <term>`| Search for a package |
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| `apt autoremove` | Remove unused dependencies |
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> 🔐 Use `sudo` before these commands when required.
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@@ -1,59 +0,0 @@
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# Linux Runlevels Guide
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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.
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---
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## 📊 Runlevels Overview
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| Runlevel | Description | Red Hat | Debian |
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| -------- | ------------------------------------ | ------------ | --------------- |
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| 0 | Halt | ✅ Supported | ✅ Supported |
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| 1 | Single User Mode | ✅ Supported | ✅ Supported |
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| 2 | Multi-user (No Network) | ❌ (HaveNet) | ✅ Supported |
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| 3 | Multi-user (Network, No GUI) | ✅ Supported | ✅ Supported |
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| 4 | Custom/User Defined | ✅ Supported | ✅ Supported |
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| 5 | Multi-user (GUI Mode) / Halt *(err)* | ❌ (GUI Mode) | ❌ (Halt/Error?) |
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| 6 | Reboot | ✅ Supported | ✅ Supported |
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> 💡 **Note**:
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>
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> * On **Red Hat**, runlevel 5 typically starts the system with a graphical user interface (GUI).
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> * On **Debian**, runlevel 5 is often unused or reserved for custom setups.
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> * Runlevel behavior can be customized depending on system configuration.
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---
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## 🔧 Commands
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### Check Current Runlevel
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```bash
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runlevel
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```
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### Change Runlevel
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```bash
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telinit <runlevel>
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```
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or
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```bash
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init <runlevel>
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```
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> ⚠️ Changing runlevels may stop or restart services. Use with caution, especially on production systems.
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---
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## 📚 Additional Tips
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* Modern Linux systems (especially systemd-based) may not rely on traditional runlevels. Instead, they use *targets*. Example:
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```bash
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systemctl get-default
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systemctl isolate graphical.target
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```
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