added apt doc

Linux/LPIC1/3-package-managers.md

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+# 📦 Linux Libraries & Package Management
+
+This guide provides a quick overview of **library types**, **package sources**, and **Debian-based package management** commands using `apt`.
+
+---
+
+## 📚 Library Types
+
+Linux supports two main types of libraries:
+
+1. **Static Libraries (`.a`)**  
+   - Linked at **compile-time**  
+   - Included in the final binary  
+   - Larger file size, faster execution
+
+2. **Shared Libraries (`.so`)**  
+   - Linked at **run-time**  
+   - Saved separately from the binary  
+   - Saves space and allows updates without recompiling
+
+---
+
+## 📦 Package Sources
+
+### ✅ Official Packages  
+Provided and maintained by the distribution (e.g., Debian, Ubuntu, Red Hat).
+
+### 🌐 Third-Party Packages  
+Created by external developers or organizations. Use with caution and verify trustworthiness.
+
+---
+
+## 🏬 Linux Package Managers
+
+### 📦 Debian-based Systems
+- **Package Manager**: `apt`
+- **Low-level Tool**: `dpkg`
+- **Package Format**: `.deb`
+
+### 📦 Red Hat-based Systems
+- **Package Manager**: `yum` or `dnf` (newer)
+- **Low-level Tool**: `rpm`
+- **Package Format**: `.rpm`
+
+---
+
+## 🌍 Package Mirrors
+
+Mirrors are alternative download sources for package repositories, often closer geographically for faster updates.
+
+To sync with package mirrors and get the latest updates:
+
+```bash
+sudo apt update
+```
+
+---
+
+## 🔧 Common `apt` Commands
+
+| Command                  | Description                           |
+|--------------------------|---------------------------------------|
+| `apt update`             | Refresh package lists                 |
+| `apt upgrade`            | Upgrade all installed packages        |
+| `apt list`               | List packages                         |
+| `apt show <package>`     | Show details of a package             |
+| `apt install <package>`  | Install a new package                 |
+| `apt reinstall <package>`| Reinstall an existing package         |
+| `apt remove <package>`   | Remove a package (keep config files)  |
+| `apt purge <package>`    | Remove package **and** config files   |
+| `apt-cache search <term>`| Search for a package                  |
+| `apt autoremove`         | Remove unused dependencies            |
+
+> 🔐 Use `sudo` before these commands when required.
+