🌐 1. What Is Linux?

Linux is an open‑source operating system kernel that powers countless distributions—Windows‑like environments for desktops, servers, embedded devices, and more. It manages hardware (CPU, memory, storage, network) and provides interfaces for software applications.

Key Point: An OS (Operating System) is the bridge between hardware and software. Linux’s modular design allows you to choose the components you need.

🕰️ 2. A Brief History

1970s: UNIX by Thompson & Ritchie at AT&T lays the foundation.
1983: GNU Project launches, pioneering free software and the GPL license.
1991: Linus Torvalds releases the first Linux kernel as a personal project.
Today: Over 600 distributions exist (Ubuntu, Fedora, Debian, Parrot, RedHat, Mint, Manjaro, Gentoo, etc.).

Why It Matters: Community‑driven development ensures rapid fixes, frequent updates, and strong security.

🧩 3. Linux Philosophy

Modularity: Small programs that do one job well.
Everything Is a File: Configuration, devices, processes—accessible as files.
Chain Commands: Combine simple tools (via pipes) to perform complex tasks.
Open & Transparent: Source code is freely available and auditable.

Example:

# List all open network ports, filter for listening

sudo ss -tuln | grep LISTEN

⚙️ 4. Core Components

Component	Role
Bootloader	GRUB or LILO: loads the kernel at startup
Kernel	Core engine: manages CPU scheduling, memory, I/O
Init System	systemd, SysV: starts services and manages runlevels
Shell	Bash, Zsh, Fish: command‑line interface
Daemons	Background services (e.g., cron, sshd, httpd)
Utilities	Tools (grep, awk, sed) and coreutils (cp, mv, rm, etc.)

🏛️ 5. Architecture Layers

[Hardware] <--> [Kernel] <--> [System Libraries] <--> [Shell & User Apps]

Hardware: CPU, RAM, disks, network adapters
Kernel: Interface to hardware and core services
Libraries: Shared code (glibc, libssl)
Shell & Apps: User programs, scripts, GUIs

📂 6. Filesystem Hierarchy

The Filesystem Hierarchy Standard (FHS) defines directories:

/ : Root of the filesystem
/bin : Essential command binaries (ls, cat, cp)
/boot : Kernel images and bootloader files
/dev : Device nodes (disks, terminals)
/etc : System configuration files
/home : User home directories
/lib : Shared libraries for low-level programs
/media : Mount points for removable media
/opt : Optional third-party software
/proc : Virtual filesystem: kernel & process info
/usr : Secondary hierarchy (apps, libraries)
/var : Variable data (logs, mail, cache)

Tip: Memorize five main dirs: /, /bin, /etc, /home, /var as a starting point.

🔍 7. Next Steps

Explore shell basics: file navigation, editing, permissions.
Practice system monitoring: top, htop, journalctl.
Dive into network tools: ping, traceroute, netstat, ss.

Ready to begin? Let’s move into interactive exercises and real‑world scenarios.

This course module is part of CyberSec Academy’s Pentest Execution track—designed for both beginners and seasoned professionals under NDA.

The place to learn about the latest cybersecurity trends and best practices.

Linux Structure

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