From Linus Torvalds' university project to powering 90% of the world's servers — understand what Linux is, why it matters, how the kernel and shell interact, and how to navigate the command-line interface and file system hierarchy.
Slide 1 — What Is Linux?
A Free, Open-Source, Unix-Like Operating System
Linux is an operating system kernel — the core software that manages hardware resources and provides services to user applications. Unlike Windows or macOS, Linux is open-source: its source code is freely available, modifiable, and distributable under the GNU General Public License (GPL).
More precisely, what most people call "Linux" is actually a complete operating system combining the Linux kernel (written by Linus Torvalds) with a large collection of GNU tools and utilities (created by Richard Stallman's GNU Project). The full name "GNU/Linux" reflects this collaboration.
Linux is not a single product — it is a family of operating systems called distributions (distros), each packaging the kernel with different software stacks, package managers, and desktop environments for different audiences and use cases.
96%Of the top 1 million web servers run Linux
100%Of the world's top 500 supercomputers run Linux
1991Year Linus Torvalds released the first Linux kernel (v0.01)
Slide 2 — History of Linux
1969 — Bell Labs
Unix Born
Ken Thompson and Dennis Ritchie create Unix at Bell Labs. Unix introduces foundational concepts: everything is a file, small tools that do one thing well, pipes, hierarchical file system. Linux inherits this entire philosophy.
1983 — GNU Project
Richard Stallman Launches Free Software Movement
Stallman announces the GNU Project — an effort to create a free Unix-compatible OS. GNU produces essential tools (gcc, bash, glibc, coreutils) but lacks a working kernel. The kernel gap sets the stage for Linux.
1991 — Helsinki
Linus Torvalds Posts "Just a Hobby" Announcement
21-year-old Linus Torvalds posts to the comp.os.minix newsgroup: "I'm doing a (free) operating system (just a hobby, won't be big and professional like GNU) for 386(486) AT clones." He releases Linux 0.01 — 10,239 lines of code. The kernel pairs with GNU tools to create a complete OS.
1993–2000 — Distribution Era
Slackware, Debian, Red Hat, SUSE Emerge
The first major distributions are born. Debian establishes the concept of package management and a community governance model. Red Hat commercializes Linux for enterprise — proving open source can be a business model. Linux adoption explodes on web servers.
2004 — Ubuntu
Linux Becomes User-Friendly
Mark Shuttleworth founds Canonical and releases Ubuntu — a Debian-based distro focused on desktop usability. "Linux for human beings" drives adoption beyond servers to desktops, students, and developers worldwide.
2007–Present — Everywhere
Android, Cloud, IoT, Supercomputers
Android (Linux-based) becomes the world's most-used OS. AWS, Google Cloud, and Azure run Linux at massive scale. Every Raspberry Pi, most IoT devices, and all 500 of the world's fastest supercomputers run Linux. The "hobby" project runs the modern world.
Slide 3 — Why Use Linux?
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Free and Open Source — No licensing fees. Source code is available to audit, modify, and redistribute. Critical for security professionals who need to verify what the OS is actually doing.
Security-First Design — Unix permission model, user/group separation, mandatory access controls (SELinux, AppArmor), and a vast security tool ecosystem. Linux is the platform of choice for security professionals.
Highly Customizable — Choose your kernel, desktop environment, init system, and every component. Lightweight enough to run on a $35 Raspberry Pi or powerful enough for a supercomputer cluster.
Developer's Platform — Native support for Python, C, Rust, Go, and all major languages. Package managers provide instant access to development tools. Servers run Linux — develop on what you deploy to.
WHY LINUX FOR SECURITY WORK
Every major penetration testing platform (Kali, Parrot, BlackArch) runs Linux. Security tools like nmap, Metasploit, Wireshark, and Burp Suite are native Linux applications. Understanding Linux deeply is not optional for cybersecurity professionals — it is foundational. Your target environments are Linux servers. Your attack tools run on Linux. Your forensic analysis happens on Linux. Learn it like a native language.
Slide 4 — The Linux Kernel
The Core of the Operating System
The kernel is the lowest-level software layer — the bridge between hardware and user applications. It runs in privileged "kernel space" and manages four core responsibilities:
CPU
Process Scheduling — Decides which process runs on which CPU core for how long. The scheduler balances fairness, responsiveness, and throughput across all running processes.
RAM
Memory Management — Allocates and deallocates memory for processes, implements virtual memory, manages the swap space, and enforces memory isolation between processes (preventing process A from reading process B's memory).
I/O
Device Drivers — Interfaces with hardware through loadable kernel modules. Everything from your NIC to your USB device to your GPU requires a kernel driver. "Everything is a file" — even hardware devices appear in /dev/.
FS
File System Management — Manages reading/writing to disk, filesystem formats (ext4, btrfs, xfs, tmpfs), VFS (Virtual File System) layer that provides a uniform interface regardless of the underlying filesystem type.
Slide 5 — The Shell and CLI
Your Interface to the Operating System
The shell is a command interpreter that reads your input, parses it, and executes programs. It sits between you and the kernel. When you type a command, the shell: finds the program in your PATH, forks a child process, executes the program, and waits for it to finish.
The most common shell is Bash (Bourne Again SHell) — the default on most Linux distributions. Other popular shells: Zsh (macOS default), Fish (user-friendly), Dash (minimal, used for scripts), and the original Bourne Shell (sh).
# The shell search path — where commands are found
$ echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
# The shell processes commands from left to right
$ command-options argument1 argument2
# Example: ls with options and argument
$ ls-la /home/student
# ls = the command (program to execute)# -la = options (l=long format, a=show hidden files)# /home/student = argument (directory to list)# Shell variables
$ HOME=/home/student # your home directory
$ USER=student # your username
$ SHELL=/bin/bash # current shell
Slide 6 — Essential CLI Commands
ls
List directory contents. ls -l long format, ls -a show hidden, ls -lh human-readable sizes.
cd
Change directory. cd /etc absolute path, cd .. parent dir, cd ~ home directory, cd - previous directory.
pwd
Print Working Directory — shows your current location in the filesystem. Always know where you are before running commands.
mkdir
Make Directory. mkdir -p a/b/c creates nested directories in one command.
rm
Remove files. rm -r recursive (directories), rm -f force. WARNING: no recycle bin — deletion is permanent.
Concatenate and display file contents. cat file.txt. Also used to create small files with cat > file.txt.
Slide 7 — Linux File System Hierarchy (FHS)
Everything Starts at / (Root)
Unlike Windows with drive letters (C:, D:), Linux has a single unified file system tree rooted at /. Every file, directory, device, and network resource appears as a node in this tree. The Filesystem Hierarchy Standard (FHS) defines where things go.
/Root of the entire filesystem
├── bin/Essential user binaries (ls, cp, mv, bash)
An absolute path starts from root: /home/student/scripts/backup.sh — it works from anywhere. A relative path starts from your current directory: scripts/backup.sh — only works if you're in /home/student/. The special directories . (current dir) and .. (parent dir) are always available. Security note: paths starting with . are hidden from ls without the -a flag — attackers use this to hide malicious files.
Slide 8 — Linux File Permissions
The Unix Permission Model
Every file and directory has three permission sets: user (u) — the owner, group (g) — a group of users, and other (o) — everyone else. Each set has three bits: read (r=4), write (w=2), execute (x=1).
$ ls -l /etc/passwd
-rw-r--r-- 1 root root 2847 Jan 15 09:22 /etc/passwd
↑ file type (-=file, d=dir, l=symlink) ↑↑↑ user perms (rw- = read+write, no execute) ↑↑↑ group perms (r-- = read only) ↑↑↑ other perms (r-- = read only)# Change permissions with chmod
$ chmod 755 script.sh # rwxr-xr-x (owner full, others r+x)
$ chmod 600 secret.key # rw------- (owner read+write only)
$ chmod +x script.sh # Add execute for all# Change ownership with chown
$ chown alice:devteam file.txt # Set owner:group
$ sudo chown root:root /etc/hosts
Octal
Symbolic
Meaning
Typical Use
755
rwxr-xr-x
Owner: all; Group/Other: read+execute
Executables, web directories
644
rw-r--r--
Owner: read+write; Group/Other: read
Config files, web content
600
rw-------
Owner: read+write; no others
SSH private keys, secrets
700
rwx------
Owner: all; no others
Private scripts, user dirs
777
rwxrwxrwx
Everyone: all permissions
Avoid — security risk
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