Where Linux comes from, who created it, why there are a thousand "distributions", how you install software and which certifications to take. The trade's general culture, in one page.
In 1991, a 21-year-old Finnish student posts a now-famous message: " I'm doing a (free) operating system (just a hobby, won't be big and professional) ". That hobby is the Linux kernel, today at the heart of most of the world's servers, of Android and of supercomputers.
He didn't just make a kernel: in 2005, to manage Linux's code, he created Git — the version control tool you learn in chapter 17. Two founding tools, one single man.
His mascot? Tux, the penguin 🐧, chosen because Linus likes penguins. The kernel is free: anyone can read it, modify it, redistribute it ( GPL).
Linux didn't come out of nowhere: it builds on twenty years of ideas. Three or four names to know, and the dates that changed everything.
They invented Unix, then the C language to write it. Linux's whole philosophy — small tools, "everything is a file", the shell — comes from them.
Father of free software: he launched GNU, wrote the GPL license and major tools (Emacs, GCC). The "free" in Linux is his fight.
He assembled the missing piece — the kernel — and released it freely. GNU + Linux = a complete system at last. Later, he created Git.
He wrote Bash, the shell you use in the Dojo — the default command interpreter of most distributions.
init.The kernel alone isn't enough to use a computer. A distribution ("distro") packages the kernel with tools, an installer, a desktop and a software manager. That's why there are hundreds of them.
People often say "GNU/Linux": Linus's kernel + the tools of Richard Stallman's GNU project (launched in 1983), which also gave the free license GPL. Together, they form a complete operating system.
👉 Good news for you: the Dojo's commands (ls, cd, chmod, grep…) work identically on every distro. Only the way to install software changes.
People think Linux is "for geeks". In reality, you use it every day without knowing it.
Almost every distro descends from 3-4 "ancestors". Knowing the family means knowing its package manager and its philosophy.
The most widespread. Stable, a huge software library. The gateway for the general public and for servers.
The corporate world. Commercial support, stability, in high demand on the job market.
The "do it yourself" one. Always up to date (rolling release), minimalist, for those who want to master everything.
Specialized distros you quickly run into as soon as you touch servers and containers.
The distro of ethical hackers and pentesters, published by OffSec. Based on Debian, it comes with 600+ security tools preinstalled (Nmap, Metasploit, Wireshark, Burp…). To be used in a legal, authorized setting only.
The best known to the general public. Published by Canonical, it brought Linux to the desktop and dominates servers and the cloud. The LTS (Long Term Support) version = 5 years of updates, ideal for a server you keep.
The reference in the enterprise. You mostly pay for the support and the certifications. Many "Linux admin" job offers mention RHEL. Its free clones Rocky and AlmaLinux are identical, without the support.
The stable "grandfather", 100% community-driven (since 1993). Nothing flashy, but legendary reliability — which is why so many other distros (Ubuntu included) are built on it.
On Linux, you don't install by downloading an .exe: you ask a package manager to fetch the software from an official library, with its dependencies. Same move everywhere, different command depending on the family.
| Action | apt (Debian/Ubuntu) | dnf (Fedora/RHEL) | pacman (Arch) | zypper (openSUSE) |
|---|---|---|---|---|
| Install package | apt install package | dnf install package | pacman -S package | zypper install package |
| Search | apt search word | dnf search word | pacman -Ss word | zypper search word |
| Update everything | apt update && apt upgrade | dnf upgrade | pacman -Syu | zypper update |
| Remove | apt remove package | dnf remove package | pacman -R package | zypper remove package |
➕ There are also universalformats, which work on (almost) every distro: Snap (Canonical), Flatpak and AppImage. Handy for desktop apps, a bit heavier. On the container side, Alpine uses apk, ultra-light.
When you open a site, your request almost always goes through middlemen before reaching the application: a proxy (from the Latin procurator, "one who acts on behalf of another"). There are two families, often confused — the difference boils down to one question: who does it represent?
It receives visitors in place of the application, hidden behind it. The visitor doesn't even know it exists.
It goes out to the Internet on behalf of a network's machines. The visited site sees the proxy's address, not yours.
Born from "patches" added to an older server (hence the joke: a patchy server), Apache httpd served most of the web for fifteen years. One process (or thread) per connection: simple and robust, but greedy when visitors number in the thousands.
How do you hold 10,000 simultaneous connections on a single machine? To meet this challenge, the Russian Igor Sysoev wrote nginx ("engine-x"): an event loop that handles thousands of clients with a few processes. It's now one of the most used web servers in the world.
Born from an American research project (Harvest), Squid has been the reference outgoing proxy for nearly 30 years. In the modem era, keeping a page in cache avoided downloading it again: a real saving of time and money.
Cloudflare, Akamai, Fastly are reverse proxies spread across the whole planet. A large share of the web goes through them: they serve pages from a server close to you and absorb DDoS attacks.
| Tool | Role | Why you pick it |
|---|---|---|
| nginx | reverse proxy + web server | Fast, light, everywhere. It's the one you configure in chapter 21 of the Dojo. |
| Apache httpd | web server (+ mod_proxy) | The historic one, very modular, still massively deployed. |
| HAProxy | reverse proxy / load balancer | Load-balancing specialist, very high performance. |
| Traefik · Caddy | reverse proxy modern | Automatic configuration, effortless HTTPS (Let's Encrypt). Ideal with Docker. |
| Envoy | reverse proxy cloud-native | At the heart of Kubernetes architectures and "service meshes". |
| Squid | forward proxy + cache | The reference outgoing proxy in companies and education. |
💡 The reflex to remember: a 502 Bad Gateway means the proxy works, but the application behind it doesn't answer. Now you know where to look.
Banks, hospitals, telecom operators, industry: when a big company runs Linux, it's very often the Red Hatfamily. Same kernel as on Ubuntu, same ls and cd — but other tools, another culture… and the certification recruiters ask for most.
Each new feature is first tested in Fedora, stabilized in CentOS Stream, then shipped in RHEL — for 10 years of updates.
No multiple choice: you're given machines and a list of tasks. A script then checks what really works.
--permanent, /etc/fstab, enableman and the installed docsAt university, Marc Ewing wore his grandfather's red cap. People with a computer problem went to see "the guy in the red hat". He made it the name of his distribution (1994), then of the company founded with Bob Young. In 2019, IBM bought it for 34 billion dollars, one of the biggest acquisitions in software history.
Designed by the US security agency and released as free software in 2000, SELinux sticks a label on every file and every program. Even root can't do what the policy hasn't planned. The code is public and reviewed by the whole community: that's precisely what makes it trustworthy.
CentOS was the free clone of RHEL, used everywhere. At the end of 2020, Red Hat announced the early end of CentOS 8 in favor of CentOS Stream, placed before RHEL. The community was furious… and within a few months came Rocky Linux (by a CentOS co-founder) andAlmaLinux .
Many tools of your Linux come from Red Hat: NetworkManager (2004), systemd (2010, by Lennart Poettering), podman (2018, containers without a root daemon). Even Ubuntu uses them. Learning Red Hat means learning modern Linux.
| Task | Debian / Ubuntu | Red Hat / Rocky |
|---|---|---|
| Install a package | apt install .deb | dnf install .rpm |
| Firewall | ufw | firewall-cmd (firewalld) |
| Enhanced access control | AppArmor (per-program profiles) | SELinux (labels on everything) |
| Admins group | sudo | wheel |
| Network | netplan (Ubuntu server) | nmcli (NetworkManager) |
| Installed service | starts by itself | stays stopped: systemctl enable --now |
| Containers | Docker | podman (same commands) |
| Apache web server | apache2 | httpd |
🎩 Want to practise? The Dojo's RHCSA page connects you to a simulated Red Hat server ([kai@rhcsa ~]$), with an "Exam readiness" gauge and graded labs on your own VM.
"Free" doesn't mean "worthless". Free = you can read it, modify it, share it (it's a freedom). And most distros are also free of charge. So how does everyone make a living? Simple: you don't sell the software — you sell the service around it.
Red Hat (IBM), Canonical, SUSE sell companies assistance, guarantees and long-term updates. The software is free; peace of mind isn't.
AWS, Google, Microsoft run millions of Linux servers and sell you computing power. Linux is the foundation of the whole cloud industry.
Google, Intel, Meta, Microsoft… pay developers to improve the kernel: they depend on it. Most of the kernel's code comes from company employees.
Certifications (Red Hat, LPI), training and donations to foundations (Linux Foundation, Debian) also fund the ecosystem.
🔁 The virtuous circle: everyone improves the same common good instead of reinventing the wheel in their own corner. Result: Linux often moves faster — and more solidly — than closed software.
Useful to land a Linux admin or security job. Some are neutral (all distros), others tied to a vendor. The most respected are 100% hands-on: you're put in front of a real server to repair.
| Certification | By | Level | For whom |
|---|---|---|---|
| CompTIA Linux+ | CompTIA | Beginner | First certification, neutral (all distros). A good starting point. |
| LPIC-1 → LPIC-3 | Linux Professional Institute | Beginner → Expert | A neutral, progressive path, recognized in Europe. |
| RHCSA then RHCE | Red Hat | Inter. → Advanced | The enterprise reference, 100% hands-on. In high demand on the market. Practise on the Dojo's 🎩 RHCSA page . |
| LFCS / LFCE | Linux Foundation | Inter. → Advanced | Hands-on, neutral, backed by the foundation that hosts the kernel. |
| OSCP | OffSec (Kali) | Advanced | The star of pentesting: 24 h to hack machines. Demanding and renowned. |
💡 A logical order: Linux+ or LPIC-1 for the basics → RHCSA for admin jobs → OSCP if you aim at offensive security. But the best CV remains your projects (the Dojo's Projects chapter 😉).
The penguin mascot, drawn by Larry Ewing in 1996. You see it when many Linux machines boot.
The license that guarantees a piece of software stays free: read, modify, share. The fuel of the whole ecosystem.
« Super User DO ": run a command with administrator rights. The famous sudo apt install….
« Read The Fine Manual ": before asking, read the manual. Hence man, the built-in docs you already know.
The " ~ " means your home folder. A famous geek T-shirt: " there's no place like ~ ».
The cult joke: Arch users love to let you know. A badge of pride (and self-mockery).
You know the setting — now take control of a real terminal. The Dojo's commands work on all these distros.
Enter the Dojo →