What even is Linux?
You probably used Linux today without knowing it. It runs most websites, every Android phone, smart TVs, cars, and all 500 of the world's fastest supercomputers. It even flew on Mars, inside NASA's Ingenuity helicopter.
You will learn
- The difference between the Linux kernel and a Linux distro
- The two big families: Rocky/RHEL and Ubuntu/Debian
- How to find out which Linux you're on, using one command
Kernel vs. distro: the car analogy
The Linux kernel is the engine. It's the core program that talks to the hardware: the CPU, memory, disk and network. You never really see it, but nothing runs without it.
An engine alone isn't a car, though. You also need seats, a steering wheel, a dashboard and a radio. A distribution (or distro) is the whole car: the kernel plus all the tools, apps, settings and an installer, packaged together by someone.
Different groups build different “cars” around the same engine. That's why there are hundreds of distros, like Ubuntu, Rocky, Fedora, Mint and Kali. They're all Linux.
Linux was started in 1991 by a 21-year-old Finnish student named Linus Torvalds, as a hobby. His first announcement said it was “just a hobby, won't be big and professional.” Today it runs most of the internet.
The two big families
Most distros belong to one of two families. Distros in the same family are like cousins: they install software the same way and keep files in the same places. If you learn one Rocky-family distro, you can use all of them.
| Rocky / RHEL family | Ubuntu / Debian family | |
|---|---|---|
| Famous members | Red Hat Enterprise Linux (RHEL), Rocky Linux, AlmaLinux, CentOS Stream, Fedora | Debian, Ubuntu, Linux Mint, Raspberry Pi OS, Kali Linux |
| Package files | .rpm | .deb |
| Install software with | dnf (older name: yum) | apt (older: apt-get) |
| Who's behind it | Red Hat (owned by IBM) makes RHEL. Rocky and Alma are free, community-built copies of it. | Debian is run by volunteers. Ubuntu is built on Debian by a company called Canonical. |
| Where you'll see it | Big companies, banks, hospitals, government, universities | Cloud servers, startups, home labs, Raspberry Pi, laptops |
Older tutorials often say “CentOS.” CentOS Linux used to be the free copy of RHEL, but it was discontinued in 2024. Rocky Linux and AlmaLinux took its place. Anything written for CentOS 7/8 mostly works on Rocky.
Why learn both?
About 90% of what you'll learn works exactly the same on both: moving around, working with files, SSH, and running services. The other 10% (installing software, some file names, firewall tools) is different, and those differences are what confuse people when they follow a tutorial written for the wrong family. Knowing both makes you twice as useful.
Throughout this site you'll see three kinds of boxes:
sudo dnf install tree
sudo apt install tree
ls -la
Use the Show switch in the top bar to see both families, or hide the one you don't use.
Which Linux am I on?
Every modern distro has a small text file called /etc/os-release that says what it is. The cat command prints a file to the screen:
cat /etc/os-release
Look for the ID_LIKE line. It tells you the family: Rocky says rhel centos fedora and Ubuntu says debian.
Try it
Below is a pretend Linux server running in your browser. Click inside it and type. Use the Rocky / Ubuntu buttons in its title bar to switch to a machine from the other family.
Quick check
1. Which distro is in the same family as Rocky Linux?
✓ Right! Rocky and AlmaLinux are both free rebuilds of Red Hat Enterprise Linux. Ubuntu and Mint are in the Debian family.
2. What's the difference between the kernel and a distro?
✓ Yes. The kernel is the engine and the distro is the whole car.