What Linux is

Linux is the kernel at the center of many operating systems. It manages processes, memory, devices, filesystems, and communication with hardware.

Started by Linus Torvalds in 1991, it is open source and developed by volunteers and companies together, with no single vendor controlling it. It powers most of the servers on the internet, and Android runs on a modified version of it.

Kernel versus distribution

A kernel alone is not something you can install and use. It has no shell, no editor, no way to install software.

A distribution combines that kernel with system tools, libraries, a package manager, and a maintained software collection. Debian, Fedora, and Ubuntu are three well-known distributions, and hundreds more exist. They all run the Linux kernel; they differ in the software wrapped around it and in how it is maintained.

Ubuntu is one Linux distribution. This course uses Ubuntu because it is common on servers and has extensive documentation.

Checking what you are running

On any Linux machine, ask the kernel to identify itself:

uname -s
# Linux

Then ask which distribution wraps it:

grep PRETTY_NAME /etc/os-release
# PRETTY_NAME="Ubuntu 24.04.3 LTS"

Two answers, two layers: the kernel name is the same everywhere, the distribution name tells you which tooling to expect.

Why the distinction matters

Instructions are usually distribution-specific, not “Linux-specific”. Ubuntu and Debian install software with apt. Fedora uses dnf. Follow a Fedora tutorial on an Ubuntu server and the first install command fails:

dnf install nginx
# Command 'dnf' not found

Nothing is broken. You are reading instructions for a different distribution. When a guide’s package commands fail this way, check /etc/os-release and find the equivalent instructions for your distribution — on Ubuntu, that usually means swapping in apt.