What is Linux?

"Linux" almost never refers to one single thing. Strictly, Linux is the kernel — the core program that talks to the hardware (CPU, memory, disks, network) and manages processes. What most people call "a Linux system" is that kernel bundled with a set of userland tools (a shell, core utilities, often GNU software) into a distribution — Ubuntu, Debian, Fedora, Arch, and so on. Different look, same kernel underneath.

Why it's everywhere

Linux runs the overwhelming majority of servers, nearly all public cloud infrastructure, and Android's kernel. Three reasons that keeps compounding:

• It's free and open source — no licensing cost, and you can inspect or modify anything.
• It's stable and scriptable, which matters enormously for automated, unattended systems (servers don't get a mouse and keyboard).
• Decades of tooling assume it. If you're doing DevOps, backend infrastructure, or anything Kubernetes-adjacent, you're operating on Linux whether you see it directly or not.

The shell, briefly

You'll mostly interact with Linux through a shell — a program that reads commands you type and runs them. A handful you'll use constantly:

pwd                 # print current directory
ls -la               # list files, including hidden ones, with details
cd /var/log          # change directory
cat /etc/os-release  # print a file's contents

Everything in Linux is arranged as files, including things that aren't really "files" in the everyday sense — devices, running processes, kernel settings. That single idea ("almost everything is a file") explains a surprising amount of how the rest of the system is organized, and it's the theme the next lessons build on.