Introduction to Linux

Linux is a free, open-source operating system that runs the overwhelming majority of servers, cloud infrastructure, and security tooling, comfort with it is close to a requirement for cybersecurity work, and genuinely useful for backend development too.

Why the terminal, not a graphical interface

Most servers run with no graphical desktop at all, connecting to one remotely gives you a terminal and nothing else, so terminal fluency isn't optional the way it can feel on a personal computer with a full desktop environment. A terminal is a text interface where you type commands instead of clicking icons; that's what this whole subject covers, one command family at a time.