The Linux command line interface (CLI) is a text-based environment for interacting with the operating system by typing commands into a terminal emulator. Unlike a graphical user interface, which relies on windows, icons, and pointer-based actions, the CLI lets users accomplish tasks by issuing precise textual instructions, often faster and more reproducibly than their GUI equivalents.
Behind the CLI sits a shell — a program that reads each command, interprets it, and asks the Linux kernel to perform the corresponding action. The most common shell on Linux distributions is Bash (the Bourne Again SHell), which provides features like command history, tab completion, and scripting. Other shells such as Zsh or Fish exist, but Bash remains the default for most systems and is the focus of this textbook.
To begin using the CLI, you launch a terminal, which opens a shell session. On Ubuntu and many other distributions, pressing Ctrl+Alt+T opens a default terminal immediately. Alternatively, you can search for "Terminal" in the applications menu. Once the terminal is open, you are presented with a prompt where you can begin typing commands.