Command-Line Fundamentals

A command line lets you direct a computer by entering text. You state an operation, provide inputs, and receive output. That small interface scales from inspecting one folder to controlling remote servers and automated build systems.

The command line is not one program. A terminal provides the text interface. A shell reads your command language. A command performs a task. Keeping those layers separate makes unfamiliar systems easier to understand.

The execution model

In a traditional shell, a command line usually has this shape:

command option argument

The shell first parses your text. It recognizes words and operators, performs expansions, applies redirections, finds the command, and runs it. The command then returns output and an exit status.

That sequence explains many surprises. Spaces separate words unless quoting protects them. Wildcards may expand before the command starts. A redirection is handled by the shell, not passed to the command as an ordinary argument.