A command-line interface (CLI) is a means of interaction between a human user and a computer program, or between two programs, where the user (or client) passes commands in the form of a line of text (a command line) to a computer program.

The command-line interface evolved from a form of dialog once conducted by humans over teleprinter machines, in which human operators remotely exchanged information, usually one line of text at a time. Early computer systems often used teleprinter machines as the means of interaction with a human operator. The computer became one end of the human-to-human teleprinter model. So instead of a human communicating with another human over a teleprinter, a human communicated with a computer.

In time, the actual mechanical teleprinter was replaced by a glass tty (keyboard and screen, but emulating the teleprinter), and then by a terminal (where the computer software could address all of the screen, rather than only print successive lines).

Due to its text-based nature, a command-line interface is sometimes confused with the text-based user interface, a kind of user interface which uses only text, but not necessarily presented in successive lines. Text may be formatted and appear in fixed locations on a computer terminal display, as opposed to only appearing in successive lines.