The interface to a program that consists entirely of text, as opposed to (although not necessarily mutually exclusive to) a GUI, or Graphical User Interface.
A command line interface (CLI) is an interface to a program based on a series of typed text lines, called commands, which follow a specific language structure. The command lines are then interpreted by a Command Language Interpreter (more commonly known as a Shell).
The CLI was a popular interface since the 1960's, and even with development of more modern UI experiences such as gui, it is still a ubiquitous interface today. Depending on the specific command language, the CLI could provide a quicker and more accurate way to perform commands compared to alternatives.
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