Command-line Completion

Command-line completion (also tab completion) is a common feature of command line interpreters, in which the program automatically fills in partially typed commands.

Command line interpreters are programs that allow a user to interact with the underlying operating system by typing commands at a command prompt using a command line interface (CLI), in contrast to pointing and clicking a mouse in a Graphical User Interface (GUI). Command-line completion allows the user to type the first few characters of a command, program, or filename, and press a completion key (normally Tab ↹) to fill in the rest of the item. The user then presses Return or ↵ Enter to run the command or open the file.

Command-line completion is useful in several ways, as illustrated by the animated image accompanying this article. Commonly-accessed commands, especially ones with long names, require fewer keystrokes to reach. Commands with long or difficult to spell filenames can be entered by typing the first few characters and pressing a completion key, which completes the command or filename. In the case of multiple possible completions, some command-line interpreters, especially Unix shells, will list all filenames beginning with those few characters. The user can type more characters and press Tab ↹ again to see a new, narrowed-down list if the typed characters are still ambiguous, or else complete the filename. An alternate form of completion rotates through all matching results when the input is ambiguous.

Completable elements may include commands, arguments, file names and other entities, depending on the specific interpreter and its configuration. Command-line completion generally only works in interactive mode. That is, it cannot be invoked to complete partially typed commands in scripts or batch files, even if the completion is unambiguous. The name tab completion comes from the fact that command-line completion is often invoked by pressing the tab key.