Command line interface

A command line interface or CLI is a method of interacting with a computer by giving it lines of textual commands (ie, a sequence of characters) either from keyboard input or from a script. The computer system (ie, the program then accepting such input) then carries out the command given. The result may be textual output, or the initialization and running of some program, or a modification to the graphical output on a monitor or printer, or some change in a file system (eg, delete a file, modify a file, or create a file). The term is usually used in contrast to a graphical user interface (GUI) in which commands are typically issued by moving a pointer (eg, a mouse cursor) and/or pressing a key (ie, 'clicking', often on a key mounted on a mouse).

Examples of programs implementing command line interfaces include the (various) Unix shells, VMS' DCL (Digital Command Language), and related designs like CP/M and MS-DOS's command.com, both based heavily on DEC's RSX and RSTS operating system interfaces (which were also command line interfaces). These programs are often called command line interpreters.

There are other programs which use CLIs as well. The CAD program AutoCAD is a prominent example. In some computing environments like the Oberon or Smalltalk user interface, most of the text which appears on the screen may be used for giving commands.

The commands given on a command line interface often are of the form

or

    > 

doSomething corresponds to a verb, how to an adverb (it describes how the command should be performed in this instance) and toAFile to an object (often one or more files) against which the command should be run. The standalone ">" in the second example is a redirection character, telling the operating system (ie, usually a command shell interpreter) to send the output of the previous commands (ie, those on the left of ">") to some other place (ie, that given to the right of the ">").

Advantages of a command line interface

Even though new users seem to learn GUIs more quickly to perform common operations, well designed CLIs have several advantages:

• All options and operations are invokable in a consistent form, one 'level' away from the basic command. With most GUIs, the available operations and options often appear on different menus with differing usage patterns. They may be separated on several different menu levels as well. In either case, different applications (or utilities) may have different patterns; if so there is little advantage in either approach. Both are likely to annoy users.

• All options and operations are controlled in more or less the same way. The less in this case is a common accusation against CLIs. It _should_ be no more difficult to understand and perform a rare operation than a common one. Few GUIs offer comparable access to the entire range of available options.

• CLIs often can double as scripting languages (see shell script) and can perform operations in a batch processing mode without user interaction. That means that once an operation is analyzed and understood, a 'script' implementing that understanding can be written and saved. The operation can thereafter be carried out with no further analysis and design effort. With GUIs, users must start over at the beginning every time, as GUI scripting (if available at all) is almost always more limited. Simple commands do not even need an actual script, as the completed command can usually be assigned a name (an 'alias') and executed simply by typing that name into the CLI.

External Links

• "In the Beginning, was the Command Line" By Neal Stephenson A brief and gentle introduction to the difference between command line interfaces and GUIs generally, and specifically between Windows and the Linux shells. Written by a novelist, non-technical, and often amusing.