Command line interface

A command line interface or CLI is a method of interacting with a computer by giving it lines of text commands in written form either from keyboard input or from a script. The computer then generally responds with text output to the display or to a file. It contrasts with graphical user interfaces (GUIs).

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. 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

  doSomething how toAFile

or

 doSomething how toAnInputFile > outputFile

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.

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 documented in a consistent way. It should be no more difficult to understand and perform a rare operation than a common one. Few GUIs are as accessible across the 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.