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 which use command line interfaces include the Unix
shell, VMS DCL, and related follow-on designs like CP/M and MS-DOS's
command.com. These programs are often called command line interpreters.

There are other programs which use CLIs as well. The CAD program AutoCAD has
one. In some computing environments like Oberon or Smalltalk, 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 performend) and toAFile to a direct object.

Advantages of a command line interface

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

   * All options and operations are invoked in consistent form, while with
     GUIs similar operations often appear on different menus with different
     interfaces and different applications have different approaches.

   * All options and operations are documented (or should be), meaning that
     it is no more difficult to perform a rare operation than a common one.

   * CLIs 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, it can be saved in a script
     and consistently performed without further effort. With GUIs, users
     must start over at the beginning every time, as GUI scripting is more
     limited (and often nonexistent). Simple commands do not even need a
     script, as the completed command can usually be assigned a name and
     executed simply by typing that name into the CLI.

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