Information about Command Line Interface

A command line interface or CLI is a method of interacting with an operating system or software using a command line interpreter. This command line interpreter may be a text terminal, terminal emulator, or remote shell client such as PuTTY.

The concept of the CLI originated when teletype machines (TTY) were connected to computers in the 1950s, and offered results on demand, compared to 'batch' oriented mechanical punch card input technology. Dedicated text-based CRT terminals followed, with faster interaction and more information visible at one time, then graphical terminals enriched the visual display of information. Currently personal computers encapsulate both functions in software.

The CLI continues to coevolve with graphical user interfaces (GUIs) like those provided by Microsoft Windows, Mac OS and the X Window System. In some applications, such as MATLAB, a CLI is integrated with the GUI, with the benefits of both.

Usage

A CLI is used whenever a large vocabulary of commands or queries, coupled with a wide (or arbitrary) range of options, can be entered more rapidly as text than with a pure GUI.

CLIs are often used by programmers and system administrators, in engineering and scientific environments, and by technically advanced personal computer users. CLIs are also popular among people with visual disability, since the commands and feedbacks can be displayed using Refreshable Braille displays.

A program that implements such a text interface is often called a command line interpreter or shell. Examples include the various Unix shells (sh, ksh, csh, tcsh, bash, etc.), the historical CP/M, and DOS's COMMAND.COM, the latter two based heavily on DEC's RSX and RSTS CLIs.

In November 2006, Microsoft released version 1.0 of Windows PowerShell (formerly codenamed Monad), which combined features of traditional Unix shells with their object-oriented .NET Framework. MinGW and Cygwin are open source packages for Windows that offer a Unix like CLI. Microsoft provides MKS Inc.'s ksh implementation MKS Korn shell for Windows through their Services for UNIX add-on.

The latest versions of the Macintosh operating system are based on a variation of UNIX called Darwin. On these computers, users can access a UNIX-like command line interface called Terminal found in the Applications Utilities folder.Some applications provide both a CLI and a GUI. The engineering/scientific numerical computation package MATLAB provides no GUI for some calculations, but the CLI can handle any calculation. The three-dimensional-modelling program Rhinoceros 3D (used to design the cases of most cell phones, as well as thousands of other industrial products) provides a CLI (whose language, by the way, is distinct from Rhino's scripting language). In some computing environments, such as the Oberon or Smalltalk user interface, most of the text which appears on the screen may be used for giving commands.

Anatomy of a CLI

A CLI can generally be considered as consisting of syntax and semantics. The syntax is the grammar that all commands must follow. In the case of operating systems (OS), MS-DOS and UNIX each define their own set of rules that all commands must follow. In the case of embedded systems, each vendor, such as Nortel, Juniper Networks or Cisco Systems, defines their own proprietary set of rules that all commands within their CLI conform to. These rules also dictate how a user navigates through the system of commands. The semantics define what sort of operations are possible, and on what sort of data these operations can be performed.

Two different CLIs may agree on either syntax or semantics, but it is only when they agree on both that they can be considered sufficiently similar to allow users to use both CLIs without needing to relearn anything as well as enable re-use of scripts.

A simple CLI will display a prompt, accept a "command line" typed by the user terminated by the Enter key, then execute the specified command and provide textual display of results or error messages. Advanced CLIs will validate, interpret and parameter-expand the command line before executing the specified command, and optionally capture or redirect its output.

Unlike a button or menu item in a GUI, a command line is typically self-documenting, stating exactly what the user wants done. In addition, command lines usually include many defaults that can be changed to customize the results. Useful command lines can be saved by assigning a character string or alias to represent the full command, or several commands can be grouped to perform a more complex sequence — for instance, compile the program, install it, and run it — creating a single entity, called a command procedure or script which itself can be treated as a command. These advantages mean that a user must figure out a command or series of commands only once, because they can be saved, to be used again.

The commands given to a CLI are often in one of the following forms:

• [doSomething] [how] [toFiles]
• [doSomething] [how] [sourceFile] [destinationFile]
• [doSomething] [how] < [inputFile] > [outputFile]
• [doSomething] [how] | [doSomething] [how] | [do Something] [how] > [outputFile]

doSomething is, in effect, a verb, how an adverb (for example, should the command be executed "verbosely" or "quietly") and toFiles an object or objects (typically one or more files) on which the command should act. The '>' in the third example is a redirection operator, telling the command line interpreter to send the output of the command not to the screen but to the file named on the right of the '>'. Another redirection operator is the pipe ('|'), which tells the CLI to use the output of one command as the input to the next command; this "operator-stream" mechanism can be very powerful.

CLI and Resource Protection

In some CLIs, the commands issued are not coupled to any conceptual place within a command hierarchy. A user can specify relative or absolute paths to any command or data. Examples of this include MS-DOS and UNIX/Windows, which provide forms of a change directory command which allows access to any directory in the system. Protection of resources is provided by a system of resource ownership by privileged groups, and password-protected user accounts which are members of specific groups. MS-DOS provides no such resource protection.

Other CLIs (such as those in routers) limit the set of commands that a user can perform to a subset, determined by location within a command hierarchy, grouped by association with security, system, interface, etc. The location within this hierarchy and the options available are often referred to as a mode. In these systems the user might traverse through a series of sub-hierarchies, each with their own subset of commands. For example, if the CLI had two modes called interface and system, the user would enter the word 'interface' at the command prompt and then enter an interface mode, where a certain subset of commands and data are available. At this point system commands are not accessible and would not be accessible until the user explicitly exits the interface mode.

Command prompt

A command prompt (or just prompt) is a sequence of (one or more) characters used in a command line interface to indicate readiness to accept commands. Its intent is to literally prompt the user to take action. A prompt usually ends with one of the characters $, %, #, :, > and often includes other information, such as the path of the current working directory.

It is common for prompts to be modifiable by the user. Depending on the environment, they may include colors, special characters, and other elements like the current time, in order, for instance, to make the prompt more informative or visually pleasing, to distinguish sessions on various machines, or to indicate the current level of nesting of commands.

In DOS's COMMAND.COM and in the Windows command line interpreter cmd.exe the prompt is modifiable by issuing a prompt command or by changing the value of the %PROMPT% environment variable. The default C:\> style is obtained, for instance, with "prompt $P$G".

On many Unix systems, the $PS1 variable can be used, although other variables also may have an impact on the prompt (depending on what shell is being used). In the bash shell, a prompt of the form

[time] user@host: work_dir $

could be set by issuing the command

export PS1='[\t] \u@\H: $(pwd) $'

In zsh the $RPROMPT variable controls an optional "prompt" on the right hand side of the display. It is not a real prompt in that the location of text entry does not change. It is used to display information on the same line as the prompt, but right justified.

Quotes

"IBM wanted CP/M prompts. It made me throw up."

"It was a mistake to think that GUIs ever would, could, or even should, eliminate CLIs."

References

See also

• Command line interpreter
• In the Beginning...was the Command Line
• GUI vs. CLI
• Scripting language
• Shell (computing)

External links

• Linux Command Line — a list of unix shell commands.
• Command Line Warriors — an open site about Command Line Computing.
• The Interaction-Design.org Encyclopedia entry on Interaction Styles, comparing Command Line Interfaces with other Interaction Styles
• Command lines versus GUIs
• The Windows XP Command Line, Batch Files, and Scripting
• "GetPot. Object oriented command line parsing." Library available for C++, Python, Java, and Ruby.
• "Object-Oriented Command Line Interfaces" — Article describing the use of the TE-Common CLI library for Java (also available for .NET).
• PHP Command Line Interface — All about PHP CLI SAPI (using PHP in command line).
• - Setting up the console for Windows - A basic guide to setting up and using the command line for Windows.
• The Command Line -- The Best Newbie Interface? -- essay/memoir alleging/demonstrating that the CLI is actually more intuitive than the GUI, at least for adult newbies.
• Linux By Examples - Explain every GNU/Linux command by examples.
• Linux Command line tips - Linux command lines for common operations.
• Braille Monitors - Device to display CLI as Braille-script.
• Basic Linux Commands in Tons