This article refers to codes used as commands for computing devices. Escape sequence can also refer to a sequence

Sequence

In mathematics, a sequence is an ordered list of objects . Like a set, it contains members , and the number of terms is called the length of the sequence. Unlike a set, order matters, and exactly the same elements can appear multiple times at different positions in the sequence...

 of escape character

Escape character

In computing and telecommunication, an escape character is a character which invokes an alternative interpretation on subsequent characters in a character sequence. An escape character is a particular case of metacharacters...

s used in parsing source code

Source code

In computer science, source code is text written using the format and syntax of the programming language that it is being written in. Such a language is specially designed to facilitate the work of computer programmers, who specify the actions to be performed by a computer mostly by writing source...

.

An escape sequence is a series of characters

Character (computing)

In computer and machine-based telecommunications terminology, a character is a unit of information that roughly corresponds to a grapheme, grapheme-like unit, or symbol, such as in an alphabet or syllabary in the written form of a natural language....

 used to change the state

State (computer science)

In computer science and automata theory, a state is a unique configuration of information in a program or machine. It is a concept that occasionally extends into some forms of systems programming such as lexers and parsers....

 of computer

Computer

A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...

s and their attached peripheral

Peripheral

A peripheral is a device attached to a host computer, but not part of it, and is more or less dependent on the host. It expands the host's capabilities, but does not form part of the core computer architecture....

 devices. These are also known as control sequences, reflecting their use in device control. Some control sequences are special characters that always have the same meaning. Escape sequences use an escape character

Escape character

In computing and telecommunication, an escape character is a character which invokes an alternative interpretation on subsequent characters in a character sequence. An escape character is a particular case of metacharacters...

 to change the meaning of the characters which follow it, meaning that the characters can be interpreted as a command

Command (computing)

In computing, a command is a directive to a computer program acting as an interpreter of some kind, in order to perform a specific task. Most commonly a command is a directive to some kind of command line interface, such as a shell....

 to be executed rather than as data

Data (computing)

In computer science, data is information in a form suitable for use with a computer. Data is often distinguished from programs. A program is a sequence of instructions that detail a task for the computer to perform...

.

Escape sequences are commonly used when a computer and a peripheral have only a single channel through which to send information back and forth. If the device in question is "dumb" and can only do one thing with the information being sent to it (for instance, print it) then there is no need for an escape sequence. However most devices have more than one capability, and thus need some way to distinguish information that is to be treated as data from information that is to be treated as commands.

An escape character is usually assigned to the Esc key

Esc key

On computer keyboards, the Esc key is a key labeled Esc or Escape that is used to generate the ASCII Escape character , the character code traditionally used to initiate an escape sequence...

 on a computer keyboard

Computer keyboard

In computing, a keyboard is a typewriter-style keyboard, which uses an arrangement of buttons or keys, to act as mechanical levers or electronic switches...

, and can be sent in other ways than as part of an escape sequence. For example, the Esc key may be used as an input character in editors such as EMACS

Emacs

Emacs is a class of text editors, usually characterized by their extensibility. GNU Emacs has over 1,000 commands. It also allows the user to combine these commands into macros to automate work.Development began in the mid-1970s and continues actively...

, or for backing up one level in a menu in some applications. The Hewlett Packard HP 2640

HP 2640

The HP 2640A and other HP 264X models were block-mode "smart" and intelligent ASCII standard serial terminals produced by Hewlett Packard using the Intel 8008 and 8080 microprocessors.-History:...

 terminals had a key for a "display functions" mode which would display graphics for all control characters, including Esc, to aid in debugging

Debugging

Debugging is a methodical process of finding and reducing the number of bugs, or defects, in a computer program or a piece of electronic hardware, thus making it behave as expected. Debugging tends to be harder when various subsystems are tightly coupled, as changes in one may cause bugs to emerge...

 applications.

If the Esc key and other keys that send escape sequences are both supposed to be meaningful to an application, an ambiguity

Ambiguity

Ambiguity of words or phrases is the ability to express more than one interpretation. It is distinct from vagueness, which is a statement about the lack of precision contained or available in the information.Context may play a role in resolving ambiguity...

 arises, if a terminal

Computer terminal

A computer terminal is an electronic or electromechanical hardware device that is used for entering data into, and displaying data from, a computer or a computing system...

 or terminal emulator

Terminal emulator

A terminal emulator, terminal application, term, or tty for short, is a program that emulates a video terminal within some other display architecture....

 is in use. In particular, when the application receives the ASCII

ASCII

The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...

 escape character, it is not clear whether that character is the result of the user pressing the Esc key or whether it is the initial character of an escape sequence (e.g., resulting from an arrow key press). The traditional method of resolving the ambiguity is to observe whether or not another character quickly follows the escape character. If not, it is assumed not to be part of an escape sequence. This heuristic

Heuristic

Heuristic refers to experience-based techniques for problem solving, learning, and discovery. Heuristic methods are used to speed up the process of finding a satisfactory solution, where an exhaustive search is impractical...

 can fail under some circumstances, but in practice it works reasonably well, especially with faster modern communication speeds.

Escape sequences date back at least to the 1874 Baudot code

Baudot code

The Baudot code, invented by Émile Baudot, is a character set predating EBCDIC and ASCII. It was the predecessor to the International Telegraph Alphabet No 2 , the teleprinter code in use until the advent of ASCII. Each character in the alphabet is represented by a series of bits, sent over a...

.

Modem control

The Hayes command set

Hayes command set

The Hayes command set is a specific command-language originally developed for the Hayes Smartmodem 300 baud modem in 1981. The command set consists of a series of short text strings which combine together to produce complete commands for operations such as dialing, hanging up, and changing the...

, for instance, defines a single escape sequence, +++. (In order not to interpret +++ which may be a part of data as the escape sequence the sender stops communication for one second before and after the +++) .When the modem encounters this in a stream of data, it switches from its normal mode of operation which simply sends any characters to the phone, to a command mode in which the following data is assumed to be a part of the command language. You can switch back to the online mode by sending the O command.

The Hayes command set is modal

Mode (computer interface)

In user interface design, a mode is a distinct setting within a computer program or any physical machine interface, in which the same user input will produce perceived different results than it would in other settings....

, switching from command mode to online mode. This is not appropriate in the case where the commands and data will switch back and forth rapidly. An example of a non-modal escape sequence control language is the VT100

VT100

The VT100 is a video terminal that was made by Digital Equipment Corporation . Its detailed attributes became the de facto standard for terminal emulators.-History:...

, which used a series of commands prefixed by the Control Sequence Introducer, escape-[.

Comparison with control characters

A control character

Control character

In computing and telecommunication, a control character or non-printing character is a code point in a character set, that does not in itself represent a written symbol.It is in-band signaling in the context of character encoding....

 is a character that, in isolation, has some control function, such as carriage return

Carriage return

Carriage return, often shortened to return, refers to a control character or mechanism used to start a new line of text.Originally, the term "carriage return" referred to a mechanism or lever on a typewriter...

 (CR). Escape sequences, by contrast, consist of an escape character

Escape character

In computing and telecommunication, an escape character is a character which invokes an alternative interpretation on subsequent characters in a character sequence. An escape character is a particular case of metacharacters...

 or sequence which changes the interpretation of following characters.

The earlier VT52

VT52

The VT52 was a CRT-based computer terminal produced by Digital Equipment Corporation introduced in September, 1975 . It provided a screen of 24 rows and 80 columns of text and supported all 95 ASCII characters as well as 32 graphics characters. It supported asynchronous communication at baud rates...

 terminal used simple digraph commands like escape-A: in isolation, "A" simply meant the letter "A", but as part of the escape sequence "escape-A", it had a different meaning. The VT52 also supported parameters: it was not a straightforward control language encoded as substitution.

ASCII video data terminals

The VT100

VT100

The VT100 is a video terminal that was made by Digital Equipment Corporation . Its detailed attributes became the de facto standard for terminal emulators.-History:...

 terminal implemented the more sophisticated ANSI standard (now ECMA-48) for functions such as controlling cursor movement, character set, and display enhancements. The Hewlett Packard HP 2640

HP 2640

The HP 2640A and other HP 264X models were block-mode "smart" and intelligent ASCII standard serial terminals produced by Hewlett Packard using the Intel 8008 and 8080 microprocessors.-History:...

 series had perhaps the most elaborate escape sequences for block and character modes, programming keys and their soft labels, graphics vectors, and even saving data to tape or disk files.

Use in DOS and Windows

A utility, ANSI.SYS

ANSI.SYS

ANSI.SYS is a device driver in the DOS operating system that provides extra console functions through ANSI escape sequences. It is partially based upon a subset of the text terminal control standard proposed by the ANSI X3L2 Technical Committee on Codes and Character Sets .-Usage:To use ANSI.SYS...

, can be used to enable the interpreting of the ANSI (ECMA-48) terminal escape sequences in a DOS command window in DOS or 16-bit Windows. The rise of GUI

Gui

Gui or guee is a generic term to refer to grilled dishes in Korean cuisine. These most commonly have meat or fish as their primary ingredient, but may in some cases also comprise grilled vegetables or other vegetarian ingredients. The term derives from the verb, "gupda" in Korean, which literally...

 applications, which directly write to display cards, has greatly reduced the usage of escape sequences on Microsoft platforms, but they can still be used to create interactive random-access character-based screen interfaces with the character-based library routines such as printf

Printf

Printf format string refers to a control parameter used by a class of functions typically associated with some types of programming languages. The format string specifies a method for rendering an arbitrary number of varied data type parameter into a string...

without resorting to a GUI program.