ANSI escape code

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ANSI escape sequences are a standard for in-band signaling to control the cursor location, color, and other options on video text terminals. Certain sequences of bytes, most starting with Esc and '[', are embedded into the text, which the terminal looks for and interprets as commands, not as character codes.

ANSI sequences were introduced in the 1970s to replace vendor-specific sequences and became widespread in the computer equipment market by the early 1980s. They were used in development, scientific and commercial applications and later by the nascent bulletin board systems to offer improved displays compared to earlier systems lacking cursor movement, a primary reason they became a standard adopted by all manufacturers.

Although hardware text terminals have become increasingly rare in the 21st century, the relevance of the ANSI standard persists because most terminal emulators interpret at least some of the ANSI escape sequences in output text. One notable exception was the Win32 console of Microsoft Windows before Windows 10 update TH2.

Video ANSI escape code

History

Almost all manufacturers of video terminals added vendor-specific escape sequences to perform operations such as placing the cursor at arbitrary positions on the screen. One example is the VT52 terminal, which allowed the cursor to be placed at an x,y location on the screen by sending the ESC character, a y character, and then two characters representing with numerical values equal to the x,y location plus 32 (thus starting at the ASCII space character and avoiding the control characters).

As these sequences were different for different terminals, elaborate libraries such as termcap and utilities such as tput had to be created so programs could use the same API to work with any terminal. In addition many of these terminals required sending numbers (such as row and column) as the binary values of the characters; for some programming languages, and for systems that did not use ASCII internally, it was often difficult or impossible to turn a number into the correct character.

The ANSI standard attempted to address these problems by making a command set that all terminals would use and requiring all numeric information to be transmitted as ASCII numbers. The first standard in the series was ECMA-48, adopted in 1976. It was a continuation of a series of character coding standards, the first one being ECMA-6 from 1965, a 7-bit standard from which ISO 646 originates. The name "ANSI escape sequence" dates from 1979 when ANSI adopted ANSI X3.64. The ANSI X3L2 committee collaborated with the ECMA committee TC 1 to produce nearly identical standards. These two standards were merged into an international standard, ISO 6429. In 1994, ANSI withdrew its standard in favor of the international standard.

The first popular video terminal to support these sequences was the Digital VT100, introduced in 1978. This model was very successful in the market, which sparked a variety of VT100 clones, among the earliest and most popular of which was the much more affordable Zenith Z-19 in 1979. Others included the Qume QVT-108, Televideo TVI-970, Wyse WY-99GT as well as optional "VT100" or "VT103" or "ANSI" modes with varying degrees of compatibility on many other brands. The popularity of these gradually led to more and more software (especially bulletin board systems and other online services) assuming the escape sequences worked, leading to almost all new terminals and emulator programs supporting them.

In 1981, ANSI X3.64 was adopted for use in the US government by FIPS publication 86. Later, the US government stopped duplicating industry standards, so FIPS pub. 86 was withdrawn.

ECMA-48 has been updated several times and is currently at its 5th edition, from 1991. It is also adopted by ISO and IEC as standard ISO/IEC 6429.

Maps ANSI escape code

Platform support

Unix-like systems

Although termcap/terminfo-style libraries were primarily developed on and for Unix, since about 1984 programs running on Unix-like operating systems could almost always assume they were using a terminal or emulator that supported ANSI sequences; this led to widespread use of ANSI by programs running on those platforms. For instance, many games and shell scripts (see below for colored prompt examples), and utilities such as color directory listings, directly write the ANSI sequences and thus cannot be used on a terminal that does not interpret them. Many programs, including text editors such as vi and GNU Emacs, still use termcap or terminfo, or use libraries such as curses that use termcap or terminfo, and thus in theory support non-ANSI terminals, but this is so rarely tested nowadays that they are unlikely to work with those terminals. Terminal emulators for communicating with local programs as well as remote machines and the text system console almost always support ANSI escape codes.

DOS

MS-DOS 1.x did not support the ANSI or any other escape sequences. Only a few control characters (BEL, CR, LF, BS) were interpreted by the underlying BIOS, making it almost impossible to do any kind of full-screen application. Any display effects had to be done with BIOS calls, which were notoriously slow, or by directly manipulating the IBM PC hardware.

DOS 2.0 introduced the ability to add a device driver for the ANSI escape sequences - the de facto standard being ANSI.SYS, but others like ANSI.COM, NANSI.SYS and ANSIPLUS.EXE are used as well (these are considerably faster as they bypass the BIOS). Slowness and the fact that it was not installed by default made software rarely take advantage of it; instead, applications continued to directly manipulate the hardware to get the text display needed. ANSI.SYS and similar drivers continued to work in Windows 9x up to Windows Me, and in NT-derived systems for 16-bit legacy programs executing under the NTVDM.

Concurrent DOS, Multiuser DOS and REAL/32 have built-in support for ANSI ecape sequences (plus a number of extensions) and do not require a separate ANSI driver to be loaded.

OS/2

The ANSI command in a batch file or at the command line enables the extensions.

Atari ST

The Atari ST used the command system adapted from the VT52 with some expansions for color support, rather than supporting ANSI escape codes.

AmigaOS

AmigaOS not only interprets ANSI code sequences for text output to the screen, the AmigaOS printer driver also interprets them (with extensions proprietary to AmigaOS) and translates them into the codes required for the particular printer that is actually attached.

Windows

The Win32 console did not support ANSI escape sequences at all. Some replacements or additions for the console window such as JP Software's TCC (formerly 4NT), Michael J. Mefford's ANSI.COM, Jason Hood's ANSICON and Maximus5's ConEmu interpreted ANSI escape sequences printed by programs. A Python package internally interpreted ANSI escape sequences in text being printed, translating them to calls to manipulate the color and cursor position, to make it easier to port Python code using ANSI to Windows.

In 2016 with Windows 10 "Threshold 2" Microsoft unexpectedly started supporting ANSI escape sequences in the console app, making the porting of software from (or remote access to) Unix much easier.

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Escape sequences

Sequences have different lengths. All sequences start with ESC (27 / hex 0x1B / octal 033), followed by a second byte in the range 0x40-0x5F (ASCII @A-Z[\]^_).

The standard says that in 8-bit environments these two-byte sequences can be merged into single C1 control code in the 0x80-0x9F range. However on modern devices those codes are often used for other purposes, such as parts of UTF-8 or for CP-1252 characters, so only the 2-byte sequence is used.

Other C0 codes besides ESC -- commonly BEL, BS, CR, LF, FF, TAB, VT, SO, and SI -- produce similar or identical effects to some control sequences when output.

Pressing special keys on the keyboard, as well as outputting many xterm CSI, DCS, or OSC sequences, often produces a CSI, DCS, or OSC sequence, sent from the terminal to the computer as though the user typed it.

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CSI sequences

The ESC [ is followed by any number (including none) of "parameter bytes" in the range 0x30-0x3F (ASCII 0-9:;<=>?), then by any number of "intermediate bytes" in the range 0x20-0x2F (ASCII space and !"#$%&'()*+,-./), then finally by a single "final byte" in the range 0x40-0x7E (ASCII @A-Z[\]^_`a-z{|}~).

All common sequences just use the parameters as a series of semicolon-separated numbers such as 1;2;3. Missing numbers are treated as 0 (1;;3 acts like the middle number is 0, and no parameters at all in ESC[m acts like a 0 reset code). Some sequences (such as CUU) treat 0 as 1 in order to make missing parameters useful. Bytes other than digits and semicolon seem to not be used.

A subset of arrangements was declared "private" so that terminal manufacturers could insert their own sequences without conflicting with the standard. Sequences containing the parameter bytes <=>? or the final bytes 0x70-0x7E (p-z{|}~) are private.

The behavior of the terminal is undefined in the case where a CSI sequence contains any character outside of the range 0x20-0x7E. These illegal characters are either C0 control characters (the range 0-0x1F), DEL (0x7F), or bytes with the high bit set. Possible responses are to ignore the byte, to process it immediately, and furthermore whether to continue with the CSI sequence, to abort it immediately, or to ignore the rest of it.

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SGR (Select Graphic Rendition) parameters

SGR sets display attributes. Several attributes can be set in the same sequence, separated by semicolons. Each display attribute remains in effect until a following occurrence of SGR resets it. If no codes are given, CSI m is treated as CSI 0 m (reset / normal).

In ECMA-48 SGR is called "Select Graphic Rendition". In Linux manual pages the term "Set Graphics Rendition" is used.

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Colors

3/4 bit

The original specification only had 8 colors, and just gave them names. The SGR parameters 30-37 selected the foreground color, while 40-47 selected the background. Quite a few terminals implemented "bold" (SGR code 1) as a brighter color rather than a different font, thus providing 8 additional foreground colors. Usually you could not get these as background colors, though sometimes inverse video (SGR code 7) would allow that. Examples: to get black letters on white background use ESC[30;47m, to get red use ESC[31m, to get bright red use ESC[1;31m. To reset colors to their defaults, use ESC[39;49m (not supported on some terminals), or reset all attributes with ESC[0m. Later terminals added the ability to directly specify the "bright" colors with 90-97 and 100-107.

When hardware started using 8-bit DACs several pieces of software assigned 24-bit color numbers to these names. The chart below shows values sent to the DAC for some common hardware and software.

8-bit

As 256-color lookup tables became common on graphic cards, escape sequences were added to select from a pre-defined set of 256 colors:

   ESC[ ... 38;5;<n> ... m Select foreground color     ESC[ ... 48;5;<n> ... m Select background color       0-  7:  standard colors (as in ESC [ 30-37 m)       8- 15:  high intensity colors (as in ESC [ 90-97 m)      16-231:  6 × 6 × 6 cube (216 colors): 16 + 36 × r + 6 × g + b (0 <= r, g, b <= 5)     232-255:  grayscale from black to white in 24 steps  

The ITU's T.416 Information technology - Open Document Architecture (ODA) and interchange format: Character content architectures uses ':' as separator characters instead:

   ESC[ ... 38:5:<n> ... m Select foreground color     ESC[ ... 48:5:<n> ... m Select background color  

24-bit

As "true color" graphic cards with 16 to 24 bits of color became common, Xterm, KDE's Konsole, as well as all libvte based terminals (including GNOME Terminal) support ISO-8613-3 24-bit foreground and background color setting

   ESC[ ... 38;2;<r>;<g>;<b> ... m Select RGB foreground color     ESC[ ... 48;2;<r>;<g>;<b> ... m Select RGB background color  

The ITU's T.416 Information technology - Open Document Architecture (ODA) and interchange format: Character content architectures which was adopted as ISO/IEC International Standard 8613-6 gives an alternative version that seems to be less supported:

   ESC[ ... 38:2:<Color-Space-ID>:<r>:<g>:<b>:<unused>:<CS tolerance>:<Color-Space: 0="CIELUV"; 1="CIELAB">; ... m Select RGB foreground color     ESC[ ... 48:2:<Color-Space-ID>:<r>:<g>:<b>:<unused>:<CS tolerance>:<Color-Space: 0="CIELUV"; 1="CIELAB">; ... m Select RGB background color  

Note that this uses the otherwise reserved ':' character to separate the sub-options which may have been a source of confusion for real-world implementations. It also documents using '3' as the second parameter to specify colors using a Cyan-Magenta-Yellow scheme and '4' for a Cyan-Magenta-Yellow-Black one, the latter using the position marked as "unused" in the above examples for the Black component.

Also note that many implementation that recognize ':' as the separator erroneously forget about the color space identifier parameter and hence shift the position of the remaining ones.

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Examples

CSI 2 J -- This clears the screen and, on some devices, locates the cursor to the y,x position 1,1 (upper left corner).

CSI 32 m -- This makes text green. The green may be a dark, dull green, so you may wish to enable Bold with the sequence CSI 1 m which would make it bright green, or combined as CSI 32 ; 1 m. Some implementations use the Bold state to make the character Bright.

CSI 0 ; 6 8 ; "DIR" ; 13 p -- This reassigns the key F10 to send to the keyboard buffer the string "DIR" and ENTER, which in the DOS command line would display the contents of the current directory. (MS-DOS ANSI.SYS only) This was sometimes used for ANSI bombs. This is a private-use code (as indicated by the letter p), using a non-standard extension to include a string-valued parameter. Following the letter of the standard would consider the sequence to end at the letter D.

CSI s -- This saves the cursor position. Using the sequence CSI u will restore it to the position. Say the current cursor position is 7(y) and 10(x). The sequence CSI s will save those two numbers. Now you can move to a different cursor position, such as 20(y) and 3(x), using the sequence CSI 20 ; 3 H or CSI 20 ; 3 f. Now if you use the sequence CSI u the cursor position will return to 7(y) and 10(x). Some terminals require the DEC sequences ESC 7 / ESC 8 instead which is more widely supported.

Example of use in shell scripting

ANSI escape codes are often used in UNIX and UNIX-like terminals to provide syntax highlighting. For example, on compatible terminals, the following list command color-codes file and directory names by type.

ls --color

Users can employ escape codes in their scripts by including them as part of standard output or standard error. For example, the following GNU sed command embellishes the output of the make command by displaying lines containing words starting with "WARN" in reverse video and words starting with "ERR" in bright yellow on a dark red background (letter case is ignored). The representations of the codes are highlighted.

make 2>&1 | sed -e 's/.*\bWARN.*/\x1b[7m&\x1b[0m/i' -e 's/.*\bERR.*/\x1b[93;41m&\x1b[0m/i'  

The following Bash function flashes the terminal (by alternately sending reverse and normal video mode codes) until the user presses a key.

flasher () { while true; do printf \\e[?5h; sleep 0.1; printf \\e[?5l; read -s -n1 -t1 && break; done; }  

This can be used to alert a programmer when a lengthy command terminates, such as with make ; flasher .

printf \\033c  

This will reset the console, similar to the command reset on modern Linux systems; however it should work even on older Linux systems and on other (non-Linux) UNIX variants.

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Invalid and ambiguous sequences in use

• The Linux console uses OSC P n rr gg bb to change the palette, which, if hard-coded into an application, may hang other terminals. However, appending ST will be ignored by Linux and form a proper, ignorable sequence for other terminals.
• On the Linux console, certain function keys generate sequences of the form CSI [ char. The CSI sequence should terminate on the [.
• Old versions of Terminator generate SS3 1; modifiers char when F1-F4 are pressed with modifiers. The faulty behavior was copied from GNOME Terminal.
• xterm replies CSI row ; column R if asked for cursor position and CSI 1 ; modifiers R if the F3 key is pressed with modifiers, which collide in the case of row == 1. This can be avoided by using the ? private modifier, which will be reflected in the response.
• many terminals prepend ESC to any character that is typed with the alt key down. This creates ambiguity for uppercase letters and symbols @[\]^_, which would form C1 codes.
• Konsole generates SS3 modifiers char when F1-F4 are pressed with modifiers.

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See also

• ANSI art
• Control character
• Advanced Video Attribute Terminal Assembler and Recreator (AVATAR)
• ISO/IEC JTC 1/SC 2
• C0 and C1 control codes

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Notes

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References

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External links

• Standard ECMA-48, Control Functions For Coded Character Sets. (5th edition, June 1991), European Computer Manufacturers Association, Geneva 1991 (also published by ISO and IEC as standard ISO/IEC 6429)
• vt100.net DEC Documents
• ANSI.SYS -- ansi terminal emulation escape sequences at the Wayback Machine (archived 6 February 2006)
• Xterm / Escape Sequences
• AIXterm / Escape Sequences
• A collection of escape sequences for terminals that are vaguely compliant with ECMA-48 and friends.
• ANSI Escape Sequences
• ITU-T Rec. T.416 (03/93) Information technology - Open Document Architecture (ODA) and interchange format: Character content architectures

Source of article : Wikipedia