ANSI escape code

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In computing, ANSI escape code (or escape sequences) is the method of in-band signaling to control formatting, color, and other output options on video text terminals. To encode this formatting information, it embeds certain sequences of bytes into the text, which have to be interpreted specially, not as codes of characters. Although hardware text terminals have become increasingly rare in the 21st century, the relevance of this standard persists because most terminal emulators interpret at least some of the ANSI escape sequences in the output text. One notable exception is the win32 console component of Microsoft Windows.

History[edit]

Almost all manufacturers of video terminals added vendor-specific escape sequences to do operations such as placing the cursor at arbitrary positions on the screen. As these sequences were all different, elaborate libraries such as termcap had to be created so programs could use the same API for all of them. In addition, most designs required sending numbers (such as row & 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 first standard for ANSI escape sequences was ECMA-48, adopted in 1976. It was a continuation of a series of character coding standards, the first one being ECMA-6 from 1961, a 7-bit standard from which ASCII originates. ECMA-48 has been updated several times and the current edition is the 5th from 1991. It is also adopted by ISO and IEC as standard ISO/IEC 6429. The name "ANSI escape sequence" dates from 1981 when ANSI adopted ECMA-48 as the standard ANSI X3.64 (and later, in 1997, withdrew it).[1]

The first popular video terminal to support these sequences was the Digital VT100 introduced in 1978,[2] which sparked a variety of "clones", among the earliest and most popular of which was the much more affordable Zenith Z-19 in 1979.[3] The popularity of these gradually led to more and more software (especially bulletin board systems) assuming the escape sequences worked, leading to almost all new terminals and emulator programs supporting them.

Support[edit]

Most terminal emulators running on Unix-like systems (such as xterm and the OS X Terminal) interpret ANSI escape sequences. The Linux console (the text seen when X is not running) also interprets them. Terminal programs for Microsoft Windows designed to show text from an outside source (a serial port, modem, or socket) almost always interprets them.

Many Unix console applications (e.g., ls, grep, Vim, and Emacs) can generate them. Most of these use curses or their own system to supposedly allow the update of a terminal using something other than ANSI, but this is so rarely tested nowadays that they are unlikely to work.[citation needed] Games, shell scripts (such as colored prompts), and bulletin board systems often use ANSI directly and cannot work on a terminal that does not interpret them.

Windows and DOS[edit]

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[4] impossible to do any kind of full-screen application. Any display effects had to be done with BIOS calls (or far more often 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,[5] NANSI.SYS[6] 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.[citation 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.

The Win32 console does not support ANSI escape sequences at all. Some alternate command processors such as JP Software's TCC (formerly 4NT) and Michael J. Mefford's ANSI.COM do interpret ANSI escape sequences printed by programs. Software such as Jason Hood's ANSICON[7] can act as a wrapper around the standard Win32 console and add support for ANSI escape sequences.

Software can manipulate the color and cursor position in the command output window with the ioctl-like Console API[8] interlaced with the text output (ie you must flush the text output stream before each call). Some software internally interprets ANSI escape sequences in text being printed and translates them to these calls.[9]

AmigaOS[edit]

All versions of AmigaOS on the Amiga computer interpret ANSI code sequences for text output to the screen. The AmigaOS printer driver also interprets ANSI code sequences (with extensions proprietary to AmigaOS) and translates them into the codes required for the particular printer that is actually attached.[10]

Sequence elements[edit]

Escape sequences start with the character ESC (ASCII decimal 27/hex 0x1B/octal 033). For two character sequences, the second character is in the range ASCII 6495 (@ to _).

However, most of the sequences are more than two characters, and start with the characters ESC and [ (left bracket). This sequence is called CSI for Control Sequence Introducer (or Control Sequence Initiator). The final character of these sequences is in the range ASCII 64126 (@ to ~).

There is a single-character CSI (155/0x9B /0233) as well. The ESC[ two-character sequence is more often used than the single-character alternative (for details see C0 and C1 control codes).

Only the two-character sequence is recognized by devices that support just ASCII (7-bit bytes) or devices that support 8-bit bytes, but use the 0x800x9F control character range for other purposes. On terminals that use UTF-8 encoding, both forms take 2 bytes (CSI in UTF-8 is 0xC2, 0x9B)[discuss] but the ESC[ sequence is clearer.

Though some encodings use multiple bytes per character, the following discussion is restricted to ASCII characters, and thus assumes a single byte for each character.

Non-CSI codes[edit]

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

ESC N = SS2
ESC O = SS3
Select a single character from one of the alternate character sets.
ESC ^ = PM
ESC _ = APC
These each take a single string of text, terminated by ST (ESC \ ). They are ignored by xterm.
ESC P = DCS
Device control string,
ESC ] = OSC
Operating system command — these are similar to CSI, but not limited to integer arguments. Because they are frequently used,[citation needed] in many cases BEL is an acceptable alternative to ST. E.g., in xterm, the window title can be set by: "OSC0;this is the window titleBEL"

Note: 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.

CSI codes[edit]

The general structure of most ANSI escape sequences is CSI [private mode character(s?)] n1 ; n2... [trailing intermediate character(s?)] letter. The final byte, modified by private mode characters and trailing intermediate characters, specifies the command. The numbers are optional parameters. The default value used for omitted parameters varies with the command, but is usually 1 or 0. If trailing parameters are omitted, the trailing semicolons may also be omitted.

The final byte is technically any character in the range 64126 (hex 0x400x7E, ASCII @ to ~), and may be modified with leading intermediate bytes in the range 32 to 47 (hex 0x20 to 0x2F).

The colon (0x3A) is the only character not a part of the general sequence. It was left for future standardization, so any sequence containing it should be ignored.

Although multiple private mode characters or trailing intermediates are permitted, there are no such known usages.[citation needed]

If there are any leading private mode characters, the main body of the sequence could theoretically contain any order of characters 0x300x3F instead of a well-formed semicolon-separated list of numbers,[citation needed] but all known terminals are nice and just use them as a flag.[citation needed] Sequences are also private if the final byte is in the range 112126 (hex 0x700x7E, ASCII p~).

Examples of private escape codes include the DECTCEM (DEC text cursor enable mode) shown below. It was first introduced for the VT-300 series of video terminals.

The existence of a C0 control, DEL (0x7F), or a high characters is undefined.[clarification needed] Typically, implementations will either cancel the sequence or execute the control, and then continue parsing the CSI sequence.[clarification needed]

Some ANSI escape sequences (not a complete list)
Code Name Effect

CSI n A CUU – Cursor Up Moves the cursor n (default 1) cells in the given direction. If the cursor is already at the edge of the screen, this has no effect.

CSI n B CUD – Cursor Down

CSI n C CUF – Cursor Forward

CSI n D CUB – Cursor Back
CSI n E CNL – Cursor Next Line Moves cursor to beginning of the line n (default 1) lines down. (not ANSI.SYS)
CSI n F CPL – Cursor Previous Line Moves cursor to beginning of the line n (default 1) lines up. (not ANSI.SYS)
CSI n G CHA – Cursor Horizontal Absolute Moves the cursor to column n. (not ANSI.SYS)

CSI n ; m H CUP – Cursor Position Moves the cursor to row n, column m. The values are 1-based, and default to 1 (top left corner) if omitted. A sequence such as CSI ;5H is a synonym for CSI 1;5H as well as CSI 17;H is the same as CSI 17H and CSI 17;1H

CSI n J ED – Erase Display Clears part of the screen. If n is 0 (or missing), clear from cursor to end of screen. If n is 1, clear from cursor to beginning of the screen. If n is 2, clear entire screen (and moves cursor to upper left on DOS ANSI.SYS).

CSI n K EL – Erase in Line Erases part of the line. If n is zero (or missing), clear from cursor to the end of the line. If n is one, clear from cursor to beginning of the line. If n is two, clear entire line. Cursor position does not change.

CSI n S SU – Scroll Up Scroll whole page up by n (default 1) lines. New lines are added at the bottom. (not ANSI.SYS)

CSI n T SD – Scroll Down Scroll whole page down by n (default 1) lines. New lines are added at the top. (not ANSI.SYS)

CSI n ; m f HVP – Horizontal and Vertical Position Moves the cursor to row n, column m. Both default to 1 if omitted. Same as CUP

CSI n m SGR – Select Graphic Rendition Sets SGR parameters, including text color. After CSI can be zero or more parameters separated with ;. With no parameters, CSI m is treated as CSI 0 m (reset / normal), which is typical of most of the ANSI escape sequences.

CSI 5i AUX Port On Enable aux serial port usually for local serial printer

CSI 4i AUX Port Off Disable aux serial port usually for local serial printer

CSI 6 n DSR – Device Status Report Reports the cursor position (CPR) to the application as (as though typed at the keyboard) ESC[n;mR, where n is the row and m is the column.)

CSI s SCP – Save Cursor Position Saves the cursor position.

CSI u RCP – Restore Cursor Position Restores the cursor position.
CSI ?25l DECTCEM Hides the cursor. (Note: the trailing character is lowercase L.)
CSI ?25h DECTCEM Shows the cursor.
SGR (Select Graphic Rendition) parameters
Code Effect Note
0 Reset / Normal all attributes off
1 Bold or increased intensity
2 Faint (decreased intensity) Not widely supported.
3 Italic: on Not widely supported. Sometimes treated as inverse.
4 Underline: Single
5 Blink: Slow less than 150 per minute
6 Blink: Rapid MS-DOS ANSI.SYS; 150+ per minute; not widely supported
7 Image: Negative inverse or reverse; swap foreground and background (reverse video)
8 Conceal Not widely supported.
9 Crossed-out Characters legible, but marked for deletion. Not widely supported.
10 Primary(default) font
11–19 n-th alternate font Select the n-th alternate font (14 being the fourth alternate font, up to 19 being the 9th alternate font).
20 Fraktur hardly ever supported
21 Bold: off or Underline: Double Bold off not widely supported; double underline hardly ever supported.
22 Normal color or intensity Neither bold nor faint
23 Not italic, not Fraktur
24 Underline: None Not singly or doubly underlined
25 Blink: off
26 Reserved
27 Image: Positive
28 Reveal conceal off
29 Not crossed out
30–37 Set text color (foreground) 30 + x, where x is from the color table below
38 Reserved for extended set foreground color typical supported next arguments are 5;x where x is color index (0..255) or 2;r;g;b where r,g,b are red, green and blue color channels (out of 255)
39 Default text color (foreground) implementation defined (according to standard)
40–47 Set background color 40 + x, where x is from the color table below
48 Reserved for extended set background color typical supported next arguments are 5;x where x is color index (0..255) or 2;r;g;b where r,g,b are red, green and blue color channels (out of 255)
49 Default background color implementation defined (according to standard)
50 Reserved
51 Framed
52 Encircled
53 Overlined
54 Not framed or encircled
55 Not overlined
56–59 Reserved
60 ideogram underline or right side line hardly ever supported
61 ideogram double underline or double line on the right side hardly ever supported
62 ideogram overline or left side line hardly ever supported
63 ideogram double overline or double line on the left side hardly ever supported
64 ideogram stress marking hardly ever supported
65 ideogram attributes off hardly ever supported, reset the effects of all of 6064
90–97 Set foreground text color, high intensity aixterm (not in standard)
100–107 Set background color, high intensity aixterm (not in standard)

Colors[edit]

Text colors (and SGR parameters in general) are manipulated using CSI n1 [;n2 [; ...]] m sequences, where each n1, n2, ... is an SGR parameter as shown above. Thus, for instance, you use codes 30+i to specify foreground color, 40+i to specify background color, where i is the number in the desired color's column header in the table below. The following examples can be used with the printf utility, where \x1b[ implements the CSI: To switch the foreground color to black, use \x1b[30m; to switch to red, use \x1b[31m; utilizing the "bold" parameter, gray would be \x1b[30;1m; to get bold red, use \x1b[31;1m. To reset colors to their defaults, use \x1b[39;49m (or reset all attributes with \x1b[0m).

Color table[11]
Intensity 0 1 2 3 4 5 6 7
Normal Black Red Green Yellow[12] Blue Magenta Cyan White
Bright Black Red Green Yellow Blue Magenta Cyan White

There are two other color standards CSS/HTML standard colors and X Window colors which standardize both the color names and associated RGB color values, but the escape sequence standard only specifies the color names, not RGB values. The chart below shows default RGB assignments for some common terminal programs, together with the CSS and the X Window System colors for these color names.[citation needed]

Color name Standard VGA colors Windows XP CMD Terminal.app PuTTY xterm CSS/HTML X
Normal Black 0, 0, 0 0, 0, 0 0, 0, 0 0, 0, 0 0, 0, 0 0, 0, 0 0, 0, 0
Red 170, 0, 0 128, 0, 0 194, 54, 33 187, 0, 0 205, 0, 0 255, 0, 0 255, 0, 0
Green 0, 170, 0 0, 128, 0 37, 188, 36 0, 187, 0 0, 205, 0 0, 255, 0 0, 128, 0
Brown/yellow 170, 85, 0 128, 128, 0 173, 173, 39 187, 187, 0 205, 205, 0 255, 255, 0 255, 255, 0
Blue 0, 0, 170 0, 0, 128 73, 46, 225 0, 0, 187 0, 0, 238 0, 0, 255 0, 0, 255
Magenta 170, 0, 170 128, 0, 128 211, 56, 211 187, 0, 187 205, 0, 205 255, 0, 255 255, 0, 255
Cyan 0, 170, 170 0, 128, 128 51, 187, 200 0, 187, 187 0, 205, 205 0, 255, 255 0, 255, 255
Gray 170, 170, 170 192, 192, 192 203, 204, 205 187, 187, 187 229, 229, 229 255, 255, 255 255, 255, 255
Bright/light Darkgray 85, 85, 85 128, 128, 128 129, 131, 131 85, 85, 85 127, 127, 127
Red 255, 85, 85 255, 0, 0 252,57,31 255, 85, 85 255, 0, 0
Green 85, 255, 85 0, 255, 0 49, 231, 34 85, 255, 85 0, 255, 0 144, 238, 144 144, 238, 144
Yellow 255, 255, 85 255, 255, 0 234, 236, 35 255, 255, 85 255, 255, 0 255, 255, 224 225, 255, 224
Blue 85, 85, 255 0, 0, 255 88, 51, 255 85, 85, 255 92, 92, 255 173, 216, 230 173, 216, 230
Magenta 255, 85, 255 255, 0, 255 249, 53, 248 255, 85, 255 255, 0, 255
Cyan 85, 255, 255 0, 255, 255 20, 240, 240 85, 255, 255 0, 255, 255 224, 255, 255 224, 255, 255
White 255, 255, 255 255, 255, 255 233, 235, 235 255, 255, 255 255, 255, 255

The VGA column denotes the typical colors that are used when booting PCs and leaving them in their classical 80×25 text mode. The colors are different in the EGA/VGA graphic modes.

In July 2004, the blue colors of xterm changed,[13] RGB (0,0,205) → (0,0,238) for normal and (0,0,255) → (92,92,255) for bright. As of 2010, old xterm versions still linger on many computers though.[citation needed]

Xterm[14] and recent-enough versions of KDE's Konsole program support ISO-8613-3 24-bit foreground and background color setting[15][better source needed] Quoting one of the text-files in its source-tree:[16][better source needed]

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

In 256 color mode (ESC[38;5;<fgcode>m and ESC[48;5;<bgcode>m), the color-codes are the following:[citation needed]

   0x00-0x07:  standard colors (as in ESC [ 30..37 m)
   0x08-0x0F:  high intensity colors (as in ESC [ 90..97 m)
   0x10-0xE7:  6*6*6=216 colors: 16 + 36*r + 6*g + b (0≤r,g,b≤5)
   0xE8-0xFF:  grayscale from black to white in 24 steps

Xterm allows also to set the default foreground and background colors using[17]

   ESC]10;<foreground>BEL
   ESC]11;<background>BEL

where <foreground> and <background> are X color specifications, and BEL is the ASCII BEL character (code 7). The closing bracket instead of an opening bracket reveals that it belongs to the operating system control commands.

Examples[edit]

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. On MS-DOS, normally the green would be 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. MS-DOS ANSI.SYS uses the Bold state to make the character Bright; also the Blink state can be set (via INT 10, AX 1003h, BL 00h) to render the Background in the Bright mode. MS-DOS ANSI.SYS does not support SGR codes 90–97 and 100–107 directly.

CSI 0 ; 6 8 ; "DIR" ; 13 p — This re-assigns 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[edit]

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 sed command embellishes the output of the make command by displaying lines containing words starting with "ERR" in reverse video and words starting with "WARN" in bold (letter case is ignored).

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

The representations of the codes are highlighted.[18]

Invalid and ambiguous sequences in use[edit]

  • 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.[citation needed]
  • 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.[clarification needed]
  • 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.[clarification needed]
  • Konsole generates SS3 modifiers char when F1–F4 are pressed with modifiers.[clarification needed]

See also[edit]

Notes[edit]

  1. ^ See this NIST list of withdrawn standards
  2. ^ Paul Williams (2006). "Digital's Video Terminals". VT100.net. Retrieved 17 August 2011. 
  3. ^ Heathkit Company (1979). "Heathkit Catalog 1979". Heathkit Company. Retrieved 4 November 2011. 
  4. ^ The screen display could be replaced by drawing the entire new screen's contents at the bottom, scrolling the previous screen up sufficiently to erase all the old text. The user would see the scrolling, and the hardware cursor would be left at the very bottom. Some early batch files achieved rudimentary "full screen" displays in this way.
  5. ^ Michael Mefford (7 February 1989). "ANSI.com: Download It Here". PC Magazine. Retrieved 10 August 2011. 
  6. ^ Dan Kegel, Eric Auer (28 February 1999). "Nansi and NNansi - ANSI Drivers for MS-DOS". Dan Kegel's Web Hostel. Retrieved 10 August 2011. 
  7. ^ Jason Hood (2005). "Process ANSI escape sequences for Windows console programs". Jason Hood's Home page. Retrieved 9 May 2013. 
  8. ^ "Console Reference (Windows)". Microsoft. Retrieved 17 August 2013. 
  9. ^ "colorama 0.2.5 :". Python Package Index. Retrieved 17 August 2013. 
  10. ^ "Amiga Printer Command Definitions". Commodore. Retrieved 10 July 2013. 
  11. ^ The names are standard, however the exact shade/hue/value of colors are not standardized and will depend on the device used to display them.
  12. ^ On terminals based on CGA compatible hardware, such as ANSI.SYS running on DOS, this normal intensity foreground color is rendered as Orange. CGA RGBI monitors contained hardware to modify the dark yellow color to an orange/brown color by reducing the green component. See this ansi art as an example.
  13. ^ "Patch #192 - 2004/7/12 - XFree86 4.4.99.9". 
  14. ^ "XTerm Control Sequences". invisible-island.net. 13 January 2014. Retrieved 13 April 2014. 
  15. ^ "color-spaces.pl (a copy of 256colors2.pl from xterm dated 11 July 1999)". KDE. 6 December 2006. 
  16. ^ "README.moreColors". KDE. 22 April 2010. 
  17. ^ http://invisible-island.net/xterm/ctlseqs/ctlseqs.html
  18. ^ Colorized shell echo

External links[edit]