After all the normal output processing has been done, and a stream of characters arrives at the console driver for actual printing, the first thing that happens is a translation from the code used for processing to the code used for printing.
If the console is in UTF-8 mode, then the incoming bytes are first assembled into 16-bit Unicode codes. Otherwise each byte is transformed according to the current mapping table (which translates it to a Unicode value). See the CHARACTER SETS section below for discussion.
In the normal case, the Unicode value is converted to a font index, and this is stored in video memory, so that the corresponding glyph (as found in video ROM) appears on the screen. Note that the use of Unicode (and the design of the PC hardware) allows us to use 512 different glyphs simultaneously.
If the current Unicode value is a control character, or we are currently processing an escape sequence, the value will treated specially. Instead of being turned into a font index and rendered as a glyph, it may trigger cursor movement or other control functions. See the LINUX CONSOLE CONTROLS section below for discussion.
It is generally not good practice to hard-wire terminal controls into programs. Linux supports a terminfo(5) database of terminal capabilities. Rather than emitting console escape sequences by hand, you will almost always want to use a terminfo-aware screen library or utility such as ncurses(3), tput(1), or reset(1).
If we have a control character, it is acted upon immediately and then discarded (even in the middle of an escape sequence) and the escape sequence continues with the next character. (However, ESC starts a new escape sequence, possibly aborting a previous unfinished one, and CAN and SUB abort any escape sequence.) The recognized control characters are BEL, BS, HT, LF, VT, FF, CR, SO, SI, CAN, SUB, ESC, DEL, CSI. They do what one would expect:
|ESC H||HTS||Set tab stop at current column.|
|ESC M||RI||Reverse linefeed.|
DEC private identification. The kernel
returns the string ESC [ ? 6 c, claiming
that it is a VT102.
Save current state (cursor coordinates,
attributes, character sets pointed at by G0, G1).
|ESC 8||DECRC||Restore state most recently saved by ESC 7.|
|ESC [||CSI||Control sequence introducer|
|ESC %||Start sequence selecting character set|
|ESC % @|| Select default (ISO 646 / ISO 8859-1)|
|ESC % G|| Select UTF-8|
|ESC % 8|| Select UTF-8 (obsolete)|
|ESC # 8||DECALN||DEC screen alignment test - fill screen with E's.|
|ESC (||Start sequence defining G0 character set|
|ESC ( B|| Select default (ISO 8859-1 mapping)|
|ESC ( 0|| Select VT100 graphics mapping|
|ESC ( U|| Select null mapping - straight to character ROM|
|ESC ( K|| Select user mapping - the map that is loaded by|
| the utility mapscrn(8).|
|ESC )||Start sequence defining G1|
|(followed by one of B, 0, U, K, as above).|
|ESC >||DECPNM||Set numeric keypad mode|
|ESC =||DECPAM||Set application keypad mode|
(Should be: Operating system command)
ESC ] P nrrggbb: set palette, with parameter
given in 7 hexadecimal digits after the final P :-(.
Here n is the color (0-15), and rrggbb indicates
the red/green/blue values (0-255).
ESC ] R: reset palette
However, after CSI [ (or ESC [ [) a single character is read and this entire sequence is ignored. (The idea is to ignore an echoed function key.)
The action of a CSI sequence is determined by its final character.
|@||ICH||Insert the indicated # of blank characters.|
|A||CUU||Move cursor up the indicated # of rows.|
|B||CUD||Move cursor down the indicated # of rows.|
|C||CUF||Move cursor right the indicated # of columns.|
|D||CUB||Move cursor left the indicated # of columns.|
|E||CNL||Move cursor down the indicated # of rows, to column 1.|
|F||CPL||Move cursor up the indicated # of rows, to column 1.|
|G||CHA||Move cursor to indicated column in current row.|
|H||CUP||Move cursor to the indicated row, column (origin at 1,1).|
|J||ED||Erase display (default: from cursor to end of display).|
|ESC [ 1 J: erase from start to cursor.|
|ESC [ 2 J: erase whole display.|
|K||EL||Erase line (default: from cursor to end of line).|
|ESC [ 1 K: erase from start of line to cursor.|
|ESC [ 2 K: erase whole line.|
|L||IL||Insert the indicated # of blank lines.|
|M||DL||Delete the indicated # of lines.|
|P||DCH||Delete the indicated # of characters on the current line.|
|X||ECH||Erase the indicated # of characters on the current line.|
|a||HPR||Move cursor right the indicated # of columns.|
|c||DA||Answer ESC [ ? 6 c: `I am a VT102'.|
|d||VPA||Move cursor to the indicated row, current column.|
|e||VPR||Move cursor down the indicated # of rows.|
|f||HVP||Move cursor to the indicated row, column.|
|g||TBC||Without parameter: clear tab stop at the current position.|
|ESC [ 3 g: delete all tab stops.|
|h||SM||Set Mode (see below).|
|l||RM||Reset Mode (see below).|
|m||SGR||Set attributes (see below).|
|n||DSR||Status report (see below).|
|q||DECLL||Set keyboard LEDs.|
|ESC [ 0 q: clear all LEDs|
|ESC [ 1 q: set Scroll Lock LED|
|ESC [ 2 q: set Num Lock LED|
|ESC [ 3 q: set Caps Lock LED|
|r||DECSTBM||Set scrolling region; parameters are top and bottom row.|
|s||?||Save cursor location.|
|u||?||Restore cursor location.|
|`||HPA||Move cursor to indicated column in current row.|
|0||reset all attributes to their defaults|
|2||set half-bright (simulated with color on a color display)|
set underscore (simulated with color on a color display)
(the colors used to simulate dim or underline are set
using ESC ] ...)
|7||set reverse video|
reset selected mapping, display control flag,
and toggle meta flag (ECMA-48 says "primary font").
select null mapping, set display control flag,
reset toggle meta flag (ECMA-48 says "first alternate font").
select null mapping, set display control flag,
set toggle meta flag (ECMA-48 says "second alternate font").
The toggle meta flag
causes the high bit of a byte to be toggled
before the mapping table translation is done.
|21||set normal intensity (ECMA-48 says "doubly underlined")|
|22||set normal intensity|
|27||reverse video off|
|30||set black foreground|
|31||set red foreground|
|32||set green foreground|
|33||set brown foreground|
|34||set blue foreground|
|35||set magenta foreground|
|36||set cyan foreground|
|37||set white foreground|
|38||set underscore on, set default foreground color|
|39||set underscore off, set default foreground color|
|40||set black background|
|41||set red background|
|42||set green background|
|43||set brown background|
|44||set blue background|
|45||set magenta background|
|46||set cyan background|
|47||set white background|
|49||set default background color|
|ESC [ 1 ; n ]||Set color n as the underline color|
|ESC [ 2 ; n ]||Set color n as the dim color|
|ESC [ 8 ]||Make the current color pair the default attributes.|
|ESC [ 9 ; n ]||Set screen blank timeout to n minutes.|
|ESC [ 10 ; n ]||Set bell frequency in Hz.|
|ESC [ 11 ; n ]||Set bell duration in msec.|
|ESC [ 12 ; n ]||Bring specified console to the front.|
|ESC [ 13 ]||Unblank the screen.|
|ESC [ 14 ; n ]||Set the VESA powerdown interval in minutes.|
There are two character sets, called G0 and G1, and one of them is the current character set. (Initially G0.) Typing ^N causes G1 to become current, ^O causes G0 to become current.
These variables G0 and G1 point at a translation table, and can be changed by the user. Initially they point at tables a) and b), respectively. The sequences ESC ( B and ESC ( 0 and ESC ( U and ESC ( K cause G0 to point at translation table a), b), c) and d), respectively. The sequences ESC ) B and ESC ) 0 and ESC ) U and ESC ) K cause G1 to point at translation table a), b), c) and d), respectively.
The sequence ESC c causes a terminal reset, which is what you want if the screen is all garbled. The oft-advised "echo ^V^O" will only make G0 current, but there is no guarantee that G0 points at table a). In some distributions there is a program reset(1) that just does "echo ^[c". If your terminfo entry for the console is correct (and has an entry rs1=\Ec), then "tput reset" will also work.
The user-defined mapping table can be set using mapscrn(8). The result of the mapping is that if a symbol c is printed, the symbol s = map[c] is sent to the video memory. The bitmap that corresponds to s is found in the character ROM, and can be changed using setfont(8).
The mouse tracking escape sequences generated by xterm encode numeric parameters in a single character as value+040. For example, `!' is 1. The screen coordinate system is 1-based.
The X10 compatibility mode sends an escape sequence on button press encoding the location and the mouse button pressed. It is enabled by sending ESC [ ? 9 h and disabled with ESC [ ? 9 l. On button press, xterm sends ESC [ M bxy (6 characters). Here b is button-1, and x and y are the x and y coordinates of the mouse when the button was pressed. This is the same code the kernel also produces.
Normal tracking mode (not implemented in Linux 2.0.24) sends an escape sequence on both button press and release. Modifier information is also sent. It is enabled by sending ESC [ ? 1000 h and disabled with ESC [ 1000 l. On button press or release, xterm sends ESC [ M bxy. The low two bits of b encode button information: 0=MB1 pressed, 1=MB2 pressed, 2=MB3 pressed, 3=release. The upper bits encode what modifiers were down when the button was pressed and are added together: 4=Shift, 8=Meta, 16=Control. Again x and y are the x and y coordinates of the mouse event. The upper left corner is (1,1).
VT100-like DC1/DC3 processing may be enabled by the tty driver.
The xterm program (in VT100 mode) recognizes the control characters BEL, BS, HT, LF, VT, FF, CR, SO, SI, ESC.
|ESC N||SS2||Single shift 2. (Select G2 character set for the next|
|ESC O||SS3||Single shift 3. (Select G3 character set for the next|
|ESC P||DCS||Device control string (ended by ESC \)|
|ESC X||SOS||Start of string.|
|ESC ^||PM||Privacy message (ended by ESC \)|
|ESC \||ST||String terminator|
|ESC * ...||Designate G2 character set|
|ESC + ...||Designate G3 character set|
The program xterm (in VT100 mode) recognizes ESC c, ESC # 8, ESC >, ESC =, ESC D, ESC E, ESC H, ESC M, ESC N, ESC O, ESC P ... ESC , ESC Z (it answers ESC [ ? 1 ; 2 c, `I am a VT100 with advanced video option') and ESC ^ ... ESC with the same meanings as indicated above. It accepts ESC (, ESC ), ESC *, ESC + followed by 0, A, B for the DEC special character and line drawing set, UK, and US-ASCII, respectively.
The user can configure xterm to respond to VT220-specific control sequences, and it will identify itself as a VT52, VT100, and up depending on the way it is configured and initialized.
It accepts ESC ] (OSC) for the setting of certain resources. In addition to the ECMA-48 string terminator (ST), xterm accepts a BEL to terminate an OSC string. These are a few of the OSC control sequences recognized by xterm:
|ESC ] 0 ; txt ST||Set icon name and window title to txt.|
|ESC ] 1 ; txt ST||Set icon name to txt.|
|ESC ] 2 ; txt ST||Set window title to txt.|
|ESC ] 4 ; num; txt ST||Set ANSI color num to txt.|
|ESC ] 10 ; txt ST||Set dynamic text color to txt.|
|ESC ] 4 6 ; name ST||Change log file to name (normally disabled|
|by a compile-time option)|
|ESC ] 5 0 ; fn ST||Set font to fn.|
It recognizes the following with slightly modified meaning (saving more state, behaving closer to VT100/VT220):
|ESC 7 DECSC||Save cursor|
|ESC 8 DECRC||Restore cursor|
It also recognizes
|ESC F||Cursor to lower left corner of screen (if enabled by|
|xterm's hpLowerleftBugCompat resource)|
|ESC l||Memory lock (per HP terminals).|
|Locks memory above the cursor.|
|ESC m||Memory unlock (per HP terminals).|
|ESC n||LS2||Invoke the G2 character set.|
|ESC o||LS3||Invoke the G3 character set.|
|ESC |||LS3R||Invoke the G3 character set as GR.|
|ESC }||LS2R||Invoke the G2 character set as GR.|
|ESC ~||LS1R||Invoke the G1 character set as GR.|
It also recognizes ESC % and provides a more complete UTF-8 implementation than Linux console.
The xterm program recognizes all of the DEC Private Mode sequences listed above, but none of the Linux private-mode sequences. For discussion of xterm's own private-mode sequences, refer to the Xterm Control Sequences document by Edward Moy, Stephen Gildea, and Thomas E. Dickey available with the X distribution. That document, though terse, is much longer than this manual page. For a chronological overview,
details changes to xterm.
The vttest program
demonstrates many of these control sequences. The xterm source distribution also contains sample scripts which exercise other features.
Some older kernel versions (after 2.0) interpret 8-bit control sequences. These "C1 controls" use codes between 128 and 159 to replace ESC [, ESC ] and similar two-byte control sequence initiators. There are fragments of that in modern kernels (either overlooked or broken by changes to support UTF-8), but the implementation is incomplete and should be regarded as unreliable.
Linux "private mode" sequences do not follow the rules in ECMA-48 for private mode control sequences. In particular, those ending with ] do not use a standard terminating character. The OSC (set palette) sequence is a greater problem, since xterm may interpret this as a control sequence which requires a string terminator (ST). Unlike the setterm sequences which will be ignored (since they are invalid control sequences), the palette sequence will make xterm appear to hang (though pressing the return-key will fix that). To accommodate applications which have been hardcoded to use Linux control sequences, set the xterm resource brokenLinuxOSC to true.
An older version of this document implied that Linux recognizes the ECMA-48 control sequence for invisible text. It is ignored.