The Linux console implements a large subset of the VT102 and ECMA-48/ISO 6429/ANSI X3.64 terminal controls, plus certain private-mode sequences for changing the color palette, character-set mapping, etc. In the tabular descriptions below, the second column gives ECMA-48 or DEC mnemonics (the latter if prefixed with DEC) for the given function. Sequences without a mnemonic are neither ECMA-48 nor VT102.
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.
This section describes all the control characters and escape sequences that invoke special functions (i.e. anything other than writing a glyph at the current cursor location) on the Linux console.
A character is a control character if (before transformation according to the mapping table) it has one of the 14 codes 00 (NUL), 07 (BEL), 08 (BS), 09 (HT), 0a (LF), 0b (VT), 0c (FF), 0d (CR), 0e (SO), 0f (SI), 18 (CAN), 1a (SUB), 1b (ESC), 7f (DEL). One can set a `display control characters' mode (see below), and allow 07, 09, 0b, 18, 1a, 7f to be displayed as glyphs. On the other hand, in UTF-8 mode all codes 00-1f are regarded as control characters, regardless of any `display control characters' mode.
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:
BEL (0x07, ^G) beeps;
BS (0x08, ^H) backspaces one column (but not past the beginning of the line);
HT (0x09, ^I) goes to the next tab stop or to the end of the line if there is no earlier tab stop;
LF (0x0A, ^J), VT (0x0B, ^K) and FF (0x0C, ^L) all give a linefeed;
CR (0x0D, ^M) gives a carriage return;
SO (0x0E, ^N) activates the G1 character set, and if LF/NL (new line mode) is set also a carriage return;
SI (0x0F, ^O) activates the G0 character set;
CAN (0x18, ^X) and SUB (0x1A, ^Z) interrupt escape sequences;
ESC (0x1B, ^[[) starts an escape sequence;
DEL (0x7F) is ignored;
CSI (0x9B) is equivalent to ESC [[.
ESC- but not CSI-sequences
ECMA-48 CSI sequences
CSI (or ESC [[) is followed by a sequence of parameters, at most NPAR (16), that are decimal numbers separated by semicolons. An empty or absent parameter is taken to be 0. The sequence of parameters may be preceded by a single question mark.
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.
ECMA-48 Set Graphics Rendition
The ECMA-48 SGR sequence ESC [[
ECMA-48 Mode Switches
ESC [[ 3 h
DECCRM (default off): Display control chars.
ESC [[ 4 h
DECIM (default off): Set insert mode.
ESC [[ 20 h
LF/NL (default off): Automatically follow echo of LF, VT or FF with CR.
ECMA-48 Status Report Commands
ESC [[ 5 n
Device status report (DSR): Answer is ESC [[ 0 n (Terminal OK).
ESC [[ 6 n
Cursor position report (CPR): Answer is ESC [[ y ; x R, where x,y is the cursor location.
DEC Private Mode (DECSET/DECRST) sequences.
These are not described in ECMA-48. We list the Set Mode sequences; the Reset Mode sequences are obtained by replacing the final `h' by `l'.
ESC [[ ? 1 h
DECCKM (default off): When set, the cursor keys send an ESC O prefix, rather than ESC [[.
ESC [[ ? 3 h
DECCOLM (default off = 80 columns): 80/132 col mode switch. The driver sources note that this alone does not suffice; some user-mode utility such as resizecons(8) has to change the hardware registers on the console video card.
ESC [[ ? 5 h
DECSCNM (default off): Set reverse-video mode.
ESC [[ ? 6 h
DECOM (default off): When set, cursor addressing is relative to the upper left corner of the scrolling region.
ESC [[ ? 7 h
DECAWM (default on): Set autowrap on. In this mode, a graphic character emitted after column 80 (or column 132 of DECCOLM is on) forces a wrap to the beginning of the following line first.
ESC [[ ? 8 h
DECARM (default on): Set keyboard autorepreat on.
ESC [[ ? 9 h
X10 Mouse Reporting (default off): Set reporting mode to 1 (or reset to 0) - see below.
ESC [[ ? 25 h
DECCM (default on): Make cursor visible.
ESC [[ ? 1000 h
X11 Mouse Reporting (default off): Set reporting mode to 2 (or reset to 0) - see below.
Linux Console Private CSI Sequences
The kernel knows about 4 translations of bytes into console-screen symbols. The four tables are: a) Latin1 -
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 reset__(1)? that just does __
The mouse tracking facility is intended to return xterm-compatible mouse status reports. Because the console driver has no way to know the device or type of the mouse, these reports are returned in the console input stream only when the virtual terminal driver receives a mouse update ioctl. These ioctls must be generated by a mouse-aware user-mode application such as the __gpm(8)?__ daemon.
Parameters for all 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.
Many different terminal types are described, like the Linux console, as being `VT100-compatible'. Here we discuss differences vbetween the Linux console an the two most important others, the DEC VT102 and xterm(1).
The vt102 also recognized the following control characters:
NUL (0x00) was ignored;
ENQ (0x05) triggered an answerback message;
DC1 (0x11, ^Q, XON) resumed transmission;
DC3 (0x13, ^S, XOFF) caused vt100 to ignore (and stop transmitting) all codes except XOFF and XON.
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.
VT100 console sequences not implemented on the Linux console:
The program xterm (in vt100 mode) recognizes ESC c, ESC # 8, ESC ''
It recognizes the following with slightly modified meaning:
It also recognizes
It does not recognize ESC % ...
The xterm program (as of XFree86 3.1.2G) does not recognize the blink or invisible-mode SGRs. Stock X11R6 versions do not recognize the color-setting SGRs. All other ECMA-48 CSI sequences recognized by Linux are also recognized by xterm, and vice-versa.