Linux Console Controls¶

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.

Control characters

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, and if LF/NL (new-line mode) is set also a carriage return;

CR (0x0D, ^M) gives a carriage return;

SO (0x0E, ^N) activates the G1 character set;

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 [ parameters m sets display attributes. Several attributes can be set in the same sequence, separated by semicolons. An empty parameter (between semicolons or string initiator or terminator) is interpreted as a zero.

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 autorepeat on.

ESC [ ? 9 h

X10 Mouse Reporting (default off): Set reporting mode to 1 (or reset to 0) — see below.

ESC [ ? 25 h

DECTECM (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 following sequences are neither ECMA-48 nor native VT102. They are native to the Linux console driver. Colors are in SGR parameters: 0 = black, 1 = red, 2 = green, 3 = brown, 4 = blue, 5 = magenta, 6 = cyan, 7 = white.

Character Sets¶

The kernel knows about 4 translations of bytes into console-screen symbols. The four tables are: a) Latin1 -> PC, b) VT100 graphics -> PC, c) PC -> PC, d) user-defined.

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).

Mouse Tracking¶

The mouse tracking facility is intended to return xterm(1)-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.

The mouse tracking escape sequences generated by xterm(1) 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(1) 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(1) 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).

Comparisons With Other Terminals¶

Many different terminal types are described, like the Linux console, as being "VT100-compatible". Here we discuss differences between the Linux console and the two most important others, the DEC VT102 and xterm(1).

Control-character handling

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(1) program (in VT100 mode) recognizes the control characters BEL, BS, HT, LF, VT, FF, CR, SO, SI, ESC.

Escape sequences

VT100 console sequences not implemented on the Linux console:

The program xterm(1) (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(1) 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(1) accepts a BEL to terminate an OSC string. These are a few of the OSC control sequences recognized by xterm(1):

It recognizes the following with slightly modified meaning (saving more state, behaving closer to VT100/VT220):

It also recognizes

It also recognizes ESC % and provides a more complete UTF-8 implementation than Linux console.

CSI Sequences

Old versions of xterm(1), for example, from X11R5, interpret the blink SGR as a bold SGR. Later versions which implemented ANSI colors, for example, XFree86 3.1.2A in 1995, improved this by allowing the blink attribute to be displayed as a color. Modern versions of xterm implement blink SGR as blinking text and still allow colored text as an alternate rendering of SGRs. Stock X11R6 versions did not recognize the color-setting SGRs until the X11R6.8 release, which incorporated XFree86 xterm. All ECMA-48 CSI sequences recognized by Linux are also recognized by xterm, however xterm(1) implements several ECMA-48 and DEC control sequences not recognized by Linux.

The xterm(1) program recognizes all of the DEC Private Mode sequences listed above, but none of the Linux private-mode sequences. For discussion of xterm(1)'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,

http://invisible-island.net/xterm/xterm.log.html

details changes to xterm.

The vttest program

http://invisible-island.net/vttest/

demonstrates many of these control sequences. The xterm(1) source distribution also contains sample scripts which exercise other features.