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).
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.
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 c | RIS | Reset. |
| ESC D | IND | Linefeed. |
| ESC E | NEL | Newline. |
| ESC H | HTS | Set tab stop at current column. |
| ESC M | RI | Reverse linefeed. |
| ESC Z | DECID | DEC private identification. The kernel |
| returns the string ESC [ ? 6 c, claiming | ||
| that it is a VT102. | ||
| ESC 7 | DECSC | 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 |
| ESC ] | OSC | (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 |
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.
| @ | 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. |
The ECMA-48 SGR sequence ESC [ <parameters> m sets display attributes. Several attributes can be set in the same sequence.
| par | result |
| 0 | reset all attributes to their defaults |
| 1 | set bold |
| 2 | set half-bright (simulated with color on a color display) |
| 4 | set underscore (simulated with color on a color display) |
| (the colors used to simulate dim or underline are set | |
| using ESC ] ...) | |
| 5 | set blink |
| 7 | set reverse video |
| 10 | reset selected mapping, display control flag, |
| and toggle meta flag. | |
| 11 | select null mapping, set display control flag, |
| reset toggle meta flag. | |
| 12 | select null mapping, set display control flag, |
| set toggle meta flag. (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 (this is not compatible with ECMA-48) |
| 22 | set normal intensity |
| 24 | underline off |
| 25 | blink off |
| 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. |
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).
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.
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).
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).
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 |
| character only.) | ||
| ESC O | SS3 | Single shift 3. (Select G3 character set for the next |
| character only.) | ||
| 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 USASCII, respectively. It accepts ESC ] for the setting of certain resources:
| ESC ] 0 ; txt BEL | Set icon name and window title to txt. |
| ESC ] 1 ; txt BEL | Set icon name to txt. |
| ESC ] 2 ; txt BEL | Set window title to txt. |
| ESC ] 4 6 ; name BEL | Change log file to name (normally disabled |
| by a compile-time option) | |
| ESC ] 5 0 ; fn BEL | Set font to fn. |
It recognizes the following with slightly modified meaning:
| 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 | |
| the 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. |
| Has no visible effect in xterm. | ||
| ESC } | LS2R | Invoke the G2 character set as GR. |
| Has no visible effect in xterm. | ||
| ESC ~ | LS1R | Invoke the G1 character set as GR. |
| Has no visible effect in xterm. |
It does not recognize ESC % ...
The xterm program will recognize 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 and Stephen Gildea, available with the X distribution.
In 2.0.23, CSI is broken, and NUL is not ignored inside escape sequences.