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Linux console
Simplified Structure of the Linux Kernel: VT subsystem.
Framebuffer console showing
Knoppix
booting. The presence of
the penguin graphic
indicates this is a framebuffer console as opposed to text mode console.
The Linux console provides a way for the kernel and other processes to
send text output to the user, and to receive text input from the user.
The user typically enters text with a computer keyboard and reads the output text on a computer monitor. The Linux kernel supports virtual consoles - consoles that are logically separate, but which access the same physical keyboard and display.[2] The Linux console (and Linux virtual consoles) are implemented by the VT subsystem of the Linux kernel, and do not rely on any user space software.[3] This is in contrast to a terminal emulator, which is a user space process that emulates a terminal, and is typically used in a graphical display environment.
The Linux console was one of the first features of the kernel and was originally written by Linus Torvalds in 1991[4] (see history of Linux). There are two main implementations: framebuffer and text mode. The framebuffer implementation is the default in modern Linux distributions, and together with kernel mode setting, provides kernel-level support for display hardware and features such as showing graphics while the system is booting.[5] The legacy text mode implementation was used in PC-compatible systems with CGA, EGA, MDA and VGA graphics cards. Non-x86 architectures used framebuffer mode because their graphics cards did not implement text mode.[5] The Linux console uses fixed-size bitmap, monospace fonts, usually defaulting to 8x16 pixels per character.[5]
The Linux console is an optional kernel feature, and most embedded Linux
systems do not enable it. These systems typically provide an
alternative user interface (e.g. web based), or boot immediately into a graphical user interface and use this as the primary means of interacting with the user. Other implementations of the Linux console include the Braille console to support refreshable Braille displays.[6] and the serial port console.[7]
Purpose[edit]
TheLinux console provides a way for the kernel and other processes to
output text-based messages to the user, and to receive text-based input
from the user. In Linux, several devices can be used as system console: a
[8] virtual terminal,[9] serial port,[10] USB serial port,[11] VGA in text-mode,[12]
framebuffer. Some modern Linux-based systems have deprecated kernel
based text-mode input and output, and instead show a graphical logo or progress bar while the system is booting, followed by the immediate start of a graphical user interface (e.g. the X.Org Server on desktop distributions, or SurfaceFlinger on Android).
During
kernel boot, the console is commonly used to display the boot log of
the kernel. The boot log includes information about detected hardware,
and updates on the status of the boot procedure. At this point in time,
the kernel is the only software running, and hence logging via
user-space (e.g. syslog)
is not possible, so the console provides a convenient place to output
this information. Once the kernel has finished booting, it runs the init
process (also sending output to the console), which handles booting of
the rest of the system including starting any background daemons.
After the init boot process is complete, the console will be used to multiplex multiple virtual terminals (accessible by pressing Ctrl-Alt-F1, Ctrl-Alt-F2 etc., Ctrl-Alt-LeftArrow, Ctrl-Alt-RightArrow, or using chvt[13]). On each virtual terminal, a getty process is run, which in turn runs /bin/login to authenticate a user. After authentication, a command shell will be run. Virtual terminals, like the console, are supported at the Linux kernel level.[14]
The Linux console implements a terminal type of "linux" and the escape sequences it uses are in the console_codes man page.[15]
Virtual consoles[edit]
Virtualconsoles allow the storage of multiple text buffers, enabling different
console programs to run simultaneously but interact with the user in
different contexts. From the user's point of view, this creates the
illusion of several independent consoles.
Each virtual console can
have its own character set and keyboard layout. Linux 2.6 introduced
the ability to load a different font for each virtual console (kernel
versions predating 2.6 change the font only on demand).
Text mode console[edit]
The text modeimplementation is used on PC-based systems with a legacy
CGA/EGA/MDA/VGA video card that implements text-based video modes. In
text mode, the kernel sends a 2D array of characters to the video card,
and the video card converts the characters to pixels for display.
Font, character set and keyboard layout[edit]
The text buffer is a part of VGA memory which describes the content of a text screen in terms of code points and character attributes. Code points in the text buffer and font are generally not the same as encoding used in text terminal semantics to put characters on the screen. The set of glyphs on the screen is determined by the current font. The text screen is handled by console.c and consolemap.c drivers. There is a utility for altering fonts and terminal encodings called consolechars.The Linux kernel (keyboard.c driver) has almost complete support for keyboard input (keyboard layouts), but it remains a bit inconsistent because it interacts badly with different character sets. Layouts are loaded by the loadkeys utility.
These two utilities and corresponding data files are packed in Linux Console Tools http://lct.sourceforge.net/ shipped with many Linux distributions.
Efforts on the internationalization of Linux at the kernel level started as early as in 1994 by Markus Kuhn and Andries Brouwer.
Text modes[edit]
TheLinux console is capable of supporting any VGA-style text mode, but the
kernel itself has very limited means to set these modes up. SVGATextMode
helps to enable more complex text modes than the standard EGA and VGA
modes. It is fully compatible with Console Tools, but has some conflicts
with dosemu, SVGAlib and display servers.
Currently, there is no support for different modes on different virtual consoles.
Comparison to Windows and DOS[edit]
Microsoft Windows(of any version) does not have a fully functional support of the
console. The comparable feature there, but for application software
only, is the Win32 console.
| Feature | Linux | Windows | DOS |
|---|---|---|---|
| VGA text attributes | Yes | Yes | Yes |
| Custom fonts | Possible | Possible | Possible |
| Character set | 512 glyphs max. (on VGA text), any code page or UTF-8 |
223 or 256 characters (depends on access method), any code page, no Unicode[n 1] |
|
| Run-time character set switching | Possible[n 2] | Impossible[n 1] | Depends on PoV |
| Terminal emulation | Yes, ANSI-compatible (TERM=linux) |
No | Yes, ANSI (with ANSI.SYS) |
| Run-time switching between text mode and GUI | Technically possible, but usually impractical[n 3] | No way to get back to GUI | Application dependent |
| Run-time changing numbers of rows and columns | Possible | Possible | Application dependent |
| Switching between applications | Possible | Possible | Limited (some TSRs may be activated) |
| Non-standard modes | Possible | Impossible | Possible |
| Mouse support | Yes (with gpm or similar) | Application dependent | System and application dependent |
- a b As for Windows 9x. Windows NT
based systems allow to switch code pages and use Unicode, but only in
window mode. Also, NT systems use own text buffer format incompatible
with VGA, which produces an overhead in hardware text modes. - a As non-ASCII keyboard layout should be reloaded because of flawed implementation.
- a Kernel mode settings in recent kernels make this more practical for some video hardware.
Linux framebuffer console[edit]
TheLinux framebuffer (fbdev) is a graphic hardware-independent abstraction
layer, which was originally implemented to allow the Linux kernel to
emulate a text console on systems such as the Apple Macintosh that do
not have a text-mode display. Now it offers a kernel space
text mode emulation on any platform. Its advantage over (currently
unmaintained) SVGATextMode is a reliance and better hardware
compatibility. It also permits to overpass all technical restrictions of VGA text modes.
A
Linux framebuffer console differs from a VGA one only in ways of
drawing characters. The processing of keyboard events and virtual
consoles’ support are exactly the same.
Linux serial port console[edit]
Linux serial console is a console implementation via serial port, enabled by option CONFIG_SERIAL_CONSOLE in the kernel configuration. It may be used in some embedded systems, and on servers, where a direct interaction with operatoris not expected. The serial console allows the same mode of access for
the system, but usually at a slower speed due to the small bandwidth of RS-232. A serial console is often used during development of software for embedded systems, and is sometimes left accessible via a debug port.
Control characters[edit]
The console responds to a number of control characters.[16]| Control character | ASCII name | Description |
|---|---|---|
| ^G | BEL | Bell sound |
| ^H | BS | Backspace |
| ^I | HT | Horizontal tab |
| ^J | LF | Line feed |
| ^K | VT | Vertical tab |
| ^L | FF | Form feed |
| ^M | CR | Carriage return |
| ^N | SO | Shift out |
| ^O | SI | Shift in |
| ^X | CAN | Cancel escape sequence |
| ^Z | SUB | Cancel escape sequence |
| ^[ | ESC | Escape / begin escape sequence |
| ^? | DEL | Nothing |
| ALT-^[ | n/a | Start command sequence |
| Control sequence | Description |
|---|---|
| ^[M | Reverse line feed |
| ^[D | Line feed |
| ^[E | Carriage return and line feed |
| ^[H | Set tab stop |
| ^[7 | Store cursor |
| ^[8 | Restore cursor |
| ^[> | Switch keypad to numeric mode |
| ^[= | Switch keypad to application mode |
| ^[c | Reset terminal settings |
| ^[Z | Print terminal ID |
The console also supports extended escape sequences, ANSI CSI Mode sequences, and DEC Private Mode sequences.[15][16]
These extended sequences can control colors, visual effects like
blinking, underline, intensity and inverse video, bell tone frequency
and duration, VESA screen blanking interval. Aside from the textual
blanking, there is no known way to place the VGA adapter into standby.
Future plans[edit]
The Kmscon projects aims to create a modern user-space replacement for the Linux console.[17][18] Development priorities include support for multi-monitor setups, Unicode font rendering with Pango, XKB keyboard handling, and GPU OpenGL acceleration.[19]Complaints about the current kernel implementation include "that it's a
user-interface in kernel-space, the code is poorly maintained, handles
keyboards badly, produces bad font rendering, misses out on mode-setting
and multi-head support, contains no multi-seat awareness, and only has
limited hot-plugging handling, limited to VT102 compliance."[3]
[edit]
| Name | Major | Minor | Description |
|---|---|---|---|
| /dev/tty1 … /dev/tty63 |
c 4 | 1 … 63 |
virtual consoles (keyboard controlled) |
| /dev/vcs | c 7 | 0 | Virtual Console Screen /dev/vcs is the contents of the current virtual screen. |
| /dev/vcs1 … /dev/vcs63 |
c 7 | 1 … 63 |
The text (the character pointer table) of a virtual screen. |
| /dev/vcsa1 … /dev/vcsa63 |
c 7 | 129 … 191 |
Virtual Console Screen with Attributes Full image of a virtual text buffer; first 4 bytes contain numbers of rows, columns and cursor position |
| /dev/ttyS0 … |
c 4 | 64 … |
Serial ports, suitable for system console |
| /dev/tty0 | c 4 | 0 | “current console” |
References[edit]
- Jump up ^ "config VT_CONSOLE: Support for console on virtual terminal". Linus Torvalds.
The system console is the device which receives all kernel messages and warnings and which allows logins in single user mode.
- Jump up ^ "config VT: Virtual terminal". Linus Torvalds.
If
you say Y here, you will get support for terminal devices with display
and keyboard devices. These are called "virtual" because you can run
several virtual terminals (also called virtual consoles) on one physical
terminal. This is rather useful, for example one virtual terminal can
collect system messages and warnings, another one can be used for a
text-mode user session, and a third could run an X session, all in
parallel. Switching between virtual terminals is done with certain key
combinations, usually Alt-<function key>. - ^ Jump up to: a b David Herrmann (2012-08-12). "Deprecating CONFIG_VT".
- Jump up ^ "Replacing CONFIG_VT/Linux-Console". FOSDEM. 2013-02-02.
CONFIG_VT
is the kernel configuration option that enables virtual terminals in
the kernel. Initially written by Linus himself, it has been around since
1991. - ^ Jump up to: a b c "The Framebuffer Console". kernel.org.
The
framebuffer console (fbcon), as its name implies, is a text console
running on top of the framebuffer device. It has the functionality of
any standard text console driver, such as the VGA console, with the
added features that can be attributed to the graphical nature of the
framebuffer. In the x86 architecture, the framebuffer console is
optional, and some even treat it as a toy. For other architectures, it
is the only available display device, text or graphical. What are the
features of fbcon? The framebuffer console supports high resolutions,
varying font types, display rotation, primitive multihead, etc.
Theoretically, multi-colored fonts, blending, aliasing, and any feature
made available by the underlying graphics card are also possible. - Jump up ^ "Documentation/braille-console.txt". kernel.org.
- Jump up ^ "Documentation/serial-console.txt". kernel.org.
- Jump up ^ "CONFIG_VT_CONSOLE: Support for console on virtual terminal".
The
system console is the device which receives all kernel messages and
warnings and which allows logins in single user mode. If you answer Y
here, a virtual terminal (the device used to interact with a physical
terminal) can be used as system console. - Jump up ^ "CONFIG_SERIAL_CONSOLE: Support for console on serial port".
If
you say Y here, it will be possible to use a serial port as the system
console (the system console is the device which receives all kernel
messages and warnings and which allows logins in single user mode). This
could be useful if some terminal or printer is connected to that serial
port. - Jump up ^ "CONFIG_USB_SERIAL_CONSOLE: USB Serial Console device support".
If
you say Y here, it will be possible to use a USB to serial converter
port as the system console (the system console is the device which
receives all kernel messages and warnings and which allows logins in
single user mode). This could be useful if some terminal or printer is
connected to that serial port. - Jump up ^ "CONFIG_VGA_CONSOLE: VGA text console".
Saying
Y here will allow you to use Linux in text mode through a display that
complies with the generic VGA standard. Virtually everyone wants that. - Jump up ^ "CONFIG_FRAMEBUFFER_CONSOLE: Framebuffer Console support".
- Jump up ^ "chvt(1) - Linux man page: chvt - change foreground virtual terminal".
- Jump up ^ "console(4) - Linux man page: console - console terminal and virtual consoles".
A Linux system has up to 63 virtual consoles
- ^ Jump up to: a b "console_codes(4) - Linux man page: console_codes - Linux console escape and control sequences".
- ^ Jump up to: a b Michael K. Johnson and Erik W. Troan. Linux Application Development - The Linux Console. ASCII books. ISBN 0321563220.
- Jump up ^ David Herrmann (2012-08-11). "KMSCON: Linux KMS/DRM based Virtual Console".
- Jump up ^ Michael Larabel (2013-03-28). "KMSCON: A DRM-Based Terminal Emulator". Phoronix.
Announced
yesterday was the release of kmscon, a terminal emulator for Linux
that's similar to what's offered inside the kernel, but instead it's in
user-space and relies upon the kernel's DRM interfaces as well as Mesa. - Jump up ^ Michael Larabel (2013-02-08). "The Linux Kernel Console Is Being Killed Off". Phoronix.
CONFIG_VT
has been part of the Linux kernel going back to the early 90's but
hasn't really advanced much in that time. David Herrmann, a developer
that got going on this new initiative as a student part of Google Summer
of Code, wants a new solution that's built with multi-seat and multiple
monitors in mind, incorporates Unicode font rendering, XKB-like
keyboard handling, graphics hardware acceleration, VT220-VT510
compatibility, and other features.
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