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 is a system console internal to the Linux kernel. A system console is the device which receives all kernel messages and warnings and which allows logins in single user mode.[1] 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 (virtual terminal) 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

The Linux 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 virtual terminal,[8] serial port,[9] USB serial port,[10] VGA in text-mode,[11] framebuffer.[12] 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

Virtual consoles 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

The text mode implementation 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

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

The Linux 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

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, or Unicode[n 1]
Run-time character set switching Possible[n 2] Impossible[n 1] Depends on PoV
Terminal emulation Yes, ANSI-compatible
(TERM=linux)		 Yes, since Windows 10 "Threshold 2"[16] Yes, ANSI
(with ANSI.SYS)
Run-time switching between text mode and GUI With kernel mode setting, hindered by some hardware (i.e. proprietary drivers) [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 terminate-and-stay-resident programs may be activated)
Non-standard modes Possible Impossible Possible
Mouse support Yes (with gpm or similar) Application dependent System and application dependent
only AltKey pressed -> programm noticed Impossible Possible Possible
  1. a b Unicode supported since Windows NT based systems, which 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. No Unicode support in non-NT versions of Windows
  2. a As non-ASCII keyboard layout should be reloaded because of flawed implementation.
  3. a Kernel mode settings in recent kernels make this more practical for some video hardware.

Linux framebuffer console

The Linux 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 overcome 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

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 operator is 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

The console responds to a number of control characters:[17]

Control characterASCII nameDescription
^GBELBell sound
^HBSBackspace
^IHTHorizontal tab
^JLFLine feed
^KVTVertical tab
^LFFForm feed
^MCRCarriage return
^NSOShift out
^OSIShift in
^XCANCancel escape sequence
^ZSUBCancel escape sequence
^[ESCEscape / begin escape sequence
^?DELNothing
ALT-^[n/aStart command sequence
Control sequenceDescription
^[MReverse line feed
^[DLine feed
^[ECarriage return and line feed
^[HSet tab stop
^[7Store cursor
^[8Restore cursor
^[>Switch keypad to numeric mode
^[=Switch keypad to application mode
^[cReset terminal settings
^[ZPrint terminal ID

For ^[ press the Escape key.

The console also supports extended escape sequences, ANSI CSI Mode sequences, and DEC Private Mode sequences.[15][17] 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

The Kmscon projects aims to create a modern user-space replacement for the Linux console.[18][19] Development priorities include support for multi-monitor setups, Unicode font rendering with Pango, XKB keyboard handling, and GPU OpenGL acceleration.[20] 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]

Name Major Minor Description
/dev/tty0 c   4 0 "current console"
/dev/tty1
...
/dev/tty63		 c   4 1
...
63		 virtual consoles
(keyboard controlled)
/dev/ttyS0
...		 c   4 64
...		 Serial ports, suitable for system console
/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

See also

References

  1. ↑ "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.
  2. ↑ "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>.
  3. 1 2 David Herrmann (2012-08-12). "Deprecating CONFIG_VT".
  4. ↑ "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.
  5. 1 2 3 "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.
  6. ↑ "Documentation/braille-console.txt". kernel.org. Archived from the original on 2015-09-06. Retrieved 2014-07-04.
  7. ↑ "Documentation/serial-console.txt". kernel.org. Archived from the original on 2013-05-22. Retrieved 2013-05-30.
  8. ↑ "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.
  9. ↑ "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.
  10. ↑ "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.
  11. ↑ "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.
  12. ↑ "CONFIG_FRAMEBUFFER_CONSOLE: Framebuffer Console support".
  13. ↑ "chvt(1) - Linux man page: chvt - change foreground virtual terminal".
  14. ↑ "console(4) – Linux man page: console – console terminal and virtual consoles". A Linux system has up to 63 virtual consoles
  15. 1 2 "console_codes(4) – Linux man page: console_codes – Linux console escape and control sequences".
  16. ↑ Grehan, Oisin (2016-02-04). "Windows 10 TH2 (v1511) Console Host Enhancements". Archived from the original on 2016-02-09. Retrieved 2016-02-10.
  17. 1 2 Michael K. Johnson and Erik W. Troan (2005). Linux Application Development – The Linux Console. ASCII books. ISBN 0321563220. Archived from the original on 2017-06-26. Retrieved 2013-09-03.
  18. ↑ David Herrmann (2012-08-11). "KMSCON: Linux KMS/DRM based Virtual Console".
  19. ↑ 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.
  20. ↑ 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 90s 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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