Virtual console (computer user-interface)

Poll without page-refresh

Article on other languages:

	del.icio.us
	Digg
	Furl
	Reddit
	Rojo
	Add to OnlyWire

Virtual console showing Knoppix boot messages

In computing, some operating systems such as UnixWare, Linux and BSD, feature a virtual console (VC, sometimes virtual terminal, VT) — a conceptual combination of the keyboard and the display for a user interface. The concrete combination is the system console of the computer, where the user can switch between the virtual consoles to access multiple unrelated user interfaces. Virtual consoles date back at least to Xenix in the 1980s.^[1]

Usually in Linux, the first six virtual consoles provide a text terminal with a login prompt to a Unix shell. The graphical X Window System starts in the seventh virtual console. In Linux, the switching is performed with a key combination of Alt plus a function key -- for example Alt+F1 to access the virtual console number 1. Alt+Left arrow changes to the previous virtual console and Alt+Right arrow to the next virtual console. To switch from the X Window System, Ctrl+Alt+function key works. (Note that users can re-define these default key-combinations.)

The need for virtual consoles has lessened now that most applications work in the graphical framework of the X Window System, where each program has a window and the text mode programs can be run in terminal-emulator windows. If several sessions of the X Window System are required to run in parallel, such as in the case of fast user switching or when debugging X programs on a separate X server, each X session usually runs in a separate virtual console. GNU Screen is a program that can change between several text-mode programs in one textual login. There are also other graphical frameworks such as FrameBuffer UI, Y Window System, and Fresco.

1 Interface
2 Unix systems
3 See also
4 Notes
5 References
6 External links

Interface

The virtual consoles are represented by device special files /dev/tty1, /dev/tty2 etc. There are also special files /dev/console, /dev/tty and /dev/tty0. (Compare the devices using the patterns vcs ("virtual console screen") and vcsa ("virtual console screen with attributes") such as /dev/vcs1 and /dev/vcsa1.^[2] ) The virtual consoles can be configured in the file /etc/inittab read by init -- typically it starts the text mode login process getty for several virtual consoles. X Window System can be configured in /etc/inittab or by an X display manager.

Programs used to access the virtual consoles typically include:

chvt to switch the current virtual console
openvt to run a program on a new virtual console
deallocvt to close a currently unused virtual console.

The program startx starts the X Window System on a new virtual console. There are also other graphical programs that can start from the console, such as LinuxTV and MPlayer.

Programs can access the virtual consoles by the device special files. In text mode, writing to the file displays text on the virtual console and reading from the file returns text the user writes to the virtual console. As with other text terminals, there are also special escape sequences, control characters and termios functions that a program can use, most easily via a library such as ncurses. For more complex operations, the programs can use console and terminal special ioctl system calls. Graphical programs can use libraries such as DirectFB, DRI, SDL or the earlier SVGALib.

Unix systems

Unix workstations, such as those manufactured by Sun or Silicon Graphics, did not include virtual consoles. The only purpose of console would be fixing the system so that graphical environment could start.

Sun Niagara-based servers running virtualization with Logical Domains get virtual console services from the Control domain.

Notes

^ "Trusted path mechanism for virtual terminal environments". FreePatentsOnline.com. Retrieved on 2008-04-09.
United States Patent 4945468 lists Xenix as prior art in this area.
^ "Screen dumps". The Linux keyboard and console HOWTO. The Linux Documentation Project. Retrieved on 2008-07-31. "The current contents of the screen of /dev/ttyN can be accessed using the device /dev/vcsN (where `vcs' stands for `virtual console screen'). [...] From a program it is usually better to use /dev/vcsaN (`virtual console screen with attributes') instead - it starts with a header giving the number of rows and columns and the location of the cursor. See vcs(4)."