[xrandr][] is a simple command line interface to X's [RandR][]
extension, allowing you to manipulate the root window and map it onto
different devices.  I use it to switch between my laptop's builtin LCD
screen and external monitors when I'm giving presentations, or when
I'm at home and just want to take advantage of a larger display.

You can get a list of available devices with

    $ xrandr
    Screen 0: minimum 320 x 200, current 800 x 480, maximum 4096 x 4096
    LVDS1 connected 800x480+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
       800x480        60.6*+
       640x480        85.0     72.8     75.0     59.9  
       720x400        85.0  
       640x400        85.1  
       640x350        85.1  
    VGA1 disconnected (normal left inverted right x axis y axis)
    TV1 disconnected (normal left inverted right x axis y axis)

Once I've plugged an external monitor into the VGA port on my laptop, I get

    $ xrandr
    Screen 0: minimum 320 x 200, current 800 x 480, maximum 4096 x 4096
    LVDS1 connected 800x480+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
       800x480        60.6*+
       640x480        85.0     72.8     75.0     59.9  
       720x400        85.0  
       640x400        85.1  
       640x350        85.1  
    VGA1 connected (normal left inverted right x axis y axis)
       1280x1024      60.0 +   75.0  
       1152x864       75.0  
       1024x768       85.0     75.1     60.0  
       832x624        74.6  
       800x600        85.1     75.0     60.3  
       640x480        85.0     75.0     60.0  
       720x400        70.1  
    TV1 disconnected (normal left inverted right x axis y axis)

Shift output to the external monitor at its full resolution with

    $ xrandr --fb 1280x1024 --output VGA1 --auto
    $ xrandr --output LVDS1 --off

When you're done, shift it back with

    $ xrandr --fb 800x480 --output LVDS1 --auto
    $ xrandr --output VGA1 --off

If you want to split your screen across both monitors, use something like

    $  xrandr --output VGA1 --auto --right-of LVDS

For more information, check out the excellent [tutorial at
ThinkWiki][tutorial].

[xrandr]: http://www.x.org/wiki/Projects/XRandR
[RandR]: http://en.wikipedia.org/wiki/RandR
[tutorial]: http://www.thinkwiki.org/wiki/Xorg_RandR_1.2

[[!tag tags/linux]]