Xwd

xwd is an application that is standard among the X utilities, so it will come in any full X distribution.

xwd dumps the output of a window into a file that can be viewed with xwud, or converted with convert (part of ImageMagick package), xv or another tool to a more usable image type, like png.

There are also more convenient tools to make a Screenshot. You should use more reliable import (part of ImageMagick package) instead of xwd.

Examples
Next command will ask you to select a window (the cursor will become a + sign), then it will take a picture of the window (as it appears on the screen), the &quot;-frame&quot; options will have it also include the window managers decorative borders, then the file is converted to png

To take a complete screen shot (no need to tell it what window to select):

Making screenshot and conversion at once:

Sometimes one need some delay in order to make some windows visible before taking screenshot: