imagemagick software suite is used to processing images on Linux. It can be used to capture screen as well.

$ sudo aptitude install imagemagick

To capture screen, use "import" command.

$ import screen_shot.png  // then click on a window to capture

To capture a whole screen, type 

$ sleep 10; import -window root screen_shot.png  // sleep 10; is optional

To view the capture from command line, type:

$ eog screen_shot.png