to take screenshot in ubuntu
Print Screen to take a screenshot of the desktop.
Alt+Print Screen to take a screenshot of a window.
Shift+Print Screen to take a screenshot of an area you select.
When you use a keyboard shortcut, the image is automatically saved in your Pictures folder in your home folder with a file name that begins with Screenshot and includes the date and time it was taken.
If you do not have a Pictures folder, the images will be saved in your home folder instead.
You can also hold down Ctrl with any of the above shortcuts to copy the screenshot image to the clipboard instead of saving it.