xwd

In the X Window System, the program xwd (X Window dump) captures the content of a screen or of a window and optionally saves it into a file.

xwd runs in one of two ways: if a user specifies the whole screen or the name or identifier of a window as an argument, the program captures the content of the window; otherwise, it changes the shape of the cursor and waits for the user to click in a window, whose content is then captured.