fbdump (captures the visible portion of framebuffer to stdout)

fbdump is a simple tool that captures the contents of the visible portion of
the Linux framebuffer device and writes it to the standard output as a PPM
file. In other words, it takes a screenshot of anything running on the
framebuffer. It currently has fairly complete support for packed-pixel
framebuffer types and also works with the VGA16 framebuffer driver.

To get a popular image format, issue a command like:

   fbdump | pnmtopng > shot.png

Note: fbdump is rather slow on modern systems with high-resolution
framebuffers at 32-bit color depth. On the maintainer's system, it takes
8 seconds to dump a 1920x1080x32 framebuffer. Be patient.