I read a very short description of how to make a Screenshot by making use of the file which represents the framebuffer /dev/fb0.
in order to write the screen content:
cat /dev/fb0 > file
in order to restore the screenshot:
cat file > /dev/fb0
I did so, but nothing happened, not even a short blink, so I find a possible hint to an answer:
But, there was only an answer I did not understand which required knowledge in C programming language (at least the Code looked to me like C )