In der Regel sollte reichen: (ImageMagick)
$ sleep 5; import -window root screen.png
Wobei 'root' keinesfalls der User ist, sondern das root vom XWindow-server.
Wenn das nicht geht, kann man auch tiefer gehen:
Quelle1)
$ echo $DISPLAY
Let your display number returns :0.0, Then the command to grab the screenshot is:
$ chvt 7; sleep 5; import -window root screen.png -display :0.0 ; chvt 3
Three actions will take place following to this,
process of switching to the X terminal grabbing the screenshot switching back to your current command-line terminal.
The first command (chvt 7) switches to the X terminal say gnome (usually the 7th terminal is the X), and then the second command (sleep 5) pauses the process for two seconds to switch to the 7th terminal. Then the import command is used to grab the root window on display :0.0 and save the shot as screen.png. Finally, the terminal is switched back to terminal 2 (chvt 2).
You can change the virtual terminal by changing the number after the chvt.