I use xwd, on Debian type stuff you have to add the "retro" X Windows stuff to get it. Works in the BSDs, most things just fine. I'm writing this on a Pi 3b, very useful little boxes.
I define this alias in my bashrc or whereever. I just type snap into some free terminal emulator then I have 10 seconds to get the screen arranged picture perfect. In some situations it beeps at the end of the 10 seconds. I never tried assigning it to a key. alias snap='sleep 10 ; xwd -root -out /tmp/snap.xwd' Then I do 'gimp /tmp/snap.xwd' because I usually want to crop anyway. -- Credit is the root of all evil. - AB1JX