And for Linux it's direct the free pro (crastinator) version even for commercial use 
and is over-there https://gnomecoder.wordpress.com/screenruler/
or
or
your graphical repository AKA app store
OK, it does not do all the thing like the @denzjos link, does not do angle, way point, triangle, etc...
and is over-there https://gnomecoder.wordpress.com/screenruler/or
Code:
sudo apt install screenruleryour graphical repository AKA app store
OK, it does not do all the thing like the @denzjos link, does not do angle, way point, triangle, etc...
Patrice