slop
Get a selection of the screen.
TLDR
Wait for the user to make a selection and output its geometry to standard output
$ slop
Double click, rather than click and drag, to draw a selection
$ slop -D
Highlight the selection rather than outlining it
$ slop -l
Specify the output format
$ slop -f [format_string]
Specify the selection rectangle's color
$ slop -c [red],[green],[blue],[alpha]