slop
Get a selection of the screen. tldr-pages
On Linux
Linux
Wait for the user to make a selection and output its geometry to stdout
slopIdentifyDouble click, rather than click and drag, to draw a selection
slop --nodragIdentifyHighlight the selection rather than outlining it
slop --highlightIdentifySpecify the output format
slop --format format_stringIdentifySpecify the selection rectangle's color
slop --color red,green,blue,alphaIdentify