focused-screen-index

#!/usr/bin/env bash
# if there is more than one monitor connected, uses
# xdotool to get current mouse location and infer the
# which monitor is currently being used.
# From: https://superuser.com/a/1238384/887225
# Prints the index of the connected monitor (zero indexed).

readonly OFFSET_RE="\+([-0-9]+)\+([-0-9]+)"

# Get the window position
# eval sets these as shell variables
eval "$(xdotool getmouselocation --shell)"

# Loop through each screen and compare the offset with the window
# coordinates.
monitor_index=0
while read -r name width height xoff yoff; do
	if [ "${X}" -ge "$xoff" ] &&
		[ "${Y}" -ge "$yoff" ] &&
		[ "${X}" -lt "$((xoff + width))" ] &&
		[ "${Y}" -lt "$((yoff + height))" ]; then
		monitor="${name}"
		break
	fi
	((monitor_index++))
done < <(xrandr | grep -w connected |
	sed -r "s/^([^ ]*).*\b([-0-9]+)x([-0-9]+)$OFFSET_RE.*$/\1 \2 \3 \4 \5/" |
	sort -nk4,5)

# If we found a monitor, echo it out, otherwise print an error.
if [[ -n "${monitor}" ]]; then
	echo "${monitor_index}"
	exit 0
else
	echo "Couldn't find any monitor for the current window." >&2
	exit 1
fi