Extend non-HiDPI external display left to HiDPI internal display

extend.sh

#!/bin/sh
# extend non-HiDPI external display on DP* on the left side of the HiDPI internal display eDP*
#
# run the script a second time if you get an error like:
#   xrandr: specified screen 6400x2160 not large enough for output HDMI1 (5120x2880+0+0)
#
# see also https://wiki.archlinux.org/index.php/HiDPI
# you may run into https://bugs.freedesktop.org/show_bug.cgi?id=39949
#                  https://bugs.launchpad.net/ubuntu/+source/xorg-server/+bug/883319

SCALE=2

EXT=`xrandr --current | sed 's/^\(.*\) connected.*$/\1/p;d' | grep -v ^eDP | head -n 1`
INT=`xrandr --current | sed 's/^\(.*\) connected.*$/\1/p;d' | grep -v ^DP | head -n 1`

ext_w=`xrandr | sed 's/^'"${EXT}"' [^0-9]* \([0-9]\+\)x.*$/\1/p;d'`
ext_h=`xrandr | sed 's/^'"${EXT}"' [^0-9]* [0-9]\+x\([0-9]\+\).*$/\1/p;d'`
int_w=`xrandr | sed 's/^'"${INT}"' [^0-9]* \([0-9]\+\)x.*$/\1/p;d'`
int_h=`xrandr | sed 's/^'"${INT}"' [^0-9]* [0-9]\+x\([0-9]\+\).*$/\1/p;d'`
off_w=`echo $(( ($int_w-$ext_w)/2 )) | sed 's/^-//'`

pan_w=`echo $(( $ext_w*$SCALE )) | sed 's/^-//'`
pan_h=`echo $(( $ext_h*$SCALE )) | sed 's/^-//'`

full_w=`echo $(( $ext_w+$int_w )) | sed 's/^-//'`

if (( ext_h > int_h )); then
	used_h=$ext_h
else
	used_h=$int_h
fi

xrandr --output "${INT}" --auto --pos ${ext_w}x0 --output "${EXT}" --scale ${SCALE}x${SCALE} --auto --pos 0x0 --fb ${full_w}x${used_h}