vincentbernat.i3wm-configuration/bin/xsettingsd-setup

#!/bin/sh

# Compute DPI. We extract the value from xrandr output. The screen
# size is correctly reported (while the X server is lying to respect
# the provided DPI). We round to the nearest quarter of 96 DPI (96,
# 120, 144, 168, 192...).
eval $(xrandr --current | sed -n 's/.* connected primary \([0-9]*\)x.* \([0-9]*\)mm x .*/wd=\1\nwm=\2/p')
if [ -n "$wd" -a -n "$wm" ]; then
    dpi=$(echo "$wd/($wm*0.03937)" | bc)
    dpi=$(printf "%.0f" $(echo "scale=1;$dpi*4/96" | bc))
    dpi=$((dpi*96/4))
else
    dpi=96
fi

# Build xsettingsd.local
cp  ~/.config/awesome/xsettingsd ~/.config/awesome/xsettingsd.local
echo Xft/DPI $(( $dpi * 1024 )) >> ~/.config/awesome/xsettingsd.local

# Signal xsettingsd
pid=$(xprop -name xsettingsd _NET_WM_PID 2> /dev/null | awk '{print $NF}')
if [ x"$pid" = x ]; then
    xsettingsd -c ~/.config/awesome/xsettingsd.local &
else
    kill -HUP $pid
fi

# Also use xrdb for very old stuff (you know, LibreOffice)
echo Xft.dpi: $dpi | xrdb -merge