Script to start xephyr xorg server and desktop session under another user

startxephyr.sh

#!/usr/bin/env bash

set -xe

target_user="$1"
target_home=$(getent passwd $target_user | cut -f6 -d:)

# Find free display number
for ((display_number=1 ; display_number <= 100 ; display_number++)) ; do
  [ -e /tmp/.X11-unix/X$display_number ] || break
done

xcache_folder=/tmp/Xephyr_X$display_number
xsocket=/tmp/.X11-unix/X$display_number
xserver_cookie=$xcache_folder/.Xauthority
xclient_cookie=$target_home/.Xauthority
clip_forwarder_pid=-1
xephir_pid=-1

[ -e "$xcache_folder" ] && rm -R "$xcache_folder"
mkdir -p $xcache_folder

# create new XAUTHORITY cookie file
touch $xserver_cookie
xauth add :$display_number . $(mcookie)

cookie=$(xauth nlist ":$display_number" | sed -e 's/^..../ffff/')
echo $cookie | xauth -f "$xserver_cookie" nmerge -

function cleanup ()
{
    kill $clip_forwarder_pid || true > /dev/null 2>/dev/null
    kill $xephir_pid || true > /dev/null 2>/dev/null
}

trap cleanup EXIT

function clipForward()
{
  CLIP1=""
  CLIP2=`xsel -o -b --display $2`
  while true; do
    CLIP1NEW=`xsel -o -b --display $1`
    CLIP2NEW=`xsel -o -b --display $2`
    if [ "x$CLIP1" != "x$CLIP1NEW" ]; then
      xsel -o -b --display $1 | XAUTHORITY=$3 xsel -i -b --display $2
      CLIP1=$CLIP1NEW
    fi;
    if [ x"$CLIP2" != x"$CLIP2NEW" ]; then
      XAUTHORITY=$3 xsel -o -b --display $2 | xsel -i -b --display $1
      CLIP2=$CLIP2NEW
    fi
    sleep 1
  done
}

Xephyr :$display_number \
    -auth $xserver_cookie \
    -extension MIT-SHM \
    -nolisten tcp \
    -screen 800x600 \
    -resizeable &
xephir_pid=$!

sleep 1

clipForward ":0" ":$display_number" "$xserver_cookie" &
clip_forwarder_pid=$!

sudo /run/current-system/sw/bin/systemd-run \
    --property PAMName=login \
    --property User=$target_user \
    --property StandardInput=tty \
    --property Environment=DISPLAY=:$display_number \
    --property Environment=XAUTHORITY=$xclient_cookie \
    --uid=$target_user \
    -t bash -c "echo $cookie | xauth -f $xclient_cookie nmerge - && ~/.xsession" > $HOME/log 2>$HOME/logerr

cleanup
wait