Based on https://coderwall.com/p/tog9eq but with SCREEN auto detect and some minor fixes

xvfb

#!/bin/bash

echo ''

XVFB=`which Xvfb`

if [ "$XVFB" == '' ]; then
        echo 'Xvfb is not installed'
        echo ''
        exit 1;
fi

XRANDR=`which xrandr`

if [ "$XRANDR" == '' ]; then
        SCREEN=0
else
        INFO="$XRANDR -q";
        EXPR='Screen ([0-9]+):'

        if [[ $INFO =~ $EXPR ]]; then
                SCREEN=$((${BASH_REMATCH[${#BASH_REMATCH[*]} - 1]} + 1))
        else
                SCREEN=0
        fi
fi

XVFBARGS=":$SCREEN -screen $SCREEN 1024x768x24 -ac +extension GLX +render -noreset"
PIDFILE=/var/run/xvfb.pid

case "$1" in
        start)
                echo 'Starting virtual X frame buffer: Xvfb'
                start-stop-daemon --chuid www-data --start --quiet --pidfile $PIDFILE --make-pidfile --background --exec $XVFB -- $XVFBARGS
                ;;
        stop)
                echo 'Stopping virtual X frame buffer: Xvfb'
                start-stop-daemon --chuid www-data --stop --quiet --pidfile $PIDFILE
                ;;
        restart)
                $0 stop
                $0 start
                ;;
        *)
                echo 'Usage: '$0' {start|stop|restart}'
                echo ''
                exit 1
esac

echo ''

exit 0

Thanks!

Here is a version for systemd: https://gist.github.com/nkm/91006178753df6f503c1

Line 34 generates an error, it works this way:
start-stop-daemon --chuid www-data --start --quiet --pidfile $PIDFILE --make-pidfile --background --exec $XVFB -- $XVFBARGS

Hope it helps.

https://gist.github.com/nkm/91006178753df6f503c1

Thanks for that.
Took me nearly an hour of debugging, before I noticed your comment 😅
Line 18 also causes an error on my ubuntu 14.04 os:

Can't open display

And then actually responds it started xvfb, when it didn't.
Simply used double quotes and it worked fine afterwards: INFO="$XRANDR -q";