ffmpeg-screencaster is a basic script to make screencasts with the simple yet excellent FFmpeg command-line software. It’s features are:

support for multi-headed display
support for specific video geometry
uses xrandr to (sort of) detect screen geometry

If you have more screens, you might need to adjust it.

#!/bin/bash

vp=$2

dest=$1

if [ -n ${dest} ]
then
	dest=`date +%Y%m%d_%H%M%S`.mp4
	echo "Using default filename: ${dest}"
fi

setViewport() {
    case "${!1}" in
        '')	viewport=`xrandr -q | head -1 | cut -f 2 -d ','`
		width=`echo $viewport | cut -f 2 -d ' '`
		height=`echo $viewport | cut -f 4 -d ' '`
		echo "Using default ${width}x${height} video size"
		viewport=${width}x${height};;
        0)	vp=`xrandr -q | grep DisplayPort-0 | cut -f 4 -d ' '`
		viewport=`echo ${vp} | cut -f 1 -d '+'`
		pos_x=`echo ${vp} | cut -f 2 -d '+'`
		pos_y=`echo ${vp} | cut -f 3 -d '+'`
		echo "Using screen 0 ${viewport} +${pos_x}+${pos_y}";;
        1)	vp=`xrandr -q | grep DisplayPort-1 | cut -f 3 -d ' '`
		viewport=`echo ${vp} | cut -f 1 -d '+'`
		pos_x=`echo ${vp} | cut -f 2 -d '+'`
		pos_y=`echo ${vp} | cut -f 3 -d '+'`
		echo "Using screen 1 ${viewport} +${pos_x}+${pos_y}";;
        2)	vp=`xrandr -q | grep DisplayPort-2 | cut -f 3 -d ' '`
		viewport=`echo ${vp} | cut -f 1 -d '+'`
		pos_x=`echo ${vp} | cut -f 2 -d '+'`
		pos_y=`echo ${vp} | cut -f 3 -d '+'`
		echo "Using screen 2 ${viewport} +${pos_x}+${pos_y}";;
        *)	vp=${!1}
		viewport=`echo ${vp} | cut -f 1 -d '+'`
		if [[ $vp == *"+"* ]]; then
			pos_x=`echo ${vp} | cut -f 2 -d '+'`
			pos_y=`echo ${vp} | cut -f 3 -d '+'`
			echo "Video position was explicitly set to +${pos_x}+${pos_y}"
		fi
		echo "Video size set to an explicit value: ${!1}";;
    esac
}

pos_x=0
pos_y=0
setViewport vp

ffmpeg -f x11grab -r 25 -s ${viewport} -i :0.0+${pos_x},${pos_y} -vcodec libx264 ${dest}

ffmpeg screencaster