Click on a window to perform screen grab at 5 FPS

record_window_ffmpeg.sh

#!/bin/sh
echo "Click on a window to record."
properties=`xwininfo | grep ":"`

width=`echo "$properties" | grep "Width: " |  tr -d '[:space:]'  | cut -d ':' -f 2`
height=`echo "$properties" | grep "Height: " | tr -d '[:space:]' | cut -d ':' -f 2`
ulx=`echo "$properties" | grep "Absolute upper-left X:" | tr -d '[:space:]' | cut -d ':' -f 2`
uly=`echo "$properties" | grep "Absolute upper-left Y:" | tr -d '[:space:]' | cut -d ':' -f 2`

ffmpeg -loglevel quiet -f x11grab \
  -framerate 5 \
  -video_size ${width}x${height} \
  -i :0.0+${ulx},${uly} \
  -vcodec libx264 \
  -qp 0 \
  -preset ultrafast \
  /var/tmp/output.mkv