transcode a given video to different bitrate versions using ffmpeg

transcode.zsh

#!/usr/bin/zsh

INPUT=${1:a}
FRACTION=${2:-0.9}

BT=$(ffprobe -i $INPUT 2>&1 | grep Video | grep -o -E '[[:digit:]]+ kb\/s' | cut -d' ' -f1)

while [[ "$BT" -gt '50' ]]
do
	BT=$(printf "%0.f" $(( $BT * $FRACTION )))
	MX=$(( $BT + 50 ))
	MM=$(( $BT - 50 ))
	BS=$(( $BT / 2 ))

	ffmpeg -y -i $INPUT -c:v libx264 -b:v ${BT}K -maxrate ${MX}K -minrate ${MM}K -bufsize ${BS}K ${INPUT:r}_bt$BT-mx$MX-mm$MM-bs$BS.mp4 &
done

wait

exit $?