use ffmpeg to compress video to a specific size in MB

compress-video.sh

#!/bin/bash

if [ "$#" -ne 2 ]; then
    echo "You must enter exactly 2 command line arguments"
    echo "Usage: "`basename "$0"`" video [target size in mb]"
    exit
fi

target_video_size_MB="$2"
origin_duration_s=$(ffprobe -v error -show_streams -select_streams a "$1" | grep -Po "(?<=^duration\=)\d*\.\d*")
origin_audio_bitrate_kbit_s=$(ffprobe -v error -pretty -show_streams -select_streams a "$1" | grep -Po "(?<=^bit_rate\=)\d*\.\d*")
target_audio_bitrate_kbit_s=$origin_audio_bitrate_kbit_s # TODO for now, make audio bitrate the same
target_video_bitrate_kbit_s=$(\
    awk \
    -v size="$target_video_size_MB" \
    -v duration="$origin_duration_s" \
    -v audio_rate="$target_audio_bitrate_kbit_s" \
    'BEGIN { print  ( ( size * 8192.0 ) / ( 1.048576 * duration ) - audio_rate ) }')

ffmpeg \
    -y \
    -i "$1" \
    -c:v libx264 \
    -b:v "$target_video_bitrate_kbit_s"k \
    -pass 1 \
    -an \
    -f mp4 \
    /dev/null \
&& \
ffmpeg \
    -i "$1" \
    -c:v libx264 \
    -b:v "$target_video_bitrate_kbit_s"k \
    -pass 2 \
    -c:a aac \
    -b:a "$target_audio_bitrate_kbit_s"k \
    "${1%.*}-$2mB.mp4"