media-tools/to_opus.sh

#!/bin/bash

# set shopt-options used by this shell-script
# Note, 0 (true) from shopt -q is "false" in a math context.
shopt -q globstar; globstar_set=$?
((globstar_set)) && shopt -s globstar
shopt -q extglob; extglob_set=$?
((extglob_set)) && shopt -s extglob

# return default shopt-options
function finish {
	((globstar_set)) && shopt -u globstar
	((extglob_set)) && shopt -u extglob
}
trap finish EXIT

print_help() {
  printf "Help: not implemented yet"
}

while getopts 'e:' flag; do
	case "${flag}" in
		e) extension+=("$OPTARG");;
		h) print_help
		exit 0;;
		*) exit 1 ;;
	esac
done

# write array into string
extension="${extension[*]}"
# remove leading space
extension="${extension##+([[:space:]])}"
# change separator
extension=${extension// /|}

for file in **/*.@($extension); do
	
	#set a channel-dependent  bitrate for every audio stream
	command=$(
		ffprobe "$file" -v 0 -show_entries stream=channels,bit_rate -select_streams a -of compact=p=0:nk=1:s="\ " |
		awk -v file="$file" -v file_output="${file%.*}.opus" '
			BEGIN{
				ORS=" ";
				print "ffmpeg -i \""file"\" -map 0:a -c:a libopus"
			}
			{	# https://wiki.xiph.org/Opus_Recommended_Settings
				if($1==1) {
					if($2>=64000) print "-b:a:"NR-1" 64k"
				}
				else if($1==2) {
					if($2>=128000) print "-b:a:"NR-1" 128k"
				}
				else if($1<=6) {
					if ($2>=256000) print "-b:a:"NR-1" 256k"
				}
				else if ($2>=450000) print "-b:a:"NR-1" 450k"
			}
			END{
				print "\""file_output"\" -v info -hide_banner"
			}')


	is_video=$(ffprobe -i Hair.m4v -v fatal -select_streams V -show_entries stream=codec_type)
	if [[ -n "$is_video" ]]; then
		echo "$file contains a video stream and will not be converted"

		#an Example for converting the audiostreams in a container while copying subtitles and videostreams.
		#ffmpeg -i Hair.m4v -map 0:a -map 0:s -map 0:v -c:a libopus -c:v copy -c:s copy Hair.mkv -hide_banner -v info

		#for extracting the audiostreams of a video file just use "eval "$command"".
		#it is recommended to check the result before deleting the original file.
	else
		eval "$command" #&& rm "$file"
	fi

done

exit 0