Script for copying file trees while converting files with some extension. Extensions to convert and conversion commands can be easily added or changed.

treecopy_flac2m4a.sh

#! /bin/sh
# Should work both in bash and dash + GNU coreutils. Probably works in other POSIX shells & coreutils.

if [ $# -ne 2 ]; then \
	echo "Usage: $0 <old_dir> <new_dir>" >&2
	exit 1
fi

# Output dir must not be descendor of input dir!
in_dir="$(realpath "$1")"
out_dir="$(realpath "$2")"

stdin="$(realpath /dev/stdin)"

convert_path() {
	# Usage: convert_path <in_path> [<old_ext>] [<new_ext>]
	local ext_changed="${1%$2}$3"
	printf '%s\n' "$out_dir${ext_changed#$in_dir}"
}

# BEGIN conversion commands
# <cmd> <in_file> <out_file>
cmd_copy() {
	cp -i "$1" "$2"
}

cmd_transcode_m4a() {
	# You may want to use 192K instead of 128K (or another bitrate)
	ffmpeg -hide_banner -i "$1" -vn -c:a aac -b:a 128K "$2"
}

cmd_playlist_m4a() {
	sed -e 's/.\(flac\)$/.m4a/' "$1" > "$2"
}

# Example for opus:
cmd_transcode_opus() {
	# For voice 43k should be enough AFAIK
	ffmpeg -hide_banner -i "$1" -vn -c:a libopus -b:a 87k -vbr on -frame_duration 60 -application audio "$2"
}

cmd_playlist_opus() {
	# Other source formats can be added
	sed -e 's/.\(flac\|m4a\|mp3\)$/.opus/' "$1" > "$2"
}
# END conversion commands

copy_or_convert() {
	# Usage: copy_or_convert <in_file>
	local copy_cmd='cmd_copy'
	local old_ext=''
	local new_ext=''
	case "$1" in
		*.flac)
			old_ext=flac
			new_ext=m4a
			copy_cmd=cmd_transcode_m4a
			;;
		# *.mp3)
		# 	old_ext=mp3
		# 	new_ext=m4a
		# 	copy_cmd=cmd_transcode_m4a
		# *.m4a)
		# 	old_ext=m4a
		# 	new_ext=opus
		# 	copy_cmd=cmd_transcode_opus
		# 	;;
		*.m3u|*.m3u8)
			copy_cmd=cmd_playlist_m4a
			;;
	esac
	"$copy_cmd" "$1" "$(convert_path "$1" "$old_ext" "$new_ext")" < "$stdin"
}

# Create dirs
find "$in_dir" -type d | while read -r file; do \
	mkdir -p "$(convert_path "$file")"
done

# Process regular files
find "$in_dir" -type f | while read -r file; do \
	copy_or_convert "$file"
done