code:

#find audio tracks in mp4 files, convert if needed and save as m4a

for file in *.mp4; do

if [[ -e "${file%.*}.m4a" ]]; then printf "${file} already exists with m4a container\n"; continue; fi

format=$(ffprobe -show_streams -select_streams a "${file}" 2>/dev/null | grep -m 1 codec_type | cut -d "=" -f2-)
codec=$(ffprobe -show_streams -select_streams a "${file}" 2>/dev/null | grep -m 1 codec_name | cut -d "=" -f2-)

if [ -z "${format}" ]; then printf "${file} does not contain an audio track\n"; continue; fi

if [[ "${format}" = "audio" && "${codec}" != "aac" ]]; then
	printf "${file%.*} audio track needs to be re-encoded\n";
	ffmpeg -i "${file}" -vn -c:a aac -b:a 192k "${file%.*}.m4a"
	if [ $? -eq 0 ]; then rm "${file}"; else printf "ffmpeg did not exit cleanly, keeping original\n"; fi
else
	printf "aac track already exists, setting mp4 container brand to m4a\n"
	ffmpeg -i "${file}" -vn -c:a copy "${file%.*}.m4a"
	if [ $? -eq 0 ]; then rm "${file}"; else printf "ffmpeg did not exit cleanly, keeping original\n"; fi
fi

done