Converts M4A to MP3 respecting the original bitrate (uses ffmpeg)

m4atomp3.sh

#!/usr/bin/env bash

# Takes a directory as parameter and converts everything inside it
# from M4A to MP3, respecting the bitrate, and creating a directory
# for the converted files.
#
# MIT Licensed by Gwyneth Llewelyn (2022)
#
# Based on https://superuser.com/a/1211902/127382 by @jbowman (2017)

if [ -d "${1}" ]
then
        echo "Doing conversions from directory \"${1}\"..."
else
        echo "Source directory not found! Please add it as a parameter, e.g.:"
        echo "  ${0} path/to/album/containing/mp3/files"
        exit 1
fi

# Get the album's name from the first parameter
ALBUM_PATH="${1}"
ALBUM=$(basename "${ALBUM_PATH}")

echo "Attempting to process album \"${ALBUM}\" in source directory
\"${ALBUM_PATH}\"..."

# Create a directory for this run
mkdir -p "mp3s"

# Check if mkdir succeeded
if [[ $? -ne 0 ]]
then
        echo "Could not create directory mp3s/${ALBUM} for output. Exiting..."
        exit 2
fi

# Loop across the filnames in that directory that end in *.m4a, invoke
# ffprobe to extract the bitrate, then use ffmpeg to convert it into MP3
# and place the result inside the mp3s/${ALBUM}/ directory:
transcode_file() {
        f=$1
        p=${f%/*}
        ALBUM=${p##*/}
        [ -d "mp3s/$ALBUM" ] || {
                echo "====================================="
                echo "starting album $ALBUM"
                echo "====================================="
                mkdir "mp3s/$ALBUM"
        }
        orgbitrate=$(ffprobe -v quiet -of flat=s=_ -show_entries format=bit_rate "${f}" | sed 's/[^0-9]*//g')
        [[ orgbitrate > 229376 ]] && bitrate=224k || bitrate=$orgbitrate
        new_filename=$(basename "${f}" .m4a).mp3
        printf "Reading %s\n\tand writing to %s\n\twith bitrate %s... (original: %s)\n" \
                "$f" "mp3s/${ALBUM}/${new_filename}" "$bitrate" "$((orgbitrate / 1024))k"
        ffmpeg -v warning -y -i "${f}" -codec:a libmp3lame -b:a "${bitrate}" -q:a 2 "mp3s/${ALBUM}/${new_filename}"
}

find ${ALBUM_PATH} -name "*.m4a" | while read m4a
do
        transcode_file "$m4a" < /dev/null
done

Use default=nk=1:nw=1 (no key, no wrappers) in place of flat=s=_ in the -of (output format) for ffprobe to get only the value and remove the need for the sed substitution.

ffprobe -v quiet -of default=nk=1:nw=1 -show_entries format=bit_rate "${f}"