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/${ALBUM}"

# 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/${ALBUM}/ directory:
for f in "${ALBUM_PATH}"/*.m4a
do
        echo "Before ffprobe... testing ${f}"
        bitrate=$(ffprobe -v quiet -of flat=s=_ -show_entries format=bit_rate "${f}" | sed 's/[^0-9]*//g')
        new_filename=$(basename "${f}" .m4a).mp3
        echo "Reading ${f} and writing to mp3s/${ALBUM}/${new_filename} with bitrate ${bitrate}..."
#       echo ffmpeg -y -i "${f}" -codec:a libmp3lame -b:a "${bitrate}" -q:a 2 "mp3s/${ALBUM}/${new_filename}"
        ffmpeg -y -i "${f}" -codec:a libmp3lame -b:a "${bitrate}" -q:a 2 "mp3s/${ALBUM}/${new_filename}"
done

You might want to change the shebang to a more universal path like /bin/env bash

You might want to change the shebang to a more universal path like /bin/env bash

Thanks for the tip! I've also replaced the pesky backticks...

Changed shebang to #!/usr/bin/env bash as per @mashuptwice's suggestion on SuperUser.com

Your ToDo is now done

Thanks for the excellent work, all I did was wrap it in a function and put a find around it to get it to work recursively. I also added a cap to 224k of the bitrate. One note, having looked at the resulting mp3s in my library: I am not entirely sure if the combination of -b:a "${bitrate}" -q:a 2 makes sense. The one tells ffmpeg to use a fixed bitrate of $bitrate, the other tells it to use a VBR of 170-210... I left it in my variant like this however.