Audibook convert m4b to mp3. This will split into chaptered mp3 files and automatically reconfigure proper ID3v2 tags. (adjusted to work on MacOS)

convert_m4b.sh

#!/bin/bash
#
# brew install id3v2 ffmpeg
#
# USAGE:
# cd /book title/
# bash ~/this_script_path.sh
# rm *.m4b (you need to manually remove the original in case something goes wrong)
#
#
# EXAMPLE:
#
#   $ ls
#   The Wheel of Time - Book 11 - Knife of Dreams, Part 1.m4b  The Wheel of Time - Book 11 - Knife of Dreams, Part 2.m4b
#   $ ~/convert_m4b.sh
#   .....
#
#   [OUTPUT]:
#
#   File: Knife of Dreams - Chapter 18 - News for the Dragon.mp3
#   Metadata: ID3v2.3
#   Title: Chapter 18 - News for the Dragon
#   Artist: Robert Jordan
#   Album: Knife of Dreams
#   Track: 23
#   Genre: Audiobooks

#initial track number
track=1

#assumes ( if there are multiple files Part 01/Part 02) that they are in alphabetical order
for i in *.m4b;
do
    name=`echo $i | cut -d'.' -f1`;
    echo $name;
    ffmpeg -i "$i" -acodec libmp3lame -ar 22050 -ab 64k "$name.mp3"
    id3v2 -2 "$name.mp3"
    full_file_path="$name.mp3"

    #split chapters
    title=$(pwd | sed 's,^\(.*/\)\?\([^/]*\),\2,' | cut -d , -f 1)

    ffmpeg -i "$full_file_path" 2> tmp.txt

    while read -r first _ _ start _ end; do
        if [[ "${first}" = "Chapter" ]]
        then
            read  # discard line with Metadata:
            read _ _ chapter
            chapter=$(sed -re ":r;s/\b[0-9]{1,$((1))}\b/0&/g;tr" <<<$chapter)
            chapter_file="${title} - ${chapter}.mp3"
            echo "processing $chapter"
            </dev/null ffmpeg -loglevel error -stats -i "${full_file_path}" -ss "${start%?}" -to "${end}" -codec:a copy -metadata track="${chapter}" "${chapter_file}"
            id3v2 -2 "$chapter_file"
            id3v2 --song "$chapter" "$chapter_file"
            id3v2 --album "$title" "$chapter_file"
            id3v2 --track "$track" "$chapter_file"
            echo "$title - $chapter"
            track=$((track+1))
        fi
    done <tmp.txt

    rm tmp.txt
    rm "$full_file_path"

done;