Shell helper for retagging M4B audiobooks with FFmpeg. Copies only audio streams, preserves chapters, writes clean title/album/artist/genre/date/comment metadata, and avoids common bin_data/text stream remux failures.

fix_m4b_tags.sh

fix_m4b_tags() {
  local input_file="$1"
  local title="$2"
  local album="$3"
  local artist="${4:-}"
  local genre="${5:-Audiobook}"
  local date="${6:-}"
  local comment="${7:-}"

  if [[ -z "$input_file" || -z "$title" || -z "$album" ]]; then
    echo "Usage:"
    echo "  fix_m4b_exif input.m4b \"Title\" \"Album\" [Artist] [Genre] [Year] [Comment]"
    return 1
  fi

  if [[ ! -f "$input_file" ]]; then
    echo "Error: file not found: $input_file"
    return 1
  fi

  if ! command -v ffmpeg >/dev/null 2>&1; then
    echo "Error: ffmpeg is not installed or not in PATH."
    return 1
  fi

  local filename="${input_file##*/}"
  local dir="${input_file%/*}"

  if [[ "$dir" == "$input_file" ]]; then
    dir="."
  fi

  local name="${filename%.*}"
  local ext="${filename##*.}"
  local output_file="${dir}/${name}-fixed-tags.${ext}"

  local cmd=(
    ffmpeg
    -y
    -i "$input_file"

    # Copy only audio streams. Do not blindly use "-map 0",
    # because some M4B files contain weird text/bin_data streams
    # that the ipod/m4b muxer rejects.
    -map 0:a

    # Keep chapter metadata without copying the problematic stream directly.
    -map_chapters 0

    -c copy

    -metadata "title=$title"
    -metadata "album=$album"
    -metadata "genre=$genre"
  )

  if [[ -n "$artist" ]]; then
    cmd+=(-metadata "artist=$artist")
  fi

  if [[ -n "$date" ]]; then
    cmd+=(-metadata "date=$date")
  fi

  if [[ -n "$comment" ]]; then
    cmd+=(-metadata "comment=$comment")
  fi

  cmd+=("$output_file")

  echo "Running FFmpeg command:"
  printf '  %q' "${cmd[@]}"
  echo
  echo

  "${cmd[@]}"
}