convert m4b chapters to mp3, vorbis, opus or speex

convert_m4b.sh

#!/bin/bash

#convert_m4b.sh book.m4b --(mp3|opus|vorbis|speex)

# 1. Parse Arguments
input_file=""
format="mp3" # Default format

for arg in "$@"; do
    case "$arg" in
        --opus)   format="opus" ;;
        --vorbis) format="vorbis" ;;
        --speex)  format="speex" ;;
        *)
            if [[ -f "$arg" ]]; then
                input_file="$arg"
            fi
            ;;
    esac
done

# 2. Validation & Config
if [[ -z "$input_file" ]]; then
    echo "Usage: $0 /path/to/book.m4b [--opus | --vorbis | --speex]"
    exit 1
fi

if [[ ! -f "$input_file" ]]; then
    echo "Error: File '$input_file' not found."
    exit 1
fi

# Set encoder parameters based on selected format
if [[ "$format" == "opus" ]]; then
    echo "Mode: Opus (High Efficiency)"
    ext="opus"
    codec="libopus"
    # 64k VBR is generally transparent for speech
    enc_params="-b:a 64k -vbr on"

elif [[ "$format" == "vorbis" ]]; then
    echo "Mode: Vorbis (Ogg)"
    ext="ogg"
    codec="libvorbis"
    # Quality 5 is approx ~160kbps
    # Quality 2 is approx ~96kbps
    enc_params="-q:a 2"

elif [[ "$format" == "speex" ]]; then
    echo "Mode: Speex (Legacy Speech)"
    ext="spx"
    codec="libspeex"
    # Speex requires specific sample rates (8k, 16k, 32k).
    # Approx ~36kbps
    enc_params="-ar 32000 -q:a 10"

else
    echo "Mode: MP3 (Universal Compatibility)"
    ext="mp3"
    codec="libmp3lame"
    # Quality 2 is VBR ~190kbps (Standard High Quality)
    enc_params="-q:a 2"
fi

# 3. Path Setup
input_dir=$(dirname "$input_file")
base_name=$(basename "$input_file" .m4b)
# Remove extension if user passed a file that isn't strictly named .m4b but is valid
base_name="${base_name%.*}"
output_dir="${input_dir}/${base_name}"

echo "Input: $input_file"
echo "Output Directory: $output_dir"
mkdir -p "$output_dir"

# 4. Extract Metadata
# Get Artist/Album to tag the output files cleanly
artist=$(ffprobe -v quiet -show_entries format_tags=artist -of default=noprint_wrappers=1:nokey=1 "$input_file")
album=$(ffprobe -v quiet -show_entries format_tags=album -of default=noprint_wrappers=1:nokey=1 "$input_file")

# Fallbacks if metadata is missing
[[ -z "$artist" ]] && artist="Unknown Artist"
[[ -z "$album" ]] && album="$base_name"

echo "Metadata -> Artist: $artist | Album: $album"

# 5. Get Chapter Data
# We dump chapters to a temp file to parse them safely
chapters_file="${output_dir}/chapters.tmp"
ffprobe -i "$input_file" -print_format flat -show_chapters -loglevel error > "$chapters_file"

total_chapters=$(grep -c "chapters.chapter.*.start_time" "$chapters_file")
echo "Found $total_chapters chapters."

chapter_count=0

# 6. Loop and Convert
while IFS= read -r line; do
    # Capture start time
    if [[ "$line" =~ chapters\.chapter\.([0-9]+)\.start_time=\"([^\"]+)\" ]]; then
        start_time="${BASH_REMATCH[2]}"
    fi
    # Capture end time
    if [[ "$line" =~ chapters\.chapter\.([0-9]+)\.end_time=\"([^\"]+)\" ]]; then
        end_time="${BASH_REMATCH[2]}"
    fi
    # Capture title and execute conversion
    if [[ "$line" =~ chapters\.chapter\.([0-9]+)\.tags\.title=\"([^\"]+)\" ]]; then
        title="${BASH_REMATCH[2]}"

        ((chapter_count++))

        # Sanitize filename (remove weird chars like / : ?)
        safe_title=$(echo "$title" | tr -dc 'a-zA-Z0-9 \-')
        # Pad track number (01, 02...)
        pad_track=$(printf "%02d" "$chapter_count")

        outfile="${output_dir}/${pad_track} - ${safe_title}.${ext}"

        echo "  [${chapter_count}/${total_chapters}] Converting: ${safe_title}"

        # FFmpeg Conversion
        # -nostdin: prevents ffmpeg from swallowing loop input
        # $enc_params is unquoted to allow multiple flags to expand correctly
        ffmpeg -nostdin -v error -stats \
            -i "$input_file" \
            -ss "$start_time" -to "$end_time" \
            -map 0:a \
            -c:a "$codec" $enc_params \
            -metadata title="$title" \
            -metadata artist="$artist" \
            -metadata album="$album" \
            -metadata track="$chapter_count" \
            "$outfile"
    fi
done < "$chapters_file"

# Cleanup
rm "$chapters_file"
echo "Success! Files are located in: $output_dir"