A Simple Script to Convert a Video Into a Collection of MP3s, One File Per Chapter

chapt_2_mp3.rb

#!/usr/bin/ruby -w

#a simple script to take a video file and convert it 
#into a collection of mp3 files, one file per chapter
#requires an ffmpeg install with ffprobe and support for
#mp3 plus whatever formats the input uses

#FNG, 2025

require 'json'
require 'shellwords' # Todo: safely handle chapter titles with spaces and special characters, replace sanitize

class Chapter
  attr_reader :id, :title, :start_time, :end_time, :duration

  def initialize(id, title, start_time, end_time)
    @id = id
    @title = title
    @start_time = start_time.to_f
    @end_time = end_time.to_f
    @duration = @end_time - @start_time
  end

  def to_s
    "ID: #{@id}, Title: #{@title}, Start: #{@start_time}, End: #{@end_time}, Duration: #{@duration}"
  end
end

def extract_chapters_from_video(video_filename)
  begin
    ffprobe_command = "ffprobe -v quiet -print_format json -show_chapters \"#{video_filename}\""
    json_output = `#{ffprobe_command}`

    data = JSON.parse(json_output)
    chapters_data = data['chapters'] || []
    chapter_objects = []

    chapters_data.each do |chapter_info|
      id = chapter_info['id']
      title = chapter_info.dig('tags', 'title')
      start_time = chapter_info['start_time']
      end_time = chapter_info['end_time']
      chapter_objects << Chapter.new(id, title, start_time, end_time)
    end

    chapter_objects

  rescue Errno::ENOENT
    puts "Error: File '#{video_filename}' not found."
    return []
  rescue JSON::ParserError
    puts "Error: Could not parse ffprobe output. Make sure ffmpeg/ffprobe is installed and the video file is valid."
    return []
  rescue StandardError => e
    puts "An unexpected error occurred: #{e.message}"
    return []
  end
end

def sanitize_filename(title)
  title.gsub(/[^a-zA-Z0-9\s_-]/, '')
end

def convert_chapter_to_mp3(video_filename, chapter)
  sanitized_title = sanitize_filename(chapter.title)
  output_filename = "[#{chapter.id}] #{sanitized_title}.mp3"
  ffmpeg_command = "ffmpeg -i \"#{video_filename}\" -ss #{chapter.start_time} -t #{chapter.duration} -vn \"#{output_filename}\""

  puts "Converting chapter: #{chapter.title} (Start: #{chapter.start_time}, Duration: #{chapter.duration} seconds) to #{output_filename}"
  system(ffmpeg_command)
end

if __FILE__ == $0
  if ARGV.empty?
    puts "Usage: ruby #{$0} <video_filename>"
  else
    video_filename = ARGV[0]
    chapters = extract_chapters_from_video(video_filename)

    chapters.each do |chapter|
      convert_chapter_to_mp3(video_filename, chapter)
    end

    puts "Finished processing all chapters."
  end
end