Convert M4A into MP3 (Linux)

m4a_to_mp3.rb

#! /usr/bin/ruby

# @ubuntu: depends on ffmpeg, libavcodec-extra-53

output_dir = "output"

if File.exists? output_dir
    id = 1
    while File.exists?(output_dir + id.to_s) && id < 10
        id += 1
    end
    if id >= 10
        puts "error: can't figure out new output directory name"
        exit 0
    end
    output_dir = output_dir + id.to_s
end

Dir.mkdir output_dir

Dir.glob("*.m4a").each do | file |
    puts "processing '#{file}'"
    file.gsub! /\.m4a/, ''
    cmd = 'ffmpeg -v 1 -i "' + file + '".m4a -f mp3 -b:a 320k "' + output_dir + "/" + file + '.mp3"'
    puts " command: " + cmd
    if not system(cmd)
        puts " error"
    else
        puts " done."
    end
end