open('binary', 'wb') do |f|
  (0..100).step(10) { |b| f << b.chr }
end
s = open('binary', 'rb') { |f| f.read } 

open('binary', 'rb') { |f| f.each_byte { |b| puts b } }

0
10
20
...
90
100

0
10
20
30
40
50
60
70
80
90
100

f = open('binary')
f.pos

f.read(1)

f.pos

f.pos = 4

f.read(2)

f.pos

f.seek(8)                                    
f.pos

f.seek(-4, File::SEEK_CUR)
f.pos

f.seek(2, File::SEEK_CUR)
f.pos

f.seek(-2, File::SEEK_END)                                    
f.pos

f.read(500)

f.pos

f.eof?

f.close
f = open('binary', 'rb+')
f.read

f.pos = 2
f.write('Hello.')
f.rewind
f.read

f << 'Goodbye.'
f.rewind
f.read

f.close
def parse_id3(mp3_file)
  fields_and_sizes = [[:track_name, 30], [:artist_name, 30], 
                      [:album_name, 30], [:year, 4], [:comment, 30], 
                      [:genre, 1]]
  tag = {}
  open(mp3_file) do |f|
    f.seek(-128, File::SEEK_END)
    if f.read(3) == 'TAG' # An ID3 tag is present
      fields_and_sizes.each do |field, size|
        # Read the field and strip off anything after the first null 
        # character.
        data = f.read(size).gsub(/\000.*/, '')
        # Convert the genre string to a number.
        data = data[0] if field == :genre
        tag[field] = data
      end
    end
  end
  return tag
end
parse_id3('ID3.mp3')

{:year=>"2005", :artist_name=>"The ID Three",

Error! (Exception?) Here's stdout:
Errno::ENOENT: No such file or directory - ID3.mp3
	from (irb):40:in `initialize'
	from (irb):40:in `parse_id3'
	from (irb):55

parse_id3('Too Indie For ID3 Tags.mp3')

{}

Error! (Exception?) Here's stdout:
Errno::ENOENT: No such file or directory - Too Indie For ID3 Tags.mp3
	from (irb):40:in `initialize'
	from (irb):40:in `parse_id3'
	from (irb):56

#Returns [track, artist, album, year, comment, genre]
def parse_id3(mp3_file)
  format = 'Z30Z30Z30Z4Z30C'
  open(mp3_file) do |f|
    f.seek(-128, File::SEEK_END)
    if f.read(3) == "TAG" # An ID3 tag is present
      return f.read(125).unpack(format)
    end
  end
  return nil
end
parse_id3('ID3.mp3')

["ID 3", "The ID Three", "Binary Brain Death", "2005", "http://www.example.com/id3/", 22]

Error! (Exception?) Here's stdout:
Errno::ENOENT: No such file or directory - ID3.mp3
	from (irb):60:in `initialize'
	from (irb):60:in `parse_id3'
	from (irb):68

id3 = ["ID 3", "The ID Three", "Binary Brain Death", "2005", 
       "http://www.example.com/id3/", 22]
id3.pack 'Z30Z30Z30Z4Z30C'