$filename = 'test.dat'

[ '', 'a', 'aa' ].each do |data|
  # Write data
  File.open($filename, 'w') { |io| io.write(data) }

  # Read data
  File.open($filename, 'r') do |io|
    puts "Reading data..."
    until io.eof?
      data = io.read(1000)
      puts "Read data: #{data.inspect}"
    end
    puts "Done."
    puts
  end
end

# ===== ACTUAL OUTPUT
#
# Reading data...
# Done.
# 
# Reading data...
# Read data: nil
# Done.
# 
# Reading data...
# Read data: "aa"
# Done.
#
# ===== EXPECTED OUTPUT
#
# Reading data...
# Done.
# 
# Reading data...
# Read data: "a"
# Done.
# 
# Reading data...
# Read data: "aa"
# Done.