md5sum_file.rb

# Hex digest a file, without loading the whole file into memory:

require 'digest/md5'

def md5sum_file(path)
  digest = Digest::MD5.new
  File.open(path, 'r') do |file|
    while buffer = file.read(1024*8)
      digest << buffer
    end
  end
  digest.hexdigest
end

puts md5sum_file('/etc/hosts')