function to check for a read or write lock

flocked.rb

require 'fcntl'

class File
  def flocked? &block
    flockstruct = [Fcntl::F_RDLCK, 0, 0, 0, 0].pack("ssqqi")
    fcntl Fcntl::F_GETLK, flockstruct
    status = flockstruct.unpack("ssqqi")[0]
    case status
      when Fcntl::F_UNLCK
        return false 
      when Fcntl::F_WRLCK|Fcntl::F_RDLCK
        return true
      else
        raise SystemCallError, status
    end
  end
  alias_method "if_not_flocked", "flocked?"
end

dpkg = open("/var/lib/dpkg/lock")

if dpkg.flocked?
  p "it's locked"
else
  p "not locked"
end