Run block as another user, in subprocess

asuser.rb

require 'etc'

def as_user(user, &block)
  # Find the user in the password database.
  u = (user.is_a? Integer) ? Etc.getpwuid(user) : Etc.getpwnam(user)

  # Fork the child process. Process.fork will run a given block of code
  # in the child process.
  Process.fork do
     # We're in the child. Set the process's user ID.
     Process.uid = u.uid

    # Invoke the caller's block of code.
    block.call(user)
  end
end