A trick using a closure for memoization

github_user.rb

def github_user
  @github_user ||= begin
    github_user, shell_user = %x{git config --get github.user}.chomp, ENV['USER'].to_s

    raise Exception, 'Unable to determine github user' if (github_user + shell_user).empty?

    github_user.empty? ? shell_user : github_user
  end
end