Tino Gomes

desc "Say hi. Use USER environment variable as default"
  task :hi, :user do |t, args|
  args.with_defaults(:user => ENV["USER"])

  puts "Hi #{args[:user]}!"
end

Código disponível em http://gist.github.com/130194

logo:

celestino@macbook:~
$ echo $USER
celestino

celestino@macbook:~ $ rake hi
(in /Users/celestino)
Hi celestino!

celestino@macbook:~ $ USER=tino rake hi
(in /Users/celestino)
Hi tino!

celestino@macbook:~ $ rake hi USER=tino2
(in /Users/celestino)
Hi tino2!

celestino@macbook:~ $ rake hi[tino3]
(in /Users/celestino)
Hi tino3!

Fonte: http://nhw.pl/wp/2008/10/11/rake-and-arguments-for-tasks