File.open, write and save?

If you just need to perform a simple script like creating a file, you can simply use a Ruby script without creating a rake task.

# file origin.rb
target  = "target.rb"
content = <<-RUBY
  puts "I'm the target!"
RUBY

File.open(target, "w+") do |f|
  f.write(content)
end

And you can execute the file with

$ ruby origin.rb