Rakefile

# rake 'spec[spec/rake]' # => "spec/rake"
# rake spec # => "/bin:/usr/bin:/usr/local/bin"

task :spec, :path do |task, args|
  p args.path
end

task_arguments.rb

module Rake
  class TaskArguments
    def lookup(name)
      if @hash.has_key?(name)
        @hash[name]
      elsif ENV.has_key?(name.to_s)
        ENV[name.to_s]
      elsif ENV.has_key?(name.to_s.upcase)
        ENV[name.to_s.upcase]
      elsif @parent
        @parent.lookup(name)
      end
    end
  end
end