def remote_file_exists?(full_path)
  'true' ==  capture("if [ -e #{full_path} ]; then echo 'true'; fi").strip
end
 
namespace :remote do
  namespace :file do
    desc "test existence of missing file"
    task :missing do
      if remote_file_exists?('/dev/mull')
        raise "It's there!?"
      end
    end
 
    desc "test existence of present file"
    task :exists do
      unless remote_file_exists?('/dev/null')
        raise "It's missing!?"
      end
    end
  end
end