title: Why capistrano check non existence of rbenv directory?
body: <p>I get following message when I run <code>cap production deploy</code> in capistrano 3.</p>
  
  <pre><code>DEBUG [5021b4ce] Running /usr/bin/env [ ! -d /usr/local/rbenv/versions/2.0.0-p451 ] on example.com
  DEBUG [5021b4ce] Command: [ ! -d /usr/local/rbenv/versions/2.0.0-p451 ]
  DEBUG [5021b4ce] Finished in 0.853 seconds with exit status 1 (failed).
  </code></pre>
  
  <p>This is <code>rbenv</code> part in <code>config/deploy.rb</code>.</p>
  
  <pre><code>set :rbenv_type, :system # or :user, depends on your rbenv setup
  set :rbenv_ruby, '2.0.0-p451'
  set :rbenv_path, '/usr/local/rbenv'
  set :rbenv_prefix, "RBENV_ROOT=#{fetch(:rbenv_path)} RBENV_VERSION=#{fetch(:rbenv_ruby)} #{fetch(:rbenv_path)}/bin/rbenv exec"
  set :rbenv_map_bins, %w{rake gem bundle ruby rails}
  set :rbenv_roles, :all # default value
  </code></pre>
  
  <p>I thought <code>Capistrano</code> should check existence of <code>rbenv</code> directory.
  Why the command is <code>[ ! -d #{rbenv_dir}]</code>, instead of <code>[ -d #{rbenv_dir}]</code>?</p>