title: What is the difference between bundle and bundle update?
body: <p>I get two different results when using the two commands <code>bundle</code> and <code>bundle update</code></p>
  
  <p>If I use <code>bundle update</code>, I get the following error:</p>
  
  <pre><code>Resolving dependencies...
  Bundler could not find compatible versions for gem "railties":
    In Gemfile:
      requirejs-rails (&gt;= 0) ruby depends on
        railties (~&gt; 3.1.1) ruby
  
      rails (= 4.0.0.rc2) ruby depends on
        railties (4.0.0.rc2)
  
  Bundler could not find compatible versions for gem "rails":
    In Gemfile:
      requirejs-rails (&gt;= 0) ruby depends on
        rails (~&gt; 3.1.1) ruby
  
      rails (4.0.0.rc2)
  </code></pre>
  
  <p>But if I use just <code>bundle</code>, I get the following :</p>
  
  <pre><code>Resolving dependencies...
  Bundler could not find compatible versions for gem "activesupport":
    In snapshot (Gemfile.lock):
      activesupport (3.2.2)
  
    In Gemfile:
      rails (= 4.0.0.rc2) ruby depends on
        activesupport (= 4.0.0.rc2) ruby
  
  Running `bundle update` will rebuild your snapshot from scratch, using only
  the gems in your Gemfile, which may resolve the conflict.
  </code></pre>
  
  <p>The errors point to two different dependencie issues, but both commands are <code>fetching gem metadata from https://rubygems.org/...</code> and <code>Resolving dependencies...</code>, so how are they different?</p>
  
  <p>I have always used just <code>bundle</code>, but tried <code>bundle update</code> and noticed the difference.</p>