title: How can I compare my local forked repository with changes that may have been made to the original?
body: <p>I want to compare the local clone of a repository I have forked with the original/upstream repository to see if further commits have been made requiring me to pull/merge. I'd like to do this from the command line.</p>
  
  <p>I added the original repository to my list of remotes with this command:</p>
  
  <pre><code>git remote add upstream &lt;original repo URL&gt;
  </code></pre>
  
  <p>This is outlined in <a href="https://help.github.com/articles/fork-a-repo/" rel="noreferrer">Github's own page on the topic of forking a branch</a>.</p>
  
  <p>However, when I run <code>git diff upstream</code>  or  <code>git diff upstream/master</code> <a href="https://stackoverflow.com/a/17688732/1563507">as advised here</a> or <code>git diff master upstream/master</code> <a href="https://stackoverflow.com/a/1801150/1563507">as advised here</a>, I get this:</p>
  
  <pre><code>fatal: ambiguous argument 'upstream': unknown revision or path not in the working tree.
  Use '--' to separate paths from revisions, like this:
  'git &lt;command&gt; [&lt;revision&gt;...] -- [&lt;file&gt;...]'
  </code></pre>
  
  <p>The command <code>git diff origin/master</code> does not return an error (or anything for that matter). Have I missed something?</p>