title: How to understand diff -u in linux?
body: <p>example code </p>
  
  <pre><code>diff -r -u -P a.c b.c &gt; diff.patch
  </code></pre>
  
  <p>I've tried to search in man.</p>
  
  <p>man says that diff -u is to unify the pattern of output, what is the meaning of that and when should we use it?</p>
  
  <p>thanks a lot.</p>