title: sed replace empty line with character
body: <p>How do you replace a blank line in a file with a certain character using sed?</p>
  
  <p>I have used the following command but it still returns the original input:</p>
  
  <pre><code>sed 's/^$/&gt;/' filename
  </code></pre>
  
  <p>Original input:</p>
  
  <pre><code>ACTCTATCATC
  
  CTACTATCTATCC
  
  CCATCATCTACTC
  
  ...
  </code></pre>
  
  <p>Desired output:</p>
  
  <pre><code>ACTCTATCATC
  &gt;
  CTACTATCTATCC
  &gt;
  CCATCATCTACTC
  &gt;
  ...
  </code></pre>
  
  <p>Thanks for any help</p>