title: Read a file to variable then replace control charactors with escape charecters
body: <p>I have a file like below </p>
  
  <pre><code> # cat test
  Start   tab
  enter 
  </code></pre>
  
  <p>or  with control characters </p>
  
  <pre><code># cat -A test
  Start^Itab$
  enter$
  </code></pre>
  
  <p>I want to convert file to a variable exactly like below</p>
  
  <pre><code>file = "Start\ttab\nenter"  
  
  echo -e $file
  Start   tab
  enter
  
  </code></pre>
  
  <p>How we can achieve this  just using bash script (without using any programming languages like perl or python).</p>
  
  <p>Please note : Given file here is just sample , I really have very big files with complex control characters , so 'sed' like methods will not work here.</p>