title: Replace spaces with underscores via BASH
body: <p>Suppose i have a string, $str.
  I want $str to be edited such that all the spaces in it are replaced by underscores.</p>
  
  <p>Example</p>
  
  <pre><code>a="hello world"
  </code></pre>
  
  <p>I want the final output of </p>
  
  <pre><code>echo "$a"
  </code></pre>
  
  <p>to be <strong>hello_world</strong></p>