title: what is difference in `declare -r` and `readonly` in bash?
body: <p>In bash, what is the difference in <code>declare -r</code> and <code>readonly</code>?</p>
  
  <pre><code>$ declare -r a="a1"
  $ readonly b="b1"
  </code></pre>
  
  <p>I'm not sure which to choose.</p>