title: expr command not working in shell
body: <p>Following is the code that I tried -</p>
  
  <pre><code>nisarg@nisarg-ThinkPad-T61:~$ export a=1
  nisarg@nisarg-ThinkPad-T61:~$ export b=2
  nisarg@nisarg-ThinkPad-T61:~$ echo $a
  1
  nisarg@nisarg-ThinkPad-T61:~$ echo $b
  2
  nisarg@nisarg-ThinkPad-T61:~$ echo 'expr $a + $b'
  expr $a + $b
  </code></pre>
  
  <p>I even made sure there are spaces around <code>+</code> as they are the cause of most errors. </p>
  
  <p>Why isn't this working? </p>