title: What is echo $? in linux terminal?
body: <p>I have tried those commands. </p>
  
  <pre><code>~$top
  (ctrl + z)stopped the process
  ~$echo $?
  147
  ~$top
  (ctrl + c)killed the process
  ~$echo $?
  0
  </code></pre>
  
  <p>What happened here, please explain it and why it showing some constant value. What is the meaning of those values.</p>