title: What are the special dollar sign shell variables?
body: <p>In Bash, there appear to be several variables which hold special, consistently-meaning values. For instance,</p>
  
  <pre><code>./myprogram &amp;; echo $!
  </code></pre>
  
  <p>will return the PID of the process which backgrounded <code>myprogram</code>. I know of others, such as <code>$?</code> which I think is the current TTY. Are there others?</p>