title: What does $$ mean in the shell?
body: <p>I once read that one way to obtain a unique filename in a shell for temp files was to use a double dollar sign (<code>$$</code>).   This does produce a number that varies from time to time... but if you call it repeatedly, it returns the same number.  (The solution is to just use the time.)</p>
  
  <p>I am curious to know what <code>$$</code> actually is, and why it would be suggested as a way to generate unique filenames.</p>