title: How to check if a process id (PID) exists
body: <p>In a bash script, I want to do the following (in pseudo-code):</p>
  
  <pre><code>if [ a process exists with $PID ]; then
  
      kill $PID 
  
  fi
  </code></pre>
  
  <p>What's the appropriate expression for the conditional statement?</p>