title: While loop, username in /etc/passwd
body: <p>How would I go about writing a while loop that says: While a username is in the /etc/passwd file, do (command)? 
  I'm trying to use a command such as <code>grep -q "^{usern}:" /etc/passwd</code> but I'm not sure how to put that as the condition of the while loop.  </p>