title: Read last line of file after the file was modified
body: <p>I have a problem with reading the last line of file in Linux Ubuntu. 
  I have a file named auth.log and I'm trying to read it last line after new line was added (after file was modified).<br><br>
  I know i need to use 
  <code>tail -1 /var/log/auth.log</code> to get last line but I don't know how to check the file every time it was modified. <br><br>
  After reading the last line i need to check if it contains "login" word and if yes I need to send this line by email. As far as I know grep needs to be use with mail command.</p>
  
  <p>Any help would be awesome, thanks!</p>