For this lab you need to use awk.
First use nmap to scan the localhost and redirect the output to a file (see man nmap if you don't know how to do this).
Then process your file. Yesterday's file should be compared to todays file to see if there is any difference in the two. Use awk to clean up the output and generate JUST a list of ports and then compare that. Use the diff command to see if the two files have changed and generate an error file if so.
This is, of course, in a script.