--delete
              This tells rsync to delete any files on the receiving side that
              aren’t  on  the  sending  side.    Files that are excluded from
              transfer  are  excluded  from  being  deleted  unless  you  use
              --delete-excluded.

              This  option  has  no  effect  if  directory  recursion  is not
              selected.

              This option can be dangerous if used incorrectly!  It is a very
              good  idea  to  run  first using the dry run option (-n) to see
              what files would be deleted to make sure important files aren’t
              listed.

              If the sending side detects any I/O errors then the deletion of
              any files at the destination will  be  automatically  disabled.
              This  is  to prevent temporary filesystem failures (such as NFS
              errors) on the sending side causing a massive deletion of files
              on  the  destination.  You can override this with the --ignore-
              errors option.