Given source and destination directory listings, return the files that must be synced: those missing at the destination or differing in size or modification time.

Given two space-delimited directory listings, source and destination (permissions, username, size, modification time, filename), return the filenames that must be synced, in source order: sync when the file is missing at the destination or its size or modification time differs; skip when the destination copy is not writable or the source is not readable.