Have you ever wanted a quick way to compare two directories (folders), in order to see which files may differ between the two? There are third-party GUI tools as well, but there’s actually a free folder comparison tool built into every Mac—it just requires a quick trip to Terminal to put it to use. The program is called diff, and it’s quite simple to use.
Launch Terminal (in Applications > Utilities), and then use the cd command to change to the directory containing the folders you’d like to compare. (The folders can be located anywhere, of course, but it’s easiest if they’re in the same folder.).
Read more at Macworld.com