I have a Back-Up HDD (not TimeMachine) with only very important data backed-up there. Every once in a while, I connect the external hard disk and sync the files per hand. As the files I am syncing are always the same, it came to my mind that I could automate the whole process as a shell script.
However, I could not find a helpful command that compares A to B (which will then ask me to sync if sync is necessary). I have found rsync but the documentations did not explain how to synchronise a single directory.
I also haven't figured out how to change directory to an external HDD connected via USB.