question: if I close my CLI window - this is, without sending ctrl-c to the command - will the sync process stops?
To me, the aws s3 sync instruction has already been received by aws, so it shouldn't harm...
So, just for every other person running the same question, the response is: yes, the process does end if the shell window is closed.
Why? Because the command issues GET and PUT for each object to be synced hence, when the shell is killed, the active commands too.
HTH.