In that case, there are two strategies for synchronizing file systems -- either:
1. The operator tells the program what to do.
-- or --
2. The program gets it wrong at least some of the time.
In the case of contact list synchronization, like Google Contacts, the system assumes that the device the operator is editing at the moment is the source, and acts accordingly. Notice that a positive determination is made as to what the source and destination are, based on the operator's activity and its timing.
In all other cases where two directory trees are synchronized, are made to have the same content, and in which files may be deleted as well as added, the operator has to tell the program which is the source and which is the destination. End, full stop.
1. The operator tells the program what to do.
-- or --
2. The program gets it wrong at least some of the time.
In the case of contact list synchronization, like Google Contacts, the system assumes that the device the operator is editing at the moment is the source, and acts accordingly. Notice that a positive determination is made as to what the source and destination are, based on the operator's activity and its timing.
In all other cases where two directory trees are synchronized, are made to have the same content, and in which files may be deleted as well as added, the operator has to tell the program which is the source and which is the destination. End, full stop.