I happen to know about rsyn, and I use rsync to sync between my mac and a linux server as follows.rsync -r -t -v MAC LINUXrsync -r -t -v LINUX MACI expected to run the first command to sync, but I needed the second command also when a change is made in LINUX.Am I missing something? Does rsync have an option to sync between two directories? 
Syncing directories in both directions with rsync
rsync;synchronization
You want bi-directional sync.  Take a look at unison, which does this: http://www.cis.upenn.edu/~bcpierce/unison/For example, on Debian/Ubuntu:$ sudo apt-get install unison$ unison MAC/ LINUX/If you have trouble with permissions (example ext4 -> FAT):$ unison -perms 0 vlc-2.2.0/ /media/sf_vlc/vlcContacting server...Looking for changesReconciling changesvlc-2.2.0      vlc                new dir  ---->            /  [f] Proceed with propagating updates? [] yPropagating updates