使用dropbox自动同步自己mac上的文件夹：
ln -s /Volumes/Macintosh_HD_2/Programs/-My_Programs /Users/foreverycc/Dropbox/My_Program
# . must be full absolute path ( "~/Dropbox" doesn't work)
# . The target directory (Dropbox directory) doesn't have to be the same name as the original directory
Done！ 非常简单
Ref：
http://www.linuxidc.com/Linux/2011-05/35373.htm
http://blog.yslove.net/2012/07/mac-dropbox-sync-any-folder/