code: function (source, target, callback) {
      source = fss.slashify(source);
      target = fss.slashify(target);
  
      if (!shell.which('rsync')) {
          log('sync error', 'no rsync command found');
          callback();
      } else {
          log('rsync', fss.baseRelative(source));
          log('remote', target);
          var cmd = [config.rsync, source, target].join(' ');
          var opt = { maxBuffer: 5000*1024 };
          exec(cmd, opt, rsync_end(callback));
      }
  }
docs: /*
  Make sure rsync options are all right, and sync.