function next() {
		i += 1;
		if ( i < len ) {
			if ( file[ i ] ) {
				self._rli.write( file[ i ]+'\n' );
				self.once( 'drain', next );
			} else {
				nextTick( next );
			}
		} else {
			nextTick( clbk );
		}
	}
Callback invoked after draining the command queue.

@private
@param {string} cmd - command
@param {boolean} success - boolean indicating whether the command successfully executed