[galaxy-dev] About the output

I made a galaxy tool which is just create a txt file. When I use this tool , I wish to let it show the detail of the txt. In the xml file I use : <output> <data name="test" format="txt"><output/> But it does not work, when I click the "eye" button in galaxy, a download dialog appear, but it is just a empty file. Could somebody tell me what should I do , I want show the txt detail in the galaxy.