how to write a file via post-build groovy plug in slave

[michael chiu]

hi ALL,
          Does anyone have experience in write a file in the slave 
workspace via post-build groovy plugins ?
          I know how to read it but I don't know how to write it back .
          I always got the following error 
            groovy.lang.MissingMethdexception: No signature of method...
          thanks in advanced.
Regards,