I'm playing around with creating a Galaxy wrapper for custom tool I've created.  My tool takes a single parameter, and SRA ID.  In command block, I have something like this:

<command>
  ${GALAXY_ROOT_DIR}/tools/mycustomtools/doSomething.pl --sra $sraid
</command>

<inputs>
  <param name="sraid" ... />
</inputs>

<outputs>
  <data format="txt", name="out1", from_work_dir="${sraid}.data" />
  <data format="txt", name="out2", from_work_dir="${sraid}.txt" />
</outputs>

Galaxy doesn't like thr $sraid in the output.  I see from the docs I need to use discover_datasets.   It would be nice to use a parameter such as what I have to name output file as my script names output files use an input parameter as the base.