bash script output to param

edit

I think roslaunch/XML/param's command attribute should do what you want:

command="$(find pkg-name)/exe '$(find pkg-name)/arg.txt'" (optional)

The output of the command will be read and stored as a string. It is strongly recommended that you use the package-relative $(find)/file.txt syntax to specify file arguments. You should also quote file arguments using single quotes due to XML escaping requirements.

I'm not sure what you mean with:

The script should also run in the same launch file