They’re out there… somewhere. I summarized a few options in this post. Also read Rich’s followup. To read/write to a file, you could use executeCommandLine to do something from a shell, or use something from java.io.
Here is another example using JSR223. With your background (I noticed in another thread), you may be much happier using JSR223 with Jython or Javascript rather than Xtend and the Rules DSL. I’m getting pretty close to completing a migration from the Rules DSL to JSR223-Jython, and I’ve been very happy with it!