My brother just finished writing a forth like interpreter for a freshman CS project (in java, not Bash, thankfully). It is a really nice language for learning the basics of an interpreter without worrying about the parser side too much.
Forth is a ball of mud that you start at the beginning and at the end you end up with a shiny turd of a https://en.wikipedia.org/wiki/Dorodango.
Forth is bottom up insted of top down. The concepts teaching/learning wise start at the bottom and lead up to the top. You can't learn that starting with Python or Fortran or C or BASIC, or even Assembler. Bottom up.
Forth might not be the best Pascal/Basic/Python learning some stuff language, but Forth is the best bits in memory to HLL "full-stack" language.
I wish your brother had done it on raw hardware instead of in Java. It is a really nice language for learning.
I think any grammar needs to cover immediate words because you cannot ignore CREATE… DOES>. Those can do anything, so how do you handle them?
There’s also BASE (the current number conversion radix). It makes parsing integers context-dependent.
One of it's biggest downfalls imo is that the toeknizer is too simple. I'm making GitHub.com/civboot/fngi as a "better than forth".
It makes a nice foundation for syntactic experiments.