Parser generators aren't widely used for implementing shells (or JavaScript engines, or C/C++ compilers, for that matter). IMO they're nice for designing languages, but not necessarily implementing them.

bash is actually one of the only shells that uses yacc, and the maintainer regards it as a mistake. It uses yacc for maybe 1/4 of the language and the rest is all hand written stuff intertwined with generated code. It's pretty messy.

See http://www.aosabook.org/en/bash.html

and e.g. https://www.oilshell.org/blog/2016/10/13.html

TCL?