I just came across bed, the bash editor [0], I was really expecting a complete nightmare and a massive script that barely functioned but was pleasantly surprised by how short and clean it is. Despite being essentially useless (beyond as an exercise) you could use it as an editor, I even learned/relearned a few things from it and used it for awhile on my own project.
0: https://github.com/comfies/bed/blob/master/bed
If I would describe the vulnerability series, it is something like SQL injection but for services that run shell commands in the background.
The parsers of both CGI and shells were/are so broken that I would just never run any shell command on a web server in a non-manual manner.
[1] https://en.m.wikipedia.org/wiki/Shellshock_(software_bug)
:(){ :|:& };:
The fork bomb
> man man
But maybe you're not thinking of something that simple.
BNF / 2nd order logic combinator style programming via data files & shell sourcing as a lead in to why learn awk.