"Hmmm… he's using strtok, that's not how a real shell would work. What would a minimal shell, without scripting, pipes, redirects etc. do? Just correctly parsing legal file paths (which TFA needs to correctly implement 'cd') is well out of scope of a small article like this."
You have backslashes, single quotes, and double quotes basically. Traditionally this is done with switch statement in a loop in C.
But that is not a good approach for a real shell. Even inside double quotes you can have a fully recursive program, like:
$ echo "hi ${v1:-A${v2:-X${v3}Y}B}"
hi AXYB