Thanks for continuing your series.
This is a great illustration of how non-trivial a production quality shell is.
Parsing input is "tricky"
Each builtin needs comprehensive error handling
This is a great illustration of how non-trivial a production quality shell is.
Parsing input is "tricky"
Each builtin needs comprehensive error handling
[0]: I was going to link to Bash's lex file here, but they appear to do something funky which would require a non-trivial amount of time to find, understand, and write here. So, you'll just have to take my word on this. I give you wikipedia as a substitute: https://en.wikipedia.org/wiki/Lexical_analysis
[1]: https://git.savannah.gnu.org/cgit/bash.git/tree/parse.y
I'm not sure what you mean that shells don't do string manipulation. Almost ALL they do is string manipulation.
That's true for the shell interpreter, which has to make sense of the input program, and for user programs, which are processing argv strings like file system paths, and stdin.
There are actually a handful of different parsers inside bash, which I mention here: http://www.oilshell.org/blog/2016/10/26.html
Brace substitution is another little parser as well. And globbing, and regex, both of which need their own parsers. (bash has its own glob parser, but some shells use libc's glob implementation). bash is really at 4-7 sublanguages in one.
The annoying thing about shell is that it makes it impossible NOT to do string manipulation in your program, because there is all this implicit stuff like word splitting.
"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