On 2016-08-13 07:31, Ben Woolley wrote:
On Aug 12, 2016, at 2:41 PM, [email protected] wrote:
Hello!
GNU Bash is 138227 lines of code. I wrote a simpler shell* in 800
lines: https://notabug.org/rain1/s/
*It is not a true POSIX shell. You can't run existing scripts with it.
It's technically just a command interpreter.
With that out the way here's an overview of how it works:
Tokenization [tokenizer.c]: Instead of the strange and complex way
that normal shells work (where "$X" is different to $X for example) s
works by a strict tokenize -> variable expansion -> parse -> execute
pipeline. This makes it much easier to program with and less likely
for scripts to break simply because your CWD has a space in it.
Variable expansion [variables.c]: The expander supports both $FOO and
${FOO} syntax, it just resolves environment variables.
Parsing [parser.c]: There are just 3 binary operations |, && and ||
and '&' optional at the end of a line. There is no "if" or looping or
anything. parser.c is 85 lines of code and uses my region [region.c]
based allocator to simplify teardown of the structure when it needs to
be free'd.
[interpreter.c] The interpreter is a simple recursive process that
walks the AST, creating pipes and forking off children.
[supporting/*.c] Instead of redirection operators like <, > and >>
being part of the language they are simply provided as supporting
programs that should be added to the $PATH: < is basically just cat.
The redirection operators are all packaged together in busybox style.
Similarly glob is not part of the language, it is a 20 line script
instead. You use it like this: glob rm *py
[builtins.c] Of course a shell cannot do everything by external tools
- so the builtins cd, source, set, unset are provided (and treated
specially by the interpreter).
It can run scripts you supply, shebang works, using it in a terminal
interactively works. In theory enough for practical every day use.
Except for the low linecount (it is even smaller than execline) and
simplicity of the lexical aspect of the shell language it does not
have strong benefits over existing shells (especially since it is not
POSIX compatible) but I hope that the code may be interesting or
refreshing to others who are unhappy with the excess of bloat most
software has.
I think it's pretty cool. There are some interesting shortcuts, like
the redirection and glob not as built-ins, and trying to be just a
shell and not a scripting language. It also seems to have a consistent
ideology behind it, in a good sense.
Thanks Ben!
That's true, consistent ideology was the main thing I was going for.
We've seen a lot of high profile dangers like shellshock and the steam
software deleting your home directory come from the bad design of early
shell languages. This is why I made s have such a rigid and minimal
syntax.
