Re: Why yacc_EOF is used as a list_terminator for for_command?

[Chet Ramey]

On 5/11/21 11:16 PM, Koichi Murase wrote:
> 2021年5月11日(火) 3:03 Peng Yu <pengyu.ut@gmail.com>:
> > > Do you have a better idea? Where would you include it? Is it worth
> > > changing the current implementation which works without problems
> > > currently? It's useless to change the code that actually doesn't
> > > change any behavior. It may just introduce a new bug.
> >
> > My goal is not to change the current implementation. I am asking the
> > question from the perspective of how would one implement it from
> > scratch.
>
> Do you mean you are planning to develop your own new shell? Or, you
> suggest replacing the parsing engine of Bash with a completely
> different one developed from scratch? 

Bash's parser is unique among widely-used POSIX shells because it uses
bison/yacc. Everyone else uses a hand-rolled recursive descent parser.

One of the POSIX group's greatest achievements was to create a yacc-like
grammar for the shell, since, as Tom Duff put it, "Nobody really knows
what the Bourne shell's grammar is." And even that has lots of special
cases and context-dependent rules.


> > "Correct" means if you were to start from scratch and whether it is
> > the best of all the possible implementation choices. (Or maybe
> > "necessary" is a better word in this case.)

I'm not sure what the best word to capture your intent is, but "correct" is
not it.

> How do you define "best"... It depends on the person. I can understand
> what you want to say; indeed yacc_EOF is unneeded in `list_terminator`
> when used in the definition of `for' statement, etc. So you may
> distinguish two different types of line termination:
>
>    list_terminator_middle: '\n' | ';'
>    list_terminator_general: list_terminator_middle | yacc_EOF

On the other hand, the grammar as it exists today parses syntactically
correct `for' statements correctly and produces errors for invalid
statements. The increase in generality does not result in incorrectness.

If you can find an exception to those two points, please report it.


--
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    chet@case.edu    http://tiswww.cwru.edu/~chet/