Re: bash tries to parse comsub in quoted PE pattern

[Chet Ramey]

On 10/18/23 10:24 AM, Emanuele Torre wrote:
> On Wed, Oct 18, 2023 at 09:50:00AM -0400, Chet Ramey wrote:
> > On 10/17/23 5:55 PM, Emanuele Torre wrote:
> > > > braces.c
> > > >         - brace_gobbler: use extract_dollar_brace_string if we see ${ with
> > > >           the appropriate value of QUOTING, so we don't have to teach brace
> > > >           expansion more shell syntax.
> > > >           Report from Emanuele Torre <torreemanuele6@gmail.com>
> > > >         - brace_gobbler: call the word extraction functions with SX_NOALLOC
> > > >           so we don't have to allocate memory we're just going to free
> > >
> > > That patch fixed the bug with "${foo#'$('}", but it also broke the
> > > "${arr["{start..end}"]}" / "${arr["{foo,bar}"]}" patterns.
> >
> > How much shell syntax do you want? You want brace expansion to detect
> > some issues with parameter expansion but ignore others, and detect
> > some expansions but not others, without supplying requirements.
>
> I didn't even think that the "${foo#'$('}" had something to do with
> brace expansion. 

OK, now you know, and now you know where the fix needs to be applied.

> Now I am reporting that the patch that was supposed to fix that problem

It does fix the reported problem. You can't fully consume shell word
expansions without parsing them as word expansions, and you can't avoid
the reported problem without parsing the word expansion.

> made "${foo["{2,1}"]}" no longer expands to "${foo["2"]}" "${foo["1"]}".

Hence my point: to fix the reported problem, you have to parse the word
expansion, since simple quoted string skipping doesn't do it. To allow
this syntax, you cannot parse the word expansion.

> Brace expansion between "${foo[" and "]}" is something that has always
> worked: at least since bash 2.05b (I have not checked with earlier
> versions after the introduction of arrays).
>
> Is it really ok to break that behaviour?

That's why, if you want the first problem repaired, you have to specify
which word expansions are ok to parse and which are not. Should it be
only command and process substitution, which can independently specify
brace expansions? Should it be other parameter expansions, which can
contain quoted strings and command substitutions? Neither? Both?

--
``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/