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Re: devel: Questions about quoting in the new replacement ${var/pat/&}

From: Koichi Murase
Subject: Re: devel: Questions about quoting in the new replacement ${var/pat/&}
Date: Tue, 19 Oct 2021 20:03:19 +0900

Thank you for the reply and sorry for my late reply.  I am busy recently.

> > ----------------------------------------------------------------------
> > Suggestion / Discussion
> >
> > I suggest that '&' has the meaning of the matched part only when it is
> > not quoted in the parameter-expansion context ${...} [ Note that
> > currently, '&' has the meaning of the matched part when it is not
> > quoted by backslash in *the expanded result* ].  I expect the
> > following interpretations with this suggestion:
> The quoting outside the ${...} doesn't affect whether REP is quoted. This
> is consistent with how POSIX specifies the pattern removal expansions, and
> how bash has worked since bash-4.3.

I agree that the quoting outside ${...} (I mean something like «
"${...}" ») should not affect the treatment of PAT or REP in
${var/PAT/REP}.  In the original email, I intended the quoting
*inside* the ${...} such as « ${var/PAT/"&"} » or « ${var/PAT/\&} ».

> So both of these, for instance, will expand to `&' *because of how bash
> already works*, regardless of whether or not we attach meaning to `&' in
> the replacement string.
> > $ echo "${var/$pat/&}"    # & represents the matched part
> > $ echo "${var/$pat/\&}"   # & is treated as a literal ampersand

Yes.  As a result, both are replaced with `&' in bash-5.1.  However,
in devel, the `&'s in both cases are further replaced with the matched
parts in the final expansion results by `strcreplace', so anyway the
current devel breaks what you say *how bash already works* after
combining quote removal and the new strcreplce.  I would not request
to change *how bash already works* but would like to request
preserving *how bash already works* that is observable to users in a
bit more situations including « ${var/$pat/\&} », « ${var/$pat/"$rep"}
», etc. in a consistent way with quoting PAT in ${var/PAT}.

> This next one will expand to `\&' again due to existing behavior,
> regardless of what we do with it, due to how quote removal works.
> And so on.
> > $ echo "${var/$pat/\\&}"  # A literal backslash plus the matched part

I know why they behave like that in the current implementation in
devel, but knowing that, I have proposed a change on the design.

> > $ echo "${var/$pat/'\'&}" # A literal backslash plus the matched part
> > $ rep='A&B'
> > $ echo "${var/$pat/$rep}"   # 'A' plus the mached part plus 'B'
> > $ echo "${var/$pat/"$rep"}" # Literal 'A&B'
> Rather than dance around behind the scenes trying to invisibly quote &,
> but only in certain contexts where it would not otherwise be escaped by
> double quoting, I would be more in favor of adding an option to enable the
> feature and allowing the normal rules of double quoted strings to apply.

I thought the proposed quoting rule is well-defined as the similar
treatment is already implemented for glob characters in PAT of
${var/PAT}.  This kind of passing the quoting state to the next
process is also used in `=~' operator of the conditional command, such
as « [[ str =~ $regex ]] » vs « [[ str =~ "$literal" ]] ».

If the new behavior is introduced in the current way, which requires
extra quoting, through a new option, I would like to propose making
the default of the option disabling the new feature.

> > Here are the rationale:
> >
> > * It is consistent with the treatment of the glob special characters
> >   and anchors # and % in $pat of ${var/$pat}.
> Yeah, doing that was probably a mistake, but we have to live with it now.
> Those are really part of the pattern operator itself, not properties of
> the pattern. But nevertheless.

Oh, I have not thought of the possibility that the treatment of
quoting removal of ${var/$pat} might be considered a mistake.
However, if I understand it correctly, similar treatment is
already standardized in POSIX for ``quoting characters within the
braces'' (as ${var#"$xxx"} and ${var%"$xxx"}):

> quoted from POSIX XCU 2.6.2
> https://pubs.opengroup.org/onlinepubs/9699919799.2018edition/utilities/V3_chap02.html#tag_18_06_02
> The following four varieties of parameter expansion provide for
> substring processing. [...] Enclosing the full parameter expansion
> string in double-quotes shall not cause the following four varieties
> of pattern characters to be quoted, whereas quoting characters
> within the braces shall have this effect. [...]
> ${parameter%[word]} ...
> ${parameter%%[word]} ...
> ${parameter#[word]} ...
> ${parameter##[word]} ...
> Examples
> [...]
> The double-quoting of patterns is different depending on where the
> double-quotes are placed:
> "${x#*}"
>   The <asterisk> is a pattern character.
> ${x#"*"}
>   The literal <asterisk> is quoted and not special.

In the final example, the normal expansion result of `"*"' is `*' but
this should not be directly passed to the pattern matching engine to
achieve the behavior POSIX defines.

> > * One can intuitively quote & to make it a literal ampersand.  The
> >   distinction of the special & in ${var/$pat/&} and the literal
> >   ampersand in ${var/$pat/\&} is more intuitive than ${var/$pat/&} vs
> >   ${var/$pat/\\&}.
> Not if you take into account the word expansions the replacement
> string undergoes. For example, if you use ${var/$pat/\&} in
> bash-5.1, you're going to get a `&' in the output, not `\&'.  Now
> you invite the questions of why bash expands things differently
> whether or not there is a `&' in the replacement string, and since
> the non-special bash-5.1 expanded that to `&', why should bash-5.2
> not treat it as a replacement?
> I guess the question is why not let the normal shell word expansion
> rules apply, and work with the result.

I think that that (i.e., letting the normal expansion rules apply and
working with the result) is what the current devel does.  But I am
proposing a different behavior that is similar to the treatment of PAT
in ${var/PAT} where *the normal expansion rules plus expansion of `&'
is performed at once* in the observable behavior (but of course the
real implementation can be different as in the case of PAT in
${var/PAT} that the expansion and the pattern matching is actually
processed in separate steps but by introducing internal quoting in the
expansion step).


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