Re: string substitution broken since 5.2

[Koichi Murase]

2022年11月4日(金) 8:48 Greg Wooledge <greg@wooledge.org>:
> On Thu, Nov 03, 2022 at 05:20:52PM -0400, Greg Wooledge wrote:
> > On Fri, Nov 04, 2022 at 04:09:10AM +0900, Koichi Murase wrote:
> > > When one wants to also support Bash 4.2, one possible workaround is to
> > > assign the result to a variable (without quoting the right-hand side
> > > of the assignment):
> > >
> > > $ bash-4.2 -c 'string=\"hi\"; string=${string//\"/\&quot;}; echo 
> > > "$string"'
> > > $ bash-4.2 -c 'str="a string"; rep="a&b"; str=${str//a/"$rep"}; echo 
> > > "$str"'
> > >
> > > These work as expected in all Bash versions I can test (2.0..5.2).
> >
> > That's amazingly clever.  You're forcing the semantics of double-quoting
> > the right-hand side (implicitly, due to the simple assignment), without
> > putting actual quotes in it.
>
> Hmm, no, you *are* including quotes, so that's not why it works.
> I actually have no idea why it works.

Ah, sorry for confusing you. By "without quoting the right-hand side",
I meant that we strip the *outer* quoting of the entire right-hand
side. More explicitly,

string="${string//\"/\&quot;}"

still does not work in Bash 4.2 or with `shopt -s compat42', but

string=${string//\"/\&quot;}

works. I think you already know it, but this is because, in Bash 4.2,
quoting inside the parameter expansion (i.e., \& in this case) is
affected by the context of the parameter expansion itself (i.e.,
${string ...}). When ${string ...} is placed inside the outer double
quotes "${string ...}", the inner quoting is also treated as if it is
inside the double quotes, i.e. \& is treated as literal \&. When
${string ...} is not placed inside the double quotes, the inner
quoting is processed as if it is directly in the right-hand sides of
assignments, so \& becomes &.