Re: string substitution broken since 5.2

[Martin D Kealey]

I now very much regret not commenting on this proposal during the
pre-release period, and I apologise for not having done so.

On Fri, 4 Nov 2022, 05:50 Chet Ramey, <chet.ramey@case.edu> wrote:

>
> The option is enabled by default. If you want to restore the previous
> behavior, add `shopt -u patsub_replacement'.
>

I am very disquieted by this pattern of creating breaking changes and then
saying "just add shopt blahblah to go back to the old behaviour".

This might be justifiable to a fix for a security problem (e.g. array
subscripting), but *this* is just a nice-to-have feature.

Suggesting that an existing script should be modified implies that scripts
must necessarily have "maintainers" whose job includes tracking changes to
the language provided by Bash, or that every piece of software installed on
a system comes from a single source who can test combinations for
compatibility.

That is not the world that we live in.

The average user out there has almost zero shell scripting experience, and
even less competence to diagnose faults in their system. They rely on a
package manager to install versions of basic utilities like Bash, and
diligently apply updates to keep on top of security. They also get other
software from sources who know nothing about them or their package manager.

In this context the reasonable thing to do is to aim for maximal backwards
compatibility, and only enable breaking changes when a special option or
command is used.

Can we please have an immediate point release turning this feature off by
default, and then let's take another look at how this can be done without
gratuitous breakage of existing code.

-Martin

PS:
I'm puzzled why a bare '&' was chosen in the first place; that seems to
provide the maximal likelihood of conflict with existing code.

As an alternative that wouldn't need an enabling option, how about using
"$BASH_MATCH" or "$&"? (I suspect the latter would be easier to use as a
marker for a deferred expansion.)

Rather than inserting a modal command or option, one possibility might be
to use the same mechanism as handles POSIX "sh", and extend it to look for
a trailing version number, and use that to enable new features.