Re: string substitution broken since 5.2

[Greg Wooledge]

On Fri, Nov 04, 2022 at 01:30:16PM +0100, Alex fxmbsw7 Ratchev wrote:
> > 1) Put something like "shopt -u patsub_replacement 2>/dev/null || true"
> >    at the top of your script.
> >
> 
> there d be many such senselessnesses
> 
> >
> > 2) Assign the result of the expansion to a temporary variable, and pass
> >    the temp var to somecmd.  Every.  Single.  Time.
> >
> 
> ? i dont get that

OK.  Let me offer a quick example script.  It works as expected in
bash-5.1:

unicorn:~$ cat foo
#!/bin/bash
template='The best candy is clearly @CANDY@!'
candy='M&Ms'
printf '%s\n' "${template/@CANDY@/"$candy"}"

unicorn:~$ ./foo
The best candy is clearly M&Ms!

Now, let's run this script under bash-4.2:

unicorn:~$ bash-4.2 ./foo
The best candy is clearly "M&Ms"!

Uh oh!  The quotes are wrong for bash-4.2.  Bug #1 is filed for this issue.
As the maintainer of the script, I test a few things between bash-4.2
and bash-5.1 and I come up with this workaround:

unicorn:~$ cat foo
#!/bin/bash
template='The best candy is clearly @CANDY@!'
candy='M&Ms'
printf '%s\n' "${template/@CANDY@/$candy}"    # unquote $candy to fix bug #1

unicorn:~$ bash-4.2 ./foo
The best candy is clearly M&Ms!
unicorn:~$ bash-5.1 ./foo
The best candy is clearly M&Ms!

Now it works on older systems too.  Everything's fine... until bash-5.2.

unicorn:~$ bash-5.2 ./foo
The best candy is clearly M@CANDY@Ms!

The workaround for bug #1 causes bug #2 on bash-5.2.  To make the script
work on all three versions of bash, we need a different workaround:

unicorn:~$ cat foo
#!/bin/bash
template='The best candy is clearly @CANDY@!'
candy='M&Ms'
message=${template/@CANDY@/"$candy"}          # Work around bug #1 and #2.
printf '%s\n' "$message"

unicorn:~$ bash-4.2 ./foo
The best candy is clearly M&Ms!
unicorn:~$ bash-5.1 ./foo
The best candy is clearly M&Ms!
unicorn:~$ bash-5.2 ./foo
The best candy is clearly M&Ms!

And there you have it.  You're allowed to quote "$candy" in a variable
assignment, as long as the whole parameter expansion ${...} remains
unquoted, and it'll work properly in all 3 versions.  I'm using "message"
as a temporary variable, whose sole purpose is to work around this issue.