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From: Stephane Chazelas <stephane.chazelas@gmail.com>
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Subject: Re: Parameter expansion resulting empty treated as if it's not empty
Date: Wed, 30 Oct 2019 17:24:00 +0000
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Xref: csiph.com gnu.bash.bug:15540

2019-10-30 14:12:41 +0300, Oğuz:
[...]
> I was expecting
> 
> bash -c '${1##*"${1##*}"}' _ foo
> 
> to print an empty line too, but instead it prints foo.
[...]
> Is this a bug?

Yes,

In gdb, we see the ${1##*} expands to \177 (CTLNUL) as a result
of quote_string(). And that's used as is in the outer pattern.

It looks like an "unquoting" may be missing in that case.

See also:

$ bash -c 'printf %s "${2%%"${1##*}"*}"' bash foo $'x\177foo' | hd
00000000  78                                                |x|
00000001

It seems it's a regression, introduced in 4.0.

-- 
Stephane