Bash 5 filename expansion and nullglob

Question

The bash-5.0 manual in section 3.5.8 states the following.

After word splitting, unless the -f option has been set (see The Set Builtin), Bash scans each word for the characters ‘*’, ‘?’, and ‘[’. If one of these characters appears, then the word is regarded as a pattern, and replaced with an alphabetically sorted list of filenames matching the pattern (see Pattern Matching). If no matching filenames are found, and the shell option nullglob is disabled, the word is left unchanged. If the nullglob option is set, and no matches are found, the word is removed.

However, in my shell, activating nullglob it seems that filename expansion kicks in also in the following case.

$ echo ${BASH_VERSION}
5.0.0(1)-release
$ shopt -s nullglob
$ FOO="/home/smith"
$ echo ${FOO//\//\\\/}

$ echo "${FOO//\//\\\/}"
\/home\/smith

Interestingly, using an older bash version I have available, a different behaviour occurs.

$ echo ${BASH_VERSION}
4.3.30(1)-release
$ shopt -s nullglob
$ FOO="/home/smith"
$ echo ${FOO//\//\\\/}
\/home\/smith
$ echo "${FOO//\//\\\/}"
\/home\/smith

Question

I would have expected no filename expansion to occur. Can you explain why echo ${FOO//\//\\\/} does not print anything with bash 5.0?

Answer 1

Indeed, after some more research, it turned out that this was reverted afterwards in v5.1-alpha.

$ echo ${BASH_VERSION}
5.1.0(1)-alpha
$ shopt -s nullglob
$ FOO="/home/smith"
$ echo ${FOO//\//\\\/}
\/home\/smith
$ echo "${FOO//\//\\\/}"
\/home\/smith

which is what I was personally expecting.

This seems also to fix different cases (thanks Stéphane for pointing it out):

$ echo ${BASH_VERSION}
5.1.0(1)-alpha
$ a='/\x' bash -O nullglob -c 'echo $a'
/\x