Re: "$@" expansion when it is consists of only null strings

[Robert Elz]

    Date:        Sun, 24 Feb 2019 18:37:33 -0500
    From:        Grisha Levit <grishalevit@gmail.com>
    Message-ID:  
<CAMu=BrqKMX7pFGARH2DNaX3TBfArhuTUVvQ+O3E+MMx165sHSw@mail.gmail.com>

  | There are some what seem to be regressions (?) in bash-4.4 and
  | bash-5.0 regarding the handling of "$@" expansion when it consists
  | entirely of null strings.

I think these are actually more the insertion of explicit null
strings, rather than $@ when it contains null strings.   These
are essentially the same as the ${b+s ''} issue that was discussed
a week or so ago, and which Chet already said he would look into.


  |     $ set -- ''
  |     $ n "$@"''
  |     0

Should be 1.   It would be if the '' null string was not there.

  |     $ set -- '' ''
  |     n ''"$@"
  |     1

Should be 2.   $1 gets obliterated by the '' and $2 remains.

  |     n "$@"''
  |     1

Same (should be 2) except here $1 remains and $2 is lost.

  |     n ''"$@"''
  |     0

Again, should be 2, here both $1 and $2 are lost.

Of course if you just did

        n "$@"

you'd get 2 as you should (no explicit null strings to kill the $@ expansions).

  |     $ set -- ''
  |     $ n ${_+''"$@"''}
  |     0

Should be 1

  |     $ n ${_+"$@" "$@"}
  |     1

Should be 2.   Not sure what is up with that one as there
are no explicit null strings to confuse things.   Each $@ should
expand to (quoted) $1 (even though that means ""), which is then
field split at the (unquoted) space to leave 2 null string args.

  | On a related note, with $#=0, bash-20190220 fixed the expansion of
  | ${_"$@"''} and ${''"$@"} to produce a single field but not so with:

I assume you mean ${_+"$@"''} and ${_+''"$@"} ?   But even bash2 got
those correct (they're both 1).   bash5 gets the 2nd one wrong (0)
but I might be testing an older version than you meant had fixed it
(I do not build development versions.)  BASH_VERSION='5.0.0(1)-release'

  |     $ set --
  |     $ n ${_+''"$@"''}
  |     0

Should also be 1.


  | And these have always been the case, but I'm not sure if it should be so?
  |
  |     $ set --
  |     $ n "${_+"$@"}"
  |     1
  |     $ n "${_+$@}"
  |     1

Those are, I believe, unspecified cases.   The first is definitely
because of the strange quoting (the " chare are in order, open, close,
open, close, not open open close close).  Anything could replace the
$@ in that (even just x ) and you'd still get unspecified results.

The second because there's no real agreement whether it should produce
0 or 1 (different shells do different things for that one, and there's
no particularly good argument for one or the other, so posix, I believe,
makes that one unspecified as well.)

kre

ps: for a couple of the last of those (before the unspecified ones)
bosh and yash seem to get them wrong as well, both giving 0 for both of
n ${_+''"$@"''} and n ${_+"$@" "$@"} ... but that's because they don't
implement $_ (and good on them for that, stupid thing it is), so the
word after the "+" isn't used unless one does _=X or similar first.
bosh still gets  n ${_+"$@" "$@"} wrong though (0), even when _ is set.