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From: L A Walsh <bash@tlinx.org>
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Subject: Re: null array[*] expansion not treated as null
Date: Sun, 25 Nov 2018 12:02:41 -0800
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On 11/12/2018 1:32 PM, Grisha Levit wrote:
> When an array A has non-zero elements but the expansion of "${A[*]}" is
> still
> a null string, it is not treated as such for purposes of ${var:-X} expa=
nsion
> (though $* is treated as null in the same circumstance).
>
>     $ A=3D(''); set -- ''
>     $ echo "<${A[*]:-X}>" "<${*:-X}>"
>     <> <X>
>  =20
----
    Ah, a null string !=3D empty.  Example:
>  A=3D()
>  declare -p A
declare -a A=3D()
>  A=3D('')                #null string
>  declare -p A
declare -a A=3D([0]=3D"")    #a string is still a string, even if empty

Related:

FWIW, below, "IFS=3D" sets IFS to an empty string, like
IFS=3D''
>     $ IFS=3D
>     $ A=3D('' ''); set -- '' '';
>  =20
Typing in lines to echo the size of A and shell params:

>  echo "=3D$#"   ; echo number of parameters set by "set - '' ''"
=3D2
>  echo "size of A=3D${#A[@]}"
size of A=3D2

>     $ echo "<${A[*]:-X}>" "<${*:-X}>"
>     <> <X>
>  =20
---
    Going to use "=C2=AB=C2=BB" instead of "<>" cuz <> are shell chars th=
at
need quoting (  otherwise: bash: syntax error near unexpected token `<'  =
).

First, your echo:

>  echo "=C2=AB${A[*]:-X}=C2=BB" "=C2=AB${*:-X}=C2=BB"  =20
=C2=AB=C2=BB =C2=ABX=C2=BB            (#same)

and w/o the quotes:
>  echo =C2=AB${A[*]:-X}=C2=BB =C2=AB${*:-X}=C2=BB
=C2=AB =C2=BB =C2=AB =C2=BB

---
Notice adding quotes around your vars can change what is
echo'ed.  Looks like the output-separator ' ' (space) is
embedded in the '*'s of both expressions, but [*] undergoes
expansion, while the 2nd one doesn't.

So 1 hint to remember a variable is an internally quoted entity, such
if it is assigned to another var, it will be assigned "as-is", but
if you put double-quotes around a variable, it will replace sequences
of white space with 1 white space.

Does that address your examples' behaviors?

of some sort where '*'=C2=AB=C2=BB