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Re: null array[*] expansion not treated as null

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  Re: null array[*] expansion not treated as null L A Walsh <bash@tlinx.org> - 2018-11-25 12:02 -0800

#14853 — Re: null array[*] expansion not treated as null

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} expansion
> (though $* is treated as null in the same circumstance).
>
>     $ A=(''); set -- ''
>     $ echo "<${A[*]:-X}>" "<${*:-X}>"
>     <> <X>
>   
----
    Ah, a null string != empty.  Example:
>  A=()
>  declare -p A
declare -a A=()
>  A=('')                #null string
>  declare -p A
declare -a A=([0]="")    #a string is still a string, even if empty

Related:

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

>  echo "=$#"   ; echo number of parameters set by "set - '' ''"
=2
>  echo "size of A=${#A[@]}"
size of A=2

>     $ echo "<${A[*]:-X}>" "<${*:-X}>"
>     <> <X>
>   
---
    Going to use "«»" instead of "<>" cuz <> are shell chars that
need quoting (  otherwise: bash: syntax error near unexpected token `<'  ).

First, your echo:

>  echo "«${A[*]:-X}»" "«${*:-X}»"   
«» «X»            (#same)

and w/o the quotes:
>  echo «${A[*]:-X}» «${*:-X}»
« » « »

---
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 '*'«»

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