When does an array variable represent the entire array, and when the element indexed by `0`?

Question

Bash manual says:

Referencing an array variable without a subscript is equivalent to referencing with a subscript of 0.

In Bash or POSIX shell, is there some general rule for when an array variable represents the entire array, and when it represents the element indexed by 0?

For example,

1. It seems that after declare an array variable means the entire array, while in parameter expansion, it means the element indexed by 0:

   $ declare -p x
   declare -a x='([1]="b" [2]="c")'
   $ echo $x
   

2. In [[ -v myarr ]], does myarr mean the array or myarr[0]?

Answer 1

I think your confusion is on the word "referencing."

You're not referencing the array variable after declare at all; you're only giving the name x.

To reference a variable you have to put a $ before the name.

Try searching through the bash man page for all instances of referenc and read them.

---

As for the second question:

In [[ -v myarr ]], does myarr mean the array or myarr[0]?

By test, it means myarr[0].

$ declare -p myarr
bash: declare: myarr: not found
$ myarr=()
$ declare -p myarr
declare -a myarr='()'
$ echo $myarr

$ [[ -v myarr ]] && echo is set
$ [[ -v myarr[0] ]] && echo is set
$ [[ -v myarr[1] ]] && echo is set
$ myarr+=([1]=b)
$ declare -p myarr
declare -a myarr='([1]="b")'
$ echo $myarr

$ [[ -v myarr ]] && echo is set
$ [[ -v myarr[0] ]] && echo is set
$ [[ -v myarr[1] ]] && echo is set
is set
$ myarr+=([0]=q)
$ declare -p myarr
declare -a myarr='([0]="q" [1]="b")'
$ echo $myarr
q
$ [[ -v myarr ]] && echo is set
is set
$ [[ -v myarr[0] ]] && echo is set
is set
$ [[ -v myarr[1] ]] && echo is set
is set
$ 

Answer 2

You are mixing variables with strings that represent "variable names".

A $variable is the variable, variable is just an string that is interpreted as a variable by some commands.

Try this:

$ a=x
$ x=(111 222 333)
$ declare -p $a
declare -a x='([0]="111" [1]="222" [2]="333")'

The expansion of variable a ($a) gave string x, which was interpreted by declare as a "variable-name" and then expanded and printed.

In [[ -v myarr ]] also the test is interpreting the string myarr as a variable, therefore it is always the whole variable.

Try

$ a=myarr
$ unset myarr
$ [[ -v $a ]] && echo set || echo unset
unset

$ myarr[5]=five
$ [[ -v $a ]] && echo set || echo unset
set