why is an array variable not considered set if it is assigned `()`?

Question

bash manual says:

A parameter is an entity that stores values

A variable is a parameter denoted by a name.

A parameter is set if it has been assigned a value.

An array variable is considered set if a subscript has been assigned a value.

Is an array variable a variable and a parameter?

If yes, why is an array variable not considered set if it is assigned ()?

$ ar=()
$ echo ${ar-This is a new value}
This is a new value

Thanks.

Answer 1

In bash, like in ksh, where $var is an array, $var is actually short for ${var[0]}.

So in:

var=([1]=whatever [2]=blah)

${var+set} would not expand to set, and [[ -v var ]] would return false, because $var has no element of indice 0 (note that in bash, like in ksh, but contrary to most other shells and languages, arrays are sparse arrays, or associative arrays with keys limited to positive integers).

In bash (at least since 4.3), when you declare a variable as array with:

typeset -a var

var is initially unset. It's not even set to an empty list. In 4.3, typeset -p var would return an error. In 4.2 and before it would output:

declare -a a='()'

(as if declaring the array assigned an empty list like it does in zsh)

In 4.4, it outputs:

declare -a var

So, since 4.3, an unset (but declared) array is different from an array that is assigned an empty list.

Still, it's difficult programmatically to differentiate between both. But in practice, the distinction is not very useful. Most likely, what you want to know is how many elements are in the array:

((${#array[@]})) || echo array has no element

See zsh, rc, fish and yash for shells with better array designs.

Answer 2

The construct ${var-foo} expands to foo if var is unset or null. An empty array is exactly that.

As you quote:

An array variable is considered set if a subscript has been assigned a value.

The example you site (ar=()) does not assign a value to any subscripts.

Answer 3

The correct way to test for a whole array is:

[[ ${ar[@]+set} == set ]] && echo "array has a value may be a null"

What you wrote is the test for the value at index 0. This two are equivalent:

${ar+set}
${ar[0]+set}

But both will only test whether the array at index zero is set or not, not the whole array, which I assume is what you meant.

But, to your question: Why is an array unset if asigned ()?

Because you have not asigned any value, not even the null, to any array index.
To get some index to be set (and consequently the array) you need something like:

$ declare -a ar=([3]="value")

Or even:

$ declare -a ar=([3]="")

zsh (even in sh or ksh emulation) is the only one of all shells tested (as usual, the special one) that declare an array as set with:

$ unset ar; typeset -a ar=(); echo $(typeset -p ar) ${ar[@]+set}

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Question from comments:

Thanks. Can ar[@] be replaced with ar[*] here? Are they array variables?

No, an "${ar[*]}" is the string concatenation of all array values.

However, in this specific expansion: "${ar[*]+set}" both are equivalent.

From the manual (at Arrays):

If subscript is @ or *, the word expands to all members of name.