为什么 `nounset` shell 选项不会在未设置的数组上引发错误？

Question

Why no errors are raised when unset is set, and an undefined array is referenced?

Am i missing something, eg this is controlled by another option?

Example:

set -o nounset

echo "${foo[*]}" # no error
echo "${foo[@]}" # no error

echo "$foo" # error

I've checked the manpage , but I'm puzzled, because it mentions $@ and $* , but not arrays:

Treat unset variables and parameters other than the special parameters '@' or '*' as an error

The Bash version I'm using is 5.0.

Answer 1

I believe this is just a shortcoming of the documentation; it should mention array parameters index with @ and * as well.

The intent is to make it look like an array can be "set" to an empty value. However,

foo=()

doesn't actually assign an empty array value to the name foo . It simply sets the array attribute on the name foo , and clears any values that may already have been present. $foo and ${foo[0]} are equivalent, and both will be treated as an unset parameter. Each of ${foo[@]} and ${foo[@]} are special cased to be "set", but "empty", as far as -u is concerned.

Note, though, that the array attribute doesn't actually need to be set for the @ / * indices to override -u .

$ unset bar
$ set -o nounset
$ echo ${bar[@]}

$