Why can't I have a single quote in a bash associative array index when testing with `test -v` [duplicate]

Question

When I run the following code

declare -A X # get an associative array
index="a'b" # a somewhat weird index
X[$index]="this is set"
echo "<<${X[$index]}>>"
if test -v X[$index]; then # behaves in an unexpected way
  echo indeed, this is set
else
  echo no, it is not
fi

the result is no, it is not in my bash version 5.1.16(1). As soon as i remove the single quote from the index, it works as expected, telling that indeed, this is set.

From the bash manual:

Indexed arrays are referenced using integers [...]; associative arrays use arbitrary strings.

Given that I can properly use the index to set and retrieve the value, I wonder if this is a bug in test -v or if there is some explanation why this does not work or whether different quoting can get test -v to work. (I could use test -z in this particular case, but the question is about -v).

Answer 1

The way the expansion happens is not how you would expect.

test -v X[$index]

Does not expand to:

test -v X["a'b"]

But instead it expands like this:

test -v 'X[a'\''b]'

bash splits and escapes the array syntax in a way that prevents test -v from seeing a valid array element. test -v expects a fully valid variable name or array element as a single argument.

As Kusalananda suggested you could use test -v 'X[$index]' however I think a much more correct way would be to use the extended test syntax instead:

if [[ -v X["$index"] ]]; then