Re: setarray[unsetkey] doesn't trigger nounset in arithmetic context

[Emanuele Torre]

On Mon, May 22, 2023 at 09:27:17AM +1000, Martin D Kealey wrote:
> I just realised I expressed an opinion about associative arrays while the
> original post was about indexed arrays.
> 

I simply reported that, specifically in arithmetic contexts, when using
a variable (not as the lhs of an = assignment), bash only checks that
the variable name before "[" is bound to decide whether to trigger
nounset or not:

  $ set -u
  $ echo "$(( a[0] ))"       # correct, a is not set
  bash: a: unbound variable
  $ declare b
  $ echo "$(( b[0] ))"       # correct, b is declared, but not set
  bash: b: unbound variable
  $ c=()
  $ echo "$(( c[0] ))"       # wrong, it is supposed to error, c[0] is not set
  0
  $ echo "$(( c[1] ))"       # wrong, it is supposed to error, c[1] is not set
  0
  $ d=11
  $ echo "$(( d[0] ))"       # correct, d[0] is set
  11
  $ echo "$(( d[1] ))"       # wrong, it is supposed to error, d[1] is not set
  0
  $ e=([1]= )
  $ echo "$(( e[1] ))"       # correct, e[1] is set
  0

If the variable has a subscript, it is supposed to also check that the
array slot is set, and trigger nounset if it is not.

If you run the same commands I marked as wrong above with regular
non-unset-immune parameter expansions instead of arithmetic expansions,
you will see that nounset is triggered if z is set, but the specific
index is not.

  $ set -u
  $ unset -v z
  $ echo "${z[0]}"
  bash: z[0]: unbound variable
  $ echo "${z[10*RANDOM+1]}"
  bash: z[10*RANDOM+1]: unbound variable
  $ z=()                     # even if z is set, it still trigger nounset
  $ echo "${z[0]}"
  bash: z[0]: unbound variable
  $ echo "${z[10*RANDOM+1]}"
  bash: z[10*RANDOM+1]: unbound variable
  $ d=11
  $ echo "${d[1]}"
  bash: d[1]: unbound variable

I think this behaviour is unintented and only happens because the code I
linked in the original post is forgetting to check that the array slot
is set after expanding variables with a subscript, so it doesn't issue
the error.

This has nothing to do specifically with associative arrays, though they
are also affected by the problem. And it also has nothing to do with [*]
and [@] having special treatment when used with nounset to follow the
behaviour of $* and $@ in POSIX sh.

I am not really proposing a major change to the behaviour of nounset, I
am only pointing out the incorrect bash behaviour for that case.

 emanuele6