This was already reported 12 and a half years ago, but there have not been many replies:
https://lists.gnu.org/archive/html/bug-bash/2010-12/msg00128.html The issue seems to be caused by expr_streval() only checking for unbound identifiers at this line: https://git.savannah.gnu.org/cgit/bash.git/tree/expr.c?h=devel#n1168 if ((v == 0 || invisible_p (v)) && unbound_vars_is_error) But then, after resolving the variable if the identifier is a array or associative array variable, bash doesn't check that the result is set, and just defaults to 0 if it isn't set at this line: https://git.savannah.gnu.org/cgit/bash.git/tree/expr.c?h=devel#n1219 tval = (value && *value) ? subexpr (value) : 0; This makes bash not trigger nounset if the identifier is a set variable/array, even thought idenitifer[subscript] is not set. $ bash -uc 'printf %s\\n "$(( a ))"' bash: line 1: a: unbound variable $ bash -uc 'printf %s\\n "$(( a[1] ))"' bash: line 1: a: unbound variable $ a=xyz bash -uc 'printf %s\\n "$(( a[1] ))"' 0 $ bash -uc 'a=(1 2); let "a[10]"' 0 $ a=xyz bash -uc 'printf %s\\n "${a[1]}"' bash: line 1: a[1]: unbound variable $ bash -uc 'a=(1 2); printf %s\\n "${a[10]}"' bash: line 1: a[10]: unbound variable o/ emanuele6
