-[[ -z "$BASH_VERSION" ]] || shopt -q login_shell || return
-[[ -z "$ZSH_VERSION" ]] || [[ -o login ]] || return
+[[ -z "${BASH_VERSION+set}" ]] || shopt -q login_shell || return
+[[ -z "${ZSH_VERSION+set}" ]] || [[ -o login ]] || return
[[ "$-" == *i* ]] || return