You mean this part of my comment: >> The shell performs [...] arithmetic expansion, command substitution [...] on those words. > [...] and that command substi

You mean this part of my comment: >> The shell performs [...] arithmetic expansion, command substitution [...] on those words. > [...] and that command substitution is peformed after arithmetic expansion in `[[` [...] You're right. That doesn't apply. I was equating "arithmetic expansion" with "evaluation of arithmetic expressions", but I see now the former is specifically about `$(())`.