How to detect array start index by current shells (zsh/bash)?

Question

We all know arrays in Bash are indexed from zero, and in zsh are indexed from one.

How can the script know it should use 0 or 1 if I can't ensure the running environment is bash, zsh or something else?

Expected code sample:

#!/bin/sh

detect_array_start_index(){
  # ... how?
  echo 1
}
ARR=(ele1 ele2)
startIndex=$(detect_array_start_index) # 0 or 1
for (( i=${startIndex}; i < ${#ARR[@]} + $startIndex; i++ )); do
    echo "$i is ${ARR[$i]}"
done

I have a idea is find the index of the first value in a fixed array, I got this: Get the index of a value in a Bash array, but the accepted answer use bash variable indirection syntax ${!VAR[@]}, which is invalid in zsh.

Answer 1

Check the index 1 element of a two element array:

detect_array_start_index() {
  local x=(1 0)                                                   
  echo ${x[1]}
}

Answer 2

You can set the KSH_ARRAYS option in zsh to force array indexing to start at 0.

{ setopt KSH_ARRAYS || : ; } 2> /dev/null

ARR=(ele1 ele2)
for ((i=0; i < ${#ARR[@]}; i++ )); do
  echo "$i is ${ARR[$i]}"
done

The command group and redirection allows the entire command to act as a no-op if executed from bash. The preceding code produces the same output in zsh and bash.