write portable shell automation that accounts for zsh vs bash differences in array indexing and word splitting

Steps

1. Remember zsh arrays are 1-indexed by default ($array[1] is the first element) while bash arrays are 0-indexed (${array[0]}), so index arithmetic ported between the two shells needs explicit adjustment.
2. Account for word splitting differences: bash performs word splitting on unquoted variable expansions, while zsh does not split unquoted expansions by default, so a bash idiom relying on splitting silently stops working under zsh.
3. If a script must run under both shells, quote all variable expansions consistently and avoid relying on either shell's default splitting behavior.
4. When a zsh script intentionally needs bash-style word splitting for a specific expansion, use zsh's explicit ${=variable} splitting flag rather than assuming default behavior will match bash.
5. Prefer targeting bash specifically (with a #!/usr/bin/env bash shebang, run as `bash script.sh`) for automation meant to run unmodified across the widest range of systems, since zsh is far less consistently preinstalled across Unix-like systems than bash.