SC1087 – ShellCheck Wiki

Use braces when expanding arrays, e.g. `${array[idx]}` (or `${var}[..` to quiet).

Problematic code:

echo "$array[@]"

Correct code:

echo "${array[@]}"

Rationale:

Some languages use the syntax $array[index] to access an index of an arrays, but a shell will interpret this as $array followed by the unrelated literal string (or glob) [index].

Curly braces are needed to tell the shell that the square brackets are part of the expansion.

Exceptions

If you want the square brackets to be treated literally or as a glob, use ${var}[idx] to prevent this warning.

This does not change how the script works, but clarifies your intent to ShellCheck as well as other programmers.