SC3054 – ShellCheck Wiki

In POSIX sh, array references are undefined.

(or "In dash, ... are not supported." when using dash)

Problematic code:

#!/bin/sh
var=(foo bar)
echo "${var[1]}"

Correct code:

The easiest solution is to switch to a shell that does support arrays, like bash:

#!/bin/bash
var=(foo bar)
echo "${var[1]}"

Alternatively, rewrite the logic to use e.g. indirect variable references or set:

#!/bin/sh
set -- foo bar
echo "$2"

Rationale:

Arrays are supported in bash and ksh, but not in dash or POSIX sh. Either switch to a shell that supports them, or rewrite your script without relying on arrays. This may not be straight forward.

Exceptions:

If you only intend to target shells that supports this feature, you can change the shebang to a shell that guarantees support, or ignore this warning.

You can use # shellcheck disable=SC3000-SC4000 to ignore all such compatibility warnings.

Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!