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

#!/bin/sh
let x=2 2

Use standard $((arithmetic expansion)) instead of legacy let:

#!/bin/sh
x=$((2 2))

let is a command used for arithmetic in some shells, but it's not a standard command. Use $((..)) instead.

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.

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