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+## In POSIX sh, 'let' is undefined.
+
+(or "In dash, ... is not supported." when using `dash`)
+
+### Problematic code:
+
+```sh
+#!/bin/sh
+let x=2+2
+```
+
+### Correct code:
+
+Use standard `$((arithmetic expansion))` instead of legacy `let`:
+
+```sh
+#!/bin/sh
+x=$((2+2))
+```
+
+### Rationale:
+
+`let` is a command used for arithmetic in some shells, but it's not a standard command. Use `$((..))` instead.
+
+### 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.
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!