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+## In POSIX sh, exec flags are undefined.
+
+(or "In dash, ... are not supported." when using `dash`)
+
+### Problematic code:
+
+```sh
+#!/bin/sh
+exec -ca ls busybox
+```
+
+### Correct code:
+
+Switch to a shell that does support the `exec` flags you are using, like `bash`:
+
+```sh
+#!/bin/bash
+exec -ca ls busybox
+```
+
+In POSIX, you can use `env` to start a command with a clean environment, but there is no way to generally specifying the zeroth argument.
+
+### Rationale:
+
+Flags to `exec` are bash and ksh extensions. `dash` and POSIX `sh` does not support them.
+
+### 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!