Created SC3020 (markdown)

SC3020.md

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+## In POSIX sh, `&>` is undefined.
+
+(or "In dash, ... is not supported." when using `dash`)
+
+### Problematic code:
+
+```sh
+#!/bin/sh
+ssh host cmd &> log
+```
+
+### Correct code:
+
+Use the long form instead of the shorthand:
+
+```sh
+#!/bin/sh
+ssh host cmd > log 2>&1
+```
+
+### Rationale:
+
+`&>` is a bash and ksh extension for redirecting both stdout and stderr. In dash and POSIX sh, use `2>&1` explicitly.
+
+### 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!