Created SC3002 (markdown)

SC3002.md

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+## In POSIX sh, extglob is undefined.
+
+(or "In dash, ... is not supported." when using `dash`)
+
+### Problematic code:
+
+```sh
+#!/bin/sh
+rm !(*.hs)
+```
+
+### Correct code:
+
+Either switch the shebang to a shell that does support extglob, like `bash` or `ksh`, or rewrite in terms of a loop with a `case` or `grep` match:
+
+```sh
+#!/bin/sh
+for file in *
+do
+  case "$file" in
+    *.hs) true;;
+    *) rm "$file" ;;
+  esac
+done
+```
+
+### Rationale:
+
+Extglobs are extensions in bash and ksh, while your shebang says you're using `sh` or `dash`. 
+
+### 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!