Created SC3028 (markdown)

SC3028.md

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+## In POSIX sh, VARIABLE is undefined.
+
+(or "In dash, ... is not supported." when using `dash`)
+
+### Problematic code:
+
+```sh
+#!/bin/sh
+echo "$HOSTNAME $UID $RANDOM"
+```
+
+### Correct code:
+
+Either switch to a shell like `bash` that supports the special variable you're trying to use, or use an external command to get the information you want:
+
+```sh
+#!/bin/sh
+echo "$(hostname) $(id -u) $(awk 'BEGIN { srand(); print int(rand()*32768) }' /dev/null)"
+```
+
+### Rationale:
+
+The variable you are attempting to use is a special variable in bash or ksh. To get the same information from `dash` or POSIX `sh`, use an external command 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!