Created SC3023 (markdown)

SC3023.md

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+## In POSIX sh, FDs outside of 0-9 are undefined.
+
+(or "In dash, ... are not supported." when using `dash`)
+
+### Problematic code:
+
+```sh
+# Open FD #10 for writing
+exec 10> file
+```
+
+### Correct code:
+
+Either rewrite the script to only use FDs in the valid range, or switch to a shell that allows a larger range like `bash` or `ksh`.
+
+### Rationale:
+
+The shell grammar is only required to recognize a single digit before redirections to set the file descriptor, which limits the portable range to 0-9.
+
+### Exceptions:
+
+None.
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!