Created SC3057 (markdown)

SC3057.md

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+## In POSIX sh, string indexing is undefined.
+
+(or "In dash, ... is not supported." when using `dash`)
+
+### Problematic code:
+
+```sh
+#!/bin/sh
+echo "Your initial is ${USER:0:1}"
+```
+
+### Correct code:
+
+Either switch to a shell that does support string indexing via parameter expansion, like `bash` or `ksh`, or rewrite with `cut`:
+
+```sh
+#!/bin/sh
+echo "Your initial is $(printf '%s' "$USER" | cut -c 1)"
+```
+
+### Rationale:
+
+String indexing is a bash and ksh extension, and does not work in `dash` or POSIX `sh`.
+
+### 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!