Created SC3018 (markdown)

SC3018.md

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+## In POSIX sh, ++ is undefined.
+
+(or "In dash, ... is not supported." when using `dash`)
+
+### Problematic code:
+
+```sh
+#!/bin/sh
+i=1
+echo "$((i++))"
+```
+
+### Correct code:
+
+Write out the increment/decrement operation explicitly:
+
+```sh
+#!/bin/sh
+i=1
+echo "$i"
+i=$((i+1))
+```
+
+### Rationale:
+
+Prefix and postfix increment and decrement are extensions in bash and ksh. They're not supported in dash or POSIX sh, which is what your shebang says you're using.
+ 
+### 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!