Created SC3014 (markdown)

SC3014.md

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+## In POSIX sh, `==` in place of `=` is undefined.
+
+(or "In dash, ... is not supported." when using `dash`)
+
+### Problematic code:
+
+```sh
+#!/bin/sh
+if [ $1 == "-n" ]
+then
+  dry_run=1
+fi
+```
+
+### Correct code:
+
+```sh
+#!/bin/sh
+if [ $1 = "-n" ]
+then
+  dry_run=1
+fi
+```
+
+### Rationale:
+
+`==` is used for equality comparison in C-like languages, and is also supported by `bash` and `ksh` in `[ .. ]` and `test` expressions.
+
+`sh` and `dash` only supports `=` for equality comparison, so use that 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!