Created SC3013 (markdown)

SC3013.md

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+## In POSIX sh, `-nt` is undefined.
+
+### Problematic code:
+
+```sh
+#!/bin/sh
+if [ Foo.java -nt Foo.class ]
+then
+  javac Foo.java
+fi
+```
+
+### Correct code:
+
+The easiest fix is to switch to a shell that *does* support `-nt`, like bash:
+
+```sh
+#!/bin/bash
+if [ Foo.java -nt Foo.class ]
+then
+  javac Foo.java
+fi
+```
+
+Otherwise, `find` can be used:
+
+```sh
+#!/bin/sh
+if [ -n "$(find Foo.java -newer Foo.class)" ]
+then
+  javac Foo.java
+fi
+```
+
+### Rationale:
+
+`test -nt` is an extension in ksh, bash and dash, but it is not covered by POSIX.
+
+### 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!