create for documenting POSIX shell’s undefined standalone ((..))

SC3006.md

@@ -0,0 +1,23 @@
+## In POSIX sh, standalone ((..)) is undefined.
+
+### Problematic code:
+
+```sh
+variable=1
+if ((variable)); then
+  echo variable is not zero
+fi
+```
+
+### Correct code:
+
+[`bash` supports standalone `((..))`](https://www.gnu.org/software/bash/manual/html_node/Conditional-Constructs.html#index-select) natively.
+
+For POSIX compliance, use
+
+```sh
+variable=1
+if [ "${variable}" -ne 0 ]; then
+  echo variable is not zero
+fi
+```