Created SC2115 (markdown)

SC2115.md

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+## Use "${var:?}" to ensure this never expands to /* .
+
+### Problematic code:
+
+    rm -rf "$STEAMROOT/"*
+
+### Correct code:
+
+    rm -rf "${STEAMROOT:?}/"*
+
+### Rationale:
+
+If `STEAMROOT` is empty, this will [end up deleting everything](https://github.com/ValveSoftware/steam-for-linux/issues/3671) in the system's root directory.
+
+Using `:?` will cause the command to fail if the variable is null or unset. Similarly, you can use `:-` to set a default value if applicable.
+
+In the case command substitution, assign to a variable first and then use `:?`. This is relevant even if the command seems simple and obviously correct, since forks and execs can fail due to external system limits and conditions, resulting in a blank substitution. 
+
+### Exceptions:
+
+None.