Created SC2172 (markdown)

SC2172.md

@@ -0,0 +1,22 @@
+## Trapping signals by number is not well defined. Prefer signal names.
+
+### Problematic code:
+
+```sh
+trap myfunc 28
+```
+
+### Correct code:
+
+```sh
+trap myfunc WINCH
+```
+### Rationale:
+
+Signal numbers can vary between platforms. Prefer signal names, which are fixed.
+
+Signal numbers 1, 2, 3, 6, 9, 14 and 15 are specified as parts of the optional POSIX XSI and ShellCheck will not warn about these. 
+
+### Exceptions:
+
+None.