Created SC2139 (markdown)

SC2139.md

@@ -0,0 +1,19 @@
+## This expands when defined, not when used. Consider escaping.
+
+### Problematic code:
+
+    alias whereami="echo $PWD"
+
+### Correct code:
+
+    alias whereami='echo $PWD'
+
+### Rationale:
+
+With double quotes, this particular alias will be defined as `echo /home/me`, so it will always print the same path. This is rarely intended.
+
+By using single quotes or escaping any expansions, we define the alias as `echo $PWD`, which will be expanded when we use the alias. This is the far more common use case.
+
+### Contraindications
+
+If you don't mind that your alias definition is expanded at compile time, you can ignore this warning.