Created SC2155 (markdown)

SC2155.md

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+## Declare and assign separately to avoid masking return values.
+
+### Problematic code:
+
+    export foo="$(mycmd)"
+
+### Correct code:
+
+    export foo
+    foo=$(mycmd)
+
+### Rationale:
+
+In the original code, the return value of `mycmd` is ignored, and `export` will instead always return true. This may prevent conditionals, `set -e` and traps from working correctly.
+
+When first marked for export and assigned separately, the return value of the assignment will be that of `mycmd`. This avoids the problem.
+
+### Exceptions:
+
+If you intend to ignore the return value of an assignment, you can either ignore this warning or use
+
+    export foo
+    foo=$(mycmd) || true