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+## Brace expansion doesn't happen in [[ ]]. Use a loop.
+
+### Problematic code:
+
+```sh
+[[ "$file" = index.{htm,html,php} ]] && echo "This is the main file"
+```
+
+### Correct code:
+
+```sh
+for main in index.{htm,html,php}
+do
+  [[ "$file" = "$main" ]] && echo "This is the main file"
+done
+```
+
+### Rationale:
+
+Brace expansions doesn't happen in `[[ ]]`. They will just be interpreted literally.
+
+Instead, use a `for` loop to iterate over values, and apply your condition to each.
+
+### Exceptions:
+
+None.
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