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+## Brace expansions don't work as operands 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" = "$main" ] && echo "This is the main file"
+done
+```
+
+### Rationale:
+
+Brace expansions in `[ ]` will expand to a sequence of words. Operators work on single words.
+
+The problematic code is equivalent to `[ "$file" = index.htm index.html index.php ]`, which is invalid syntax. A typical error message is `bash: [: too many arguments` or `dash: somefile: unexpected operator`. 
+
+Instead, use a `for` loop to iterate over values, and apply your condition to each.
+
+### Exceptions:
+
+None.
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