Created SC2204 (markdown)

SC2204.md

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+## (..) is a subshell. Did you mean [ .. ], a test expression?
+
+### Problematic code:
+
+```sh
+if ( -d mydir )
+then
+  echo "It's a directory"
+fi
+```
+
+### Correct code:
+
+```sh
+if [ -d mydir ]
+then
+  echo "It's a directory"
+fi
+```
+### Rationale:
+
+Tests like `-d` to see if something is a directory or `-z` to see if it's non-empty are actually flags to the `test` command, and only work as tests in that context. `[` is an alias for `test`, so you'll frequently see them written as `[ -d mydir ]`.
+
+`( .. )` is completely unrelated, and is a subshell mostly used to scope shell modifications. They should not be used in `if` or `while` statements in shell scripts.
+
+If you wanted to test a condition, rewrite the `( .. )` to `[ .. ]`. 
+
+### Exceptions:
+
+None. 
+
+This error is triggered by having a unary test operator as the first command name in a subshell, which won't normally happen. Note that there's a similar warning [[SC2205]] with a higher false positive rate.