Created SC2091 (markdown)

SC2091.md

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+## Remove surrounding $() to avoid executing output.
+
+### Problematic code:
+
+    if $(which "false"); then echo "true"; fi
+
+### Correct code:
+
+    if which "false" >/dev/null >2&1; then echo "true"; fi
+
+### Rationale:
+
+If the `$()` subshell produces an output it will be evaluated by the `if` statement, this could result in the execution of the output, which will result in different behaviour than intended.
+
+### Exceptions:
+
+None.