Created SC2121 (markdown)

SC2121.md

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+# To assign a variable, use just 'var=value', no 'set ..'.
+
+### Problematic code:
+
+    set var=42
+    set var 42
+
+### Correct code:
+
+    var=42
+
+### Rationale:
+
+`set` is not used to set or assign variables in Bourne shells. It's used to set shell options and positional parameters.
+
+To assign variables, use `var=value` with no `set` or other qualifiers.
+
+### Contraindications
+
+If you actually do want to set positional parameters, simply quoting them or using `--` will make shellcheck stop warning, e.g. `set -- var1 var2` or `set "foo=bar"`.