Created SC2021 (markdown)

SC2021.md

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+## Don't use [] around ranges in tr, it replaces literal square brackets.
+
+### Problematic code:
+
+```sh
+tr -cd '[a-z]'
+```
+
+### Correct code:
+
+```sh
+tr -cd 'a-z'
+```
+### Rationale:
+
+Ancient System V `tr` required brackets around operands, but modern implementations including POSIX, GNU, OS X and *BSD instead treat them as literals.
+
+Unless you want to operate on literal square brackets, don't include them.
+
+### Exceptions:
+
+If you do want to replace literal square brackets, reorder the expression (e.g. `a-z[]` to make it clear that the brackets are not special).
+
+ShellCheck does not warn about correct usage of `[..]` in character and equivalence classes like `[:lower:]` and `[=e=]`.