Created SC2171 (markdown)

SC2171.md

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+## Found trailing ] outside test. Add missing [ or quote if intentional.
+
+### Problematic code:
+
+```sh
+if foo -eq bar ]; then true; fi
+```
+
+or
+
+```sh
+tr -d ]
+```
+
+### Correct code:
+
+```sh
+if [ foo -eq bar ]; then true; fi
+```
+
+or
+
+```sh
+tr -d ']'
+```
+
+### Rationale:
+
+ShellCheck found a non-test command that ends with `]` or `]]`.
+
+If this was intended to be a test expression like in the first example, add the missing `[` or `[[`. 
+
+If the `]` was intended to be literal, like in `tr -d ]`, you can quote to make this obvious.
+
+### Exceptions:
+
+`tr -d ]` is valid and not different from `tr -d ']'`, so in these cases you can ignore the error instead.
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!