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+## Test expression was opened with double `[[` but closed with single `]`. Make sure they match.
+
+(or SC1034 for vice versa)
+
+### Problematic code:
+
+```sh
+[[ -z "$var" ]
+```
+
+### Correct code:
+
+```sh
+[[ -z "$var" ]]
+```
+### Rationale:
+
+ShellCheck found a test expression `[ ... ]` (POSIX) or `[[ ... ]]` (ksh/bash), but where the opening and closing brackets did not match (i.e. `[[ .. ]` or `[ .. ]]`). The brackets need to match up to work.
+
+Note in particular that `[..]` do *not* work like parentheses in other languages. You can not do:
+
+```sh
+# Invalid
+[[ x ] || [ y ]]
+```
+Instead, use one of
+You would instead use two separate test expressions joined by `||`:
+
+```sh
+# Valid basic test expressions (sh/bash/ksh)
+[ x ] || [ y ]
+
+# Valid extended test expressions (bash/ksh)
+[[ x ]] || [[ y ]]
+```
+
+### Exceptions:
+
+None
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!
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