Created SC2166 (markdown)

SC2166.md

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+## Prefer [ p ] && [ q ] as [ p -a q ] is not well defined.
+
+And likewise, prefer `[ p ] || [ q ]` over `[ p -o q ]`.
+
+### Problematic code:
+
+    [ "$1" = "test" -a -z "$2" ]
+
+### Correct code:
+
+    [ "$1" = "test" ] && [ -z "$2" ]
+
+### Rationale:
+
+`-a` and `-o` to mean AND and OR in a `[ .. ]` test expression is not well defined. They are obsolescent extensions in [POSIX](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/test.html) and their behavior is almost always undefined.
+
+Using multiple `[ .. ]` expressions with shell AND/OR operators `&&` and `||` is well defined and therefore preferred (but note that they have equal precedence, while `-a`/`-o` is unspecified but often implemented as `-a` having higher precedence). 
+
+### Exceptions:
+
+None.