Created SC1026 (markdown)

SC1026.md

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+## If grouping expressions inside [[..]], use ( .. ).
+
+### Problematic code:
+
+```sh
+[[ [ a || b ] && c ]]
+[ [ a -o b ] -a c ]]
+```
+
+### Correct code:
+
+```sh
+[[ ( a || b ) && c ]]
+[ \( a -o b \) -a c ]]  # or  { [ a ] || [ b ]; } && [ c ]
+```
+### Rationale:
+
+`[ .. ]` should not be used to group subexpressions inside `[[ .. ]]` or `[ .. ]` statements.
+
+For `[[ .. ]]`, use regular parentheses.
+
+For `[ .. ]`, either use escaped parentheses, or preferably rewrite the expression into multiple `[ .. ]` joined with `&&`, `||` and `{ ..; }` groups. 
+
+### Exceptions:
+
+None
+
+### Related resources:
+
+* [Bash Pitfalls: `if [ [ a = b ] && [ c = d ] ]; then ...`](https://mywiki.wooledge.org/BashPitfalls#pf11)