Created SC2255 (markdown)

SC2255.md

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+##  `[ ]` does not apply arithmetic evaluation. Evaluate with `$((..))` for numbers, or use string comparator for strings.
+
+### Problematic code:
+
+```sh
+[ 2*3 -eq array[i] ]
+```
+
+### Correct code:
+
+```sh
+[ $((2*3)) -eq $((array[i])) ]
+```
+
+### Rationale:
+
+When using `[[ .. ]]` with numerical comparators (`-eq`, `-lt`, etc), the value on either side will be evaluated as an arithmetic expression. This means that `2*3` will be evaluated to `6`, and `x` will be evaluated to the contents of the variable `$x`.
+
+When using `[ .. ]`, this does not happen. `2*3` and `x` will both be considered invalid numbers. Instead, use e.g. `$((2*3))` to evaluate the expression before passing it to `[ .. ]`.
+
+Alternatively, if the expression should be considered a string, quote the expression and use a string comparison operator like `=` and `!=`. 
+
+### Exceptions:
+
+None
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!