Created SC2099 (markdown)

SC2099.md

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+## Use `$((..))` for arithmetics, e.g. `i=$((i + 2))`
+
+### Problematic code:
+
+```sh
+i=3
+i=i + 2
+```
+
+### Correct code:
+
+```sh
+i=3
+i=$((i + 2))
+```
+
+### Rationale:
+
+Unlike most languages, variable assignments in shell scripts are space sensitive and (almost) always assign strings.
+
+To evaluate a mathematical expressions, use `$((..))` as in the correct code:
+
+    i=$((i + 2))   # Spaces are fine inside $((...))
+
+In the problematic code, `i=i + 2` will give an error `+: command not found` because the expression is interpreted similar to something like `LC_ALL=C wc -c` instead of numerical addition:
+ 
+     Prefix assignment    Command  Argument
+     LC_ALL=C             wc       -c
+     i=i                  +        2
+
+### Exceptions:
+
+If you wanted to assign a literal string, quote it:
+
+    game_score="0 - 2"
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!