Created SC2100 (markdown)

SC2100.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=$((i + 2))
+```
+
+### Rationale:
+
+Unlike most languages, variable assignments (almost) always assigns strings and not expressions. In the example code, `i` will become the string `i+2` instead of the intended `5`. 
+
+To instead evaluate a mathematical expressions, use `$((..))` as in the correct code.
+
+### Exceptions:
+
+If you wanted to assign a literal string, quote it:
+
+    description="friendly-looking"
+
+ShellCheck (as of v0.5) doesn't recognize Bash/Ksh numeric variables created with `declare -i` where this syntax is valid. Using `$((..))` still works, but you can also [[ignore]] this warning.
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!