Add rules for condition evaluations

Code-Style-Guide.md

@@ -1,4 +1,4 @@
-These are the code style guidelines that should be followed when contributing to zim.
+These are the code style guidelines that should be followed when contributing to Zim.
 
 Indentation
 -----------
@@ -18,7 +18,7 @@ for x in a very very very very \
 done
 ```
 
-Line Length
+Line length
 -----------
 
 Be reasonable. Keeping within 80 characters is recommended, 120 is the maximum.
@@ -37,15 +37,21 @@ Use existing variables whenever possible:
 * Good: `${PWD}` `${USER}`
 * Bad: `$(pwd)` `$(whoami)`
 
-Flow Logic
+Flow logic
 ----------
 
 For short single commands, prefer one-liners:
-* `for` with the zsh syntax: `for x (foo bar) print ${x}`
-* `&&` instead of if: `(( ! ${+FOO} )) && FOO='bar'`
+* `for` with the Zsh syntax: `for x (foo bar) print ${x}`
+* `&&` instead of if: `[[ -d ${dir} ]] && cd ${dir}`
 
 Otherwise just place `; do` `; then` on the same line as `while`, `for ... in`, and `if`.
 
+Use `(( ))` for arithmetic conditions, and `[[ ]]` for other condition evaluations. Don't use `[ ]`.
+
+With arithmetic conditions:
+* Don't use `$` in front of variables: `(( var > 1 ))`  
+* Unless some substitution is needed: `(( ${#var} > 1 ))`
+
 Functions
 ---------
 
@@ -63,4 +69,3 @@ Misc
 
 Do **NOT** use backticks ``` ` ``` to execute something in a subshell. This GRINDS MY GEARS.
 `$(print "use parentheses")`
-