Add bash arithmetic section to notes

notes/shell/bash_arithmetic.md

@@ -0,0 +1,79 @@
+# Working with numbers in bash
+Bash interprets everything by default as a string, however it can also work with
+numbers, just be careful with your conditional operators
+
+## Storing numbers
+`declare` and `local` both can make integer variables with the `-i` flag
+
+```bash
+declare -i a=2
+
+f () { local -i a=3 ;}
+```
+
+Variables can be manipulated in double parenthesis, very similar to c/python.
+Use `$(())` to return the value instead. Edges and centers seem whitespace
+insensitive. No need to `$` in front of variables inside `(())`
+
+Work as expected: `+ - * %`
+Exponent: `**`
+Floored division: `/`
+
+```bash
+declare -i a=2 b=3  # Multiple on the same line are fine
+
+(( a++ ))    # a == 3
+(( a+=b ))   # a == 6
+(( a = 2 ))  # a == 2
+((a=4+b))    # a == 7
+(( a = 4 + b )) # Spacing is optional, same as above
+
+declare -i c=$((a**b + 4))  # c == 7**3 + 4 == 347
+```
+
+## Using numbers
+Conditionals are different for numbers in bash. Mapping c to bash:
+
+```
+ C  | Bash
+----|-----
+ == | -eq
+ != | -ne
+ >  | -gt
+ <  | -lt
+ >= | -ge
+ <= | -le
+```
+
+```bash
+declare -i a=3 b=2
+
+if [[ $((a**b)) -ge 1000 ]]; then
+  echo "Exponentiation with ${a}^${b} is pretty big"
+elif [[ $a -lt $b ]]; then
+  echo "$a is smaller than $b"
+else
+  echo "$a isn't smaller than $b"
+fi
+
+echo "You rolled a: $(( RANDOM % 6 + 1 )) on a 6 sided dice"
+```
+
+## Floats
+Bash has no support for floats, don't even try. If you need floats, use awk,
+which has full support. To still sort of use rational numbers in bash, store a
+numerator and denominator, then pass both to awk when you need the float
+
+```bash
+declare -i num_1=4 denom_1=9
+declare -i num_2=2 denom_2=9
+
+(( num_1 += num_2 ))
+
+awk -v n1=$num_1 -v d1=$denom_1 -v n2=$num_2 -v d2=$denom_2 \
+  'BEGIN { printf "%.2f\n", n1/d1 + n2/d2 }'
+```
+
+Bash has floored division `/` and remainders `%`, though the awk approach is
+easier. Note awk's syntax is identical to c, so it uses `^` for exponentiation
+and the standard `==`, `<=`... for comparison

notes/vim/vim_batch_editing.md

@@ -35,11 +35,15 @@ visual mode
 :21             Goes to line 21. ^ and $ are for the first and last line
 :10,20d         Deletes lines 10 through 20, inclusive on both ends
 :u[ndo]         Undoes the last action
+:mark a         Makes a mark at a. From vim
+:ka             Also makes a mark at a. From ex
+:'a             Go to mark a
 :g/re/p         Globally exectute a command on lines with /re/
 :v/re/p         Inverse of :g. Executes on all lines without /re/
 :3,6co$         Copy lines [3,6] to the end of the document
 :3m6            Move line 3 to line 6
 :z=3            Pretty print lines in [-2,+2]
+:norm! @l       Execute keystrokes, if all else fails
 ```
 
 Several commands can be chained with `|`, similar to `;` in bash