Bash Basics: Increment variables

increment.sh

#!/bin/bash

# 3 correct and 1 incorrect way to increment bash variables using "let" and arithmetic expansion.
#
# OUTPUT:
#
# 0
# 0+1
# 1
# 2
# 3
# 4
# 5
# 6
# 7
# 

num=0 ; echo $num

# 0 - FAIL
num=$num+1 ; echo $num ; num=0

# 1 Reassign
let "num=num+1" ; echo $num
((num=num+1)) ; echo $num
num=$((num+1)) ; echo $num

# 2 += operator
let "num+=1" ; echo $num
((num+=1)) ; echo $num

# 3 ++ operator
let "num++" ; echo $num
((num++)) ; echo $num