Increment/decrement a variable in bash

Increment:

num=$((num+1))
((num=num+1))
((num+=1))
((num++))
((++num))
let "num=num+1"
let "num+=1"
let "num++"
num=$(expr $num + 1)
num=$(bc <<< "$num+1")

Decrement:

num=$((num-1))
((num=num-1))
((num-=1))
((num--))
((--num))
let "num=num-1"
let "num-=1"
let "num--"
num=$(expr $num - 1)
num=$(bc <<< "$num-1")