Add numbers in a bash script

Typical examples:

num=$((num1 + num2))
num=$(($num1 + $num2))	# Also works
num=$((num1 + 2 + 3))	# ...
num=$[num1+num2]	# Old, deprecated arithmetic expression syntax
num=`expr $num1 + $num2`# Whitespace for expr is important
			# this is only needed for really old systems

Other examples:

((i++))
count=$[count+1]