What are Double Parentheses in Bash
The “((..))” double parentheses are similar to the let command, which you can use to perform arithmetic calculations in a script. For example, let’s create a script that will perform various arithmetic calculations:
echo "Please enter the values of A and B"
read A
read B
sum=$((A+B))
echo "Addition of $A and $B is $sum"
((sub=AB))
echo "Subtraction of $A and $B is $sub"
num1=A
num2=B
((multiply=num1*num2))
echo "Multiplication $A and $B is $multiply"
division=$((num1/num2))
echo "Division of $A and $B is $division"
Once you run the above script, the terminal will ask you to enter two numbers, and then it will perform the calculations:
Similarly, you can use the following patterns of double parentheses styles to get the required results:
echo "Please enter the values of A and B"
read A
read B
sum=$((A+B))
echo "Addition of $A and $B is $sum"
((sub=AB))
echo "Subtraction of $A and $B is $sub"
num1=A
num2=B
((multiply=num1*num2))
echo "Multiplication $A and $B is $multiply"
division=$((num1/num2))
echo "Division of $A and $B is $division"
This script provides the same result as the previous one:
You can also use the [[ rather than [ because it is an advanced type that offers a ton of enhancements such as:

 The [[ can efficiently handle the empty strings and some strings with the whitespace.
 You can use  and && logical operators with [[ but not [ because single [ can’t pass  and && operator as a commandline argument.
Wrapping Up
This was all about the double parentheses in bash, which you can try to enhance the arithmetic calculations easily. We have explained various types of examples to perform arithmetic calculations by adding double parentheses. Bash contains a ton of concepts that you can learn to become a bash expert. So make sure you check out Linuxhint to read various tutorials of bash.