Math in Bash

Bash supports a number of basic and more advanced arithmetic operations. These include addition, subtraction, multiplication, division, modulus, incrementing, decrementing, and exponentiation.

There are a few common ways to perform mathematical operations on numbers in bash. These include double parentheses, and the let and expr commands.

Basic Arithmetic in Bash

A simple way to perform arithmetic in bash is to use double parentheses $(()). These are used so that the variables are interpreted as integers rather than strings. For example:

#!/bin/bash

a=5
b=3
result=$((a + b))
echo "Result: $result"

Output:

Result: 8

In the above example, we first initialized two variables, ‘a’ and ‘b’. Then we created a third variable ‘result’ equal to the sum of ‘a’ and ‘b’.

This example demonstrated addition; the following shows how we can perform subtraction, multiplication, and division as well:

#!/bin/bash

a=5
b=3

sum=$((a + b))
echo "Sum: $sum"

difference=$((a - b))
echo "Difference: $difference"

product=$((a * b))
echo "Product: $product"

quotient=$((a / b))
echo "Quotient: $quotient"

Output:

Sum: 8
Difference: 2
Product: 15
Quotient: 1

Modulus

Bash supports the modulus operator %, which finds the remainder of a division. For example, 10 divided by 3 (10/3) is equal to three, with a remainder of 1. The modulus of this operation is 1.

#!/bin/bash

result=$((10 % 3))
echo "Modulus: $modulus"

Output:

Modulus: 1

Increment and Decrement

Bash supports the increment ((++)) and decrement ((--)) operators. Increment (++) adds one to a value, and decrement (–) subtracts one.

We need to use two sets of parentheses in order for the operation to work properly. For example:

#!/bin/bash

a=5
((a++))
echo "Incremented: $a"

Output:

Incremented: 6

The value of ‘a’ is initially 5 but we can that it has been incremented to 6.

Let’s look at decrementing as well:

#!/bin/bash

a=5
((a--))
echo "Decremented: $a"

Output:

Decremented: 4

This time, ‘a’ gets decremented down 4.

Exponents

Bash supports exponentiation using the ** operator.

For example:

result=$((2**3))
echo "Result: $result"

Output:

Result: 8

This example calculates 2³ = 2 x 2 x 2, which is 8.

Using the let Command

The let command is a built-in command in Bash that allows you to perform arithmetic operations and assignments.

The syntax for using let is:

   let expression

We can use arithmetic expressions with let, including addition (+), subtraction (-), multiplication (*), division (/), modulus (%), and exponentiation (**).

Let’s see an example:

   #!/bin/bash

   # Addition
   let result=5+3
   echo "Addition: $result"

   # Subtraction
   let result=result-2
   echo "Subtraction: $result"

   # Multiplication
   let result=result*4
   echo "Multiplication: $result"

   # Division
   let result=result/2
   echo "Division: $result"

   # Modulus
   let result=result%3
   echo "Modulus: $result"

   # Exponentiation
   let result=result**2
   echo "Exponentiation: $result"

Output:

Addition: 8
Subtraction: 6
Multiplication: 24
Division: 12
Modulus: 0
Exponentiation: 144

Using the let command, we can perform various arithmetic operations in Bash scripts. It’s a convenient way to manipulate numeric values and perform calculations within our scripts.

Using the expr Command

In Bash, we can perform arithmetic operations using the expr command. expr evaluates expressions and prints the result to stdout.

The basic syntax of expr for arithmetic is:

   expr ARG1 OPERATOR ARG2

ARG1 and ARG2 are the arguments to the arithmetic operation and OPERATOR is the arithmetic operator (+, -, *, /, %,)).

Let’s see an example:

   #!/bin/bash

   # Addition
   result=$(expr 5 + 3)
   echo "Addition: $result"

   # Subtraction
   result=$(expr $result - 2)
   echo "Subtraction: $result"

   # Multiplication
   result=$(expr $result \* 4)
   echo "Multiplication: $result"

   # Division
   result=$(expr $result / 2)
   echo "Division: $result"

   # Modulus
   result=$(expr $result % 3)
   echo "Modulus: $result"

   # Exponentiation (not supported by expr, using bc)
   result=$(echo "scale=2; $result ^ 2" | bc)
   echo "Exponentiation: $result"

Output:

Addition: 8
Subtraction: 6
Multiplication: 24
Division: 12
Modulus: 0
Exponentiation: 144.00

Note the following points:

• For multiplication (*), you need to escape the asterisk with a backslash (\) to prevent it from being interpreted as a wildcard character by the shell.
• expr does not support floating-point arithmetic. For exponentiation, you can use bc (an arbitrary precision calculator language) as shown in the example.

Floating-Point Math in Bash

You may have noticed that all of these examples used integers rather than floating-point (decimal) numbers.

Bash does not support floating-point arithmetic natively. However, it is still possible to work with floating point numbers using external tools like awk or bc. For example:

result=$(awk 'BEGIN {print 10.5 / 3}')
echo "Result: $result"

These features allow you to perform a wide range of mathematical and arithmetic operations in Bash scripts.