Using `bc` for Basic Arithmetic in Bash

The bc command (short for "Basic Calculator") in Bash provides a robust way to perform arithmetic operations, especially when dealing with floating-point calculations, which are not natively supported in Bash. Here's a comprehensive guide to using bc for basic arithmetic in Bash scripts.

Why Use `bc`?

Floating-Point Arithmetic: Bash supports only integer arithmetic by default. bc handles floating-point calculations.
Advanced Operations: It supports mathematical functions like exponentiation and can use scale to control decimal precision.
Scripting-Friendly: Easily integrates into Bash scripts.

Getting Started

The basic syntax for bc is:

echo "expression" | bc

Simple Examples

#!/bin/bash

# Addition
result=$(echo "5 + 3" | bc)
echo "5 + 3 = $result"

# Subtraction
result=$(echo "10 - 7" | bc)
echo "10 - 7 = $result"

# Multiplication
result=$(echo "4 * 2" | bc)
echo "4 * 2 = $result"

# Division
result=$(echo "20 / 5" | bc)
echo "20 / 5 = $result"

Floating-Point Arithmetic

For operations involving decimals, set the scale, which specifies the number of decimal places to retain.

Example: Using Scale

#!/bin/bash

result=$(echo "scale=2; 5 / 3" | bc)
echo "5 / 3 = $result"  # Outputs 1.66

Explanation:

scale=2: Specifies that the result should have 2 decimal places.

Using Variables

You can pass Bash variables to bc for dynamic calculations.

#!/bin/bash

a=10
b=3

# Perform division
result=$(echo "scale=2; $a / $b" | bc)
echo "$a / $b = $result"

Advanced Arithmetic

1. Exponentiation

#!/bin/bash

result=$(echo "2^3" | bc)
echo "2^3 = $result"

To handle floating-point exponents, use the -l option to load the math library:

result=$(echo "scale=2; e(2*l(3))" | bc -l)
echo "3^2 = $result"  # Calculates 3^2 with floating-point precision

2. Square Root

#!/bin/bash

result=$(echo "scale=4; sqrt(16)" | bc)
echo "Square root of 16 = $result"

Conditional Logic in `bc`

bc also supports simple conditional checks using relational and logical operators.

Example: Relational Operators

#!/bin/bash

result=$(echo "5 > 3" | bc)
echo "Is 5 greater than 3? $result"  # Outputs 1 (true)

result=$(echo "5 < 3" | bc)
echo "Is 5 less than 3? $result"  # Outputs 0 (false)

Example: Logical Operators

#!/bin/bash

result=$(echo "5 > 3 && 2 < 4" | bc)
echo "Are both conditions true? $result"  # Outputs 1 (true)

Math Library with `bc -l`

Using the -l option enables advanced mathematical functions like sine, cosine, and natural logarithms.

Examples

#!/bin/bash

# Pi constant
result=$(echo "scale=4; 4*a(1)" | bc -l)
echo "Value of π = $result"

# Sine function (angle in radians)
result=$(echo "scale=4; s(1.5708)" | bc -l)
echo "Sine of π/2 = $result"  # Outputs 1.0000

Interactive Mode

You can start bc in interactive mode by simply typing bc in the terminal. For math library functions, start with:

bc -l

In this mode, you can directly input expressions and see results.

Best Practices

Use scale for Precision: Always specify scale for consistent decimal results.
Quote Expressions: Wrap expressions in quotes to prevent parsing issues.
Validate Inputs: If using user input, ensure values are valid numbers to avoid errors.
Use -l for Advanced Math: Load the math library (-l) when needed to access trigonometric and logarithmic functions.

Common Use Cases

1. Simple Calculator

#!/bin/bash

echo "Enter first number:"
read num1

echo "Enter second number:"
read num2

echo "Enter operation (+, -, *, /):"
read op

result=$(echo "scale=2; $num1 $op $num2" | bc)
echo "Result: $result"

2. Conversion Between Units

#!/bin/bash

echo "Enter temperature in Celsius:"
read celsius

fahrenheit=$(echo "scale=2; $celsius * 9/5 + 32" | bc)
echo "$celsius°C is equal to $fahrenheit°F"

Limitations

Integer Results by Default: Without scale, bc truncates results.
Requires Explicit Scripting: You must structure expressions carefully.
No Native Support for Complex Numbers: bc only handles real numbers.

Summary

The bc command is an essential tool for performing both basic and advanced arithmetic in Bash scripts, especially when floating-point precision is required. By understanding its syntax and features like scale, the -l math library, and conditional logic, you can integrate powerful mathematical calculations seamlessly into your Bash workflows.

Why Use bc?