How to Use bc for Calculations in Bash Scripts

Use the bc command in bash to perform calculations by piping expressions to it or using here-strings. For example, echo "5 + 3" | bc outputs 8. This allows bash to handle floating-point and complex math easily.

Syntax

The basic syntax to use bc in bash is to echo a math expression and pipe it to bc. You can also use here-strings for inline calculations.

• echo "expression" | bc: Sends the expression to bc for calculation.
• bc <<< "expression": Uses here-string to pass the expression.
• scale=n: Sets decimal precision for floating-point results inside bc.

bash

echo "5 + 3" | bc
bc <<< "10 / 4"
echo "scale=2; 10 / 4" | bc

Output

8
2
2.50

Example

This example shows how to calculate the sum, division with decimals, and multiplication using bc in bash.

bash

#!/bin/bash

# Sum of two numbers
echo "5 + 7" | bc

# Division with 3 decimal places
echo "scale=3; 10 / 3" | bc

# Multiplication
result=$(bc <<< "12 * 3")
echo "$result"

Output

12
3.333
36

Common Pitfalls

Common mistakes when using bc include:

• Not setting scale for decimal precision, which causes integer division results.
• Forgetting to quote expressions, leading to shell interpretation errors.
• Trying to use bash arithmetic $(( )) for floating-point math, which only supports integers.

Correct usage requires quoting expressions and setting scale when decimals are needed.

bash

echo "10 / 4" | bc  # Wrong: no scale, outputs 2

echo "scale=2; 10 / 4" | bc  # Right: outputs 2.50

Output

2
2.50

Quick Reference

Command	Description
echo "expression" | bc	Calculate expression using bc
bc <<< "expression"	Calculate expression using here-string
scale=n	Set decimal places to n inside bc
echo "scale=2; 5 / 2" | bc	Divide with 2 decimal places
result=$(bc <<< "12 * 3")	Store calculation result in variable

Key Takeaways

Use bc to perform floating-point and complex math in bash.

Always quote math expressions when passing to bc to avoid shell errors.

Set scale inside bc to control decimal precision.

Use piping or here-strings to send expressions to bc.

Bash arithmetic $(( )) only supports integers, so use bc for decimals.