bash subtract

Guri Bee

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21-10-2024

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Bash Subtraction: A Beginner's Guide to Arithmetic Operations

Bash, the default command-line interpreter for Linux and macOS, offers a powerful way to perform basic arithmetic operations, including subtraction. While Bash doesn't directly support minus signs for subtraction, it utilizes the expr command and arithmetic expansion to achieve this functionality.

This article will guide you through the essential techniques of subtracting numbers in Bash, explaining the underlying concepts and providing practical examples.

1. Using expr Command

The expr command evaluates expressions and returns the resulting value. For subtraction, you use the minus sign (-) within the expr command.

Example:

result=$(expr 10 - 5)
echo "The result is: $result"

This code snippet first uses expr 10 - 5 to subtract 5 from 10, storing the result in the variable result. Then, it prints the value of result, which is 5.

2. Arithmetic Expansion

Arithmetic expansion allows you to evaluate arithmetic expressions within double parentheses $(( )). This method provides a more concise and efficient way to perform subtraction.

Example:

result=$(( 10 - 5 ))
echo "The result is: $result"

Similar to the previous example, this code calculates 10 - 5 and assigns the result to result, printing it subsequently.

3. Handling Negative Numbers

Both expr and arithmetic expansion can handle negative numbers. When subtracting a larger number from a smaller number, the result will be negative.

Example:

result=$(( 5 - 10 ))
echo "The result is: $result"

This code will output: The result is: -5.

4. Combining Subtraction with Other Operations

You can combine subtraction with other arithmetic operations like addition, multiplication, and division within a single expression.

Example:

result=$(( (10 + 5) * (15 - 8) ))
echo "The result is: $result"

This example combines addition, subtraction, and multiplication, resulting in The result is: 105.

5. Real-World Application: Calculating File Sizes

Let's apply Bash subtraction to a practical scenario: calculating the difference between two file sizes.

file1_size=$(du -b file1.txt | awk '{print $1}')
file2_size=$(du -b file2.txt | awk '{print $1}')

size_difference=$(( file1_size - file2_size ))

echo "The size difference is: $size_difference bytes"

This script retrieves the file sizes of file1.txt and file2.txt using the du command and then subtracts file2_size from file1_size to determine the difference.

Important Notes:

• Space: Ensure you use spaces around the minus sign (-) when using the expr command.
• Integer Arithmetic: Bash performs integer arithmetic, meaning it only handles whole numbers. If you need to work with decimal numbers, consider using tools like bc or python.
• Error Handling: It's good practice to include error handling to prevent unexpected behavior when working with expressions.

Conclusion

Bash provides versatile methods for subtracting numbers through expr and arithmetic expansion. Understanding these techniques allows you to incorporate arithmetic operations into your scripts, making your command-line tasks more dynamic and efficient.

References:

• Bash Arithmetic Expansion
• Bash expr Command
• Du Command

Note: This article is based on information found on GitHub, including this and this repository, among others.