Bash Numeric Comparisons

Numeric Comparisons in Bash

Numeric comparisons in Bash allow you to evaluate relationships between numbers, such as equality, inequality, and order. These comparisons are essential for scripting tasks like loops, conditionals, and arithmetic operations.

Numeric Comparison Operators

Operator	Description
-eq	Equal to
-ne	Not equal to
-lt	Less than
-le	Less than or equal to
-gt	Greater than
-ge	Greater than or equal to

Basic Syntax

if [ num1 -operator num2 ]; then
    # Commands if the condition is true
else
    # Commands if the condition is false
fi

Examples of Numeric Comparisons

1. Check Equality

num1=10
num2=20

if [ $num1 -eq $num2 ]; then
    echo "The numbers are equal."
else
    echo "The numbers are not equal."
fi

2. Check Inequality

if [ $num1 -ne $num2 ]; then
    echo "The numbers are not equal."
fi

Greater Than and Less Than

3. Compare Two Numbers

num1=15
num2=10

if [ $num1 -gt $num2 ]; then
    echo "$num1 is greater than $num2."
else
    echo "$num1 is not greater than $num2."
fi

4. Check for Less Than

if [ $num1 -lt $num2 ]; then
    echo "$num1 is less than $num2."
else
    echo "$num1 is not less than $num2."
fi

Combining Numeric Comparisons

You can combine conditions using logical operators like && (AND) and || (OR).

5. Logical AND

num=25

if [ $num -gt 10 ] && [ $num -lt 30 ]; then
    echo "$num is between 10 and 30."
else
    echo "$num is not in the range."
fi

6. Logical OR

if [ $num -lt 10 ] || [ $num -gt 50 ]; then
    echo "$num is out of range."
else
    echo "$num is within the range."
fi

Using Double Parentheses for Numeric Comparisons

The the (( )) syntax is a more concise and flexible way to perform numeric comparisons. It eliminates the need for -eq, -ne, etc.

7. Basic Comparison with (( ))

num=42

if (( num > 40 )); then
    echo "$num is greater than 40."
fi

8. Combining Conditions in (( ))

if (( num > 20 && num < 50 )); then
    echo "$num is between 20 and 50."
fi

Example: Validating User Input

9. Check if Input is Positive

read -p "Enter a number: " num

if [ $num -gt 0 ]; then
    echo "The number is positive."
else
    echo "The number is not positive."
fi

10. Check If Number is Even or Odd

if (( num % 2 == 0 )); then
    echo "$num is even."
else
    echo "$num is odd."
fi

Error Handling in Numeric Comparisons

1. Ensure Variables Contain Numbers: If a variable contains non-numeric data, comparisons will fail. Use regex or arithmetic expansion to validate input:

   read -p "Enter a number: " num
   
   if [[ "$num" =~ ^[0-9]+$ ]]; then
       echo "Valid number."
   else
       echo "Invalid input. Please enter a number."
   fi
   

2. Avoid Uninitialized Variables: Always initialize variables before using them in numeric comparisons:

   num=10  # Initialize before comparing
   

Common Mistakes and Tips

1. Do Not Use String Comparisons for Numbers: Numeric comparisons must use -eq, -lt, etc., or (( )).

   if [ $num = 10 ]; then  # Incorrect for numbers
   if [ $num -eq 10 ]; then  # Correct
   

2. Quote Variables in Test Commands: Quote variables in [ ] to avoid errors with empty or special values:

   if [ "$num" -eq 10 ]; then  # Correct
   

3. Avoid Spaces Around Assignment: Ensure there are no spaces around = in variable assignments:

   num=10  # Correct
   num = 10  # Incorrect
   

Conclusion

Numeric comparisons are a fundamental part of Bash scripting, enabling conditional logic for a wide range of tasks. By mastering the operators and syntax, you can write robust and efficient scripts.

Let me know if you need additional examples or further refinements!