Arithmetic: Math in Scripts

Scripts need to do math. Add. Subtract. Multiply. Divide.

Here's the thing: Arithmetic is simple. But you need to know the syntax. Learn it once. Use it forever.

Basic Arithmetic

Using $(( ))

result=$((5 + 3))                # 8
result=$((10 - 4))               # 6
result=$((6 * 2))                # 12
result=$((8 / 2))                # 4
result=$((10 % 3))               # 1 (remainder)

The $(( )): Arithmetic expansion. This is what you'll use.

My take: Use $(( )). It's simple. It works. It's what everyone uses.

Operators

+                                # Addition
-                                # Subtraction
*                                # Multiplication
/                                # Division
%                                # Modulo (remainder)
**                               # Exponentiation (bash 4+)

My take: Standard math operators. Nothing special.

Increment and Decrement

count=10
((count++))                      # Increment: 11
((count--))                      # Decrement: 10
((count+=5))                     # Add 5: 15
((count-=3))                     # Subtract 3: 12

My take: Increment/decrement is common. Use it in loops.

Real-World Examples

Counter

count=0
for i in {1..10}; do
    ((count++))
done
echo "Count: $count"

Calculate

total=$((price * quantity))
discount=$((total * 10 / 100))
final=$((total - discount))

Check Even/Odd

if (( number % 2 == 0 )); then
    echo "Even"
else
    echo "Odd"
fi

Common Mistakes (I've Made These)

1. Forgetting $(( )): result=5+3 doesn't work. Use result=$((5+3)).

2. Spaces in $(( )): $((5+3)) works. $(( 5 + 3 )) also works. Both fine.

3. Division with integers: $((10/3)) = 3 (integer division). No decimals.

4. Using let: let works, but $(( )) is better. Use $(( )).

5. Using expr: expr is old. Use $(( )). It's better.

What's Next?

Now that you can do math, scripts can calculate. Or learn about Comparison Operators to compare values.

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Personal note: When I started, I'd use expr for math. Then I learned $(( )). Now I use it for everything. It's simpler. It's better. Learn it.