Bash Math

Basic Math

Bash has some built-in functionality for really basic integer math, like addition, subtraction, multiplication, and division.

Addition

$ sum=$((2+2))  &&   echo $sum
4

$ sum=$((sum+10))  &&   echo $sum
14

Subtraction

$ diff=$((100-50))   &&   echo $diff
50

$ diff=$((diff-25))   &&   echo $diff
25

Multiplication

$ prod=$((2*8))    &&   echo $prod
16

$ a=9; b=3; prod=$((a*b))    &&   echo $prod
27

Division

$ a=14; b=2; div=$((a/b))   &&   echo $div
7

Remainder

$ a=10; b=4; z=$((a%b))   &&   echo $z
2

Exponentiation

$ z=5   &&   echo $z
5

$ zz=$((z**2))   &&   echo $zz
25

Complex Math

To do anything more complex than basic integer math, you have to use the program bc (bash calculator).

You can find the bc man page here: http://unixhelp.ed.ac.uk/CGI/man-cgi?bc+1

The gist of how you script mathematical operations with bc is, you echo the operation you want to perform, and you pipe it to bc. bc then performs the calculation and spits out the result.

For example:

$ echo "25.7 + 90.1384" | bc

115.8384

$ echo "25.7 >= 90.1384" | bc
0

$ echo "25.7 <= 90.1384" | bc
1

$ echo "sqrt(85)" | bc
9

$ echo "sqrt(85.0)" | bc
9.2

$ echo "sqrt(85.000000)" | bc
9.219544

Operators

You can use the increment and decrement operators ++ and -- as follow:

Increment++

The increment operator can be used as either a prefix or postfix operator, i.e. as ++i or i++. Example:

$ z=1   &&   echo $z
1

$ ((++z))   &&   echo $z
2

$ ((++z))   &&   echo $z
3

$ ((++z))   &&   echo $z
4

This can be used in bash for loops as follows:

for ((i=1; i <=$NUM ; i++)); do
    echo $i
done

Decrement--

Like the increment operator, this can be a prefix or postfix operator, i.e. --i or i--:

$ z=1   &&   echo $z
1

$ ((--z))   &&   echo $z

$ ((--z))   &&   echo $z
-1

$ ((--z))   &&   echo $z
-2

$ ((--z))   &&   echo $z
-3

Notice that echo $z doesn't print anything if z=0.