Unix / Linux - Shell Arithmetic Operators Example

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The following arithmetic operators are supported by Bourne Shell.

Assume variable a holds 10 and variable b holds 20 then −

Operator	Description	Example
+ (Addition)	Adds values on either side of the operator	`expr $a + $b` will give 30
- (Subtraction)	Subtracts right hand operand from left hand operand	`expr $a - $b` will give -10
* (Multiplication)	Multiplies values on either side of the operator	`expr $a \* $b` will give 200
/ (Division)	Divides left hand operand by right hand operand	`expr $b / $a` will give 2
% (Modulus)	Divides left hand operand by right hand operand and returns remainder	`expr $b % $a` will give 0
= (Assignment)	Assigns right operand in left operand	a = $b would assign value of b into a
== (Equality)	Compares two numbers, if both are same then returns true.	[ $a == $b ] would return false.
!= (Not Equality)	Compares two numbers, if both are different then returns true.	[ $a != $b ] would return true.

It is very important to understand that all the conditional expressions should be inside square braces with spaces around them, for example [ $a == $b ] is correct whereas, [$a==$b] is incorrect.

All the arithmetical calculations are done using long integers.

Example

Here is an example which uses all the arithmetic operators −

#!/bin/sh

a=10
b=20

val=`expr $a + $b`
echo "a + b : $val"

val=`expr $a - $b`
echo "a - b : $val"

val=`expr $a \* $b`
echo "a * b : $val"

val=`expr $b / $a`
echo "b / a : $val"

val=`expr $b % $a`
echo "b % a : $val"

if [ $a == $b ]
then
   echo "a is equal to b"
fi

if [ $a != $b ]
then
   echo "a is not equal to b"
fi

The above script will produce the following result −

a + b : 30
a - b : -10
a * b : 200
b / a : 2
b % a : 0
a is not equal to b

The following points need to be considered when using the Arithmetic Operators −

• There must be spaces between the operators and the expressions. For example, 2+2 is not correct; it should be written as 2 + 2.

• Complete expression should be enclosed between ‘ ‘, called the inverted commas.

• You should use \ on the * symbol for multiplication.

• if...then...fi statement is a decision-making statement which has been explained in the next chapter.