How to Use expr in Bash: Syntax and Examples

In bash, expr is a command used to evaluate expressions like arithmetic calculations and string operations. You write expressions as arguments to expr, and it outputs the result. For example, expr 3 + 4 outputs 7.

Syntax

The basic syntax of expr is:

• expr expression

Where expression can be arithmetic (like addition, subtraction), string operations, or comparisons.

Important parts:

• Operators like +, -, *, /, % for math.
• Spaces are required between operands and operators.
• Use backslash \ to escape special characters like *.

bash

expr 5 + 3
expr 10 \* 2
expr 15 / 3

Output

8
20
5

Example

This example shows how to add two numbers and compare them using expr:

bash

#!/bin/bash
num1=7
num2=5
sum=$(expr $num1 + $num2)
echo "Sum: $sum"
comparison=$(expr $num1 \> $num2)
echo "Is num1 greater than num2? $comparison"

Output

Sum: 12
Is num1 greater than num2? 1

Common Pitfalls

Common mistakes when using expr include:

• Not putting spaces around operators (e.g., expr 3+4 is wrong, use expr 3 + 4).
• For multiplication, forgetting to escape the * (use \*).
• Using expr for floating-point math (it only supports integers).
• Not quoting variables that might be empty, which can cause errors.

Example of wrong and right usage:

bash

expr 3+4
expr 3 + 4
expr 5 * 2
expr 5 \* 2

Output

expr: syntax error
7
expr: syntax error
10

Quick Reference

Operator	Description	Example	Output
+	Addition	expr 4 + 5	9
-	Subtraction	expr 10 - 3	7
\*	Multiplication (escape *)	expr 6 \* 7	42
/	Division	expr 20 / 4	5
%	Modulo	expr 10 % 3	1
>	Greater than (returns 1 if true)	expr 5 \> 3	1
<	Less than (returns 1 if true)	expr 2 \< 4	1
=	Equal (returns 1 if true)	expr 3 = 3	1

Key Takeaways

Always put spaces around operators when using expr.

Escape special characters like * with a backslash (\*).

expr only supports integer arithmetic, not floating-point.

Use expr for simple math and string comparisons in bash scripts.

Quote variables in expr to avoid errors if they are empty.