[Linux] Bash expr Usage: Evaluate expressions and perform calculations

Overview

The expr command in Bash is used to evaluate expressions and perform arithmetic calculations. It can handle integer arithmetic, string operations, and logical comparisons, making it a versatile tool for scripting and command-line operations.

Usage

The basic syntax of the expr command is as follows:

expr [options] [arguments]

Common Options

• + : Addition operator.
• - : Subtraction operator.
• * : Multiplication operator (must be escaped as \* or enclosed in quotes).
• / : Division operator.
• % : Modulus operator.
• = : String comparison operator.
• != : String inequality operator.
• > : Greater than comparison.
• < : Less than comparison.
• \ : Escape character for special characters.

Common Examples

Basic Arithmetic

To perform basic arithmetic operations:

expr 5 + 3

Output: 8

expr 10 - 4

Output: 6

Multiplication

To multiply two numbers, remember to escape the asterisk:

expr 4 \* 7

Output: 28

Division

To divide two numbers:

expr 20 / 4

Output: 5

Modulus

To find the remainder of a division:

expr 10 % 3

Output: 1

String Comparison

To compare two strings:

expr "hello" = "hello"

Output: 1 (true)

expr "hello" != "world"

Output: 1 (true)

Logical Comparison

To check if one number is greater than another:

expr 10 \> 5

Output: 1 (true)

Tips

• Always remember to escape the multiplication operator * or use quotes to avoid syntax errors.
• Use parentheses to group expressions for clarity, especially in complex calculations.
• For string operations, ensure that the strings are enclosed in quotes to prevent word splitting.
• Consider using $(( )) for arithmetic operations in Bash, as it is often more straightforward and supports more complex expressions.