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The expr command

The expr command evaluates a given expression and displays its corresponding output. It is used for basic operations like addition, subtraction, multiplication, division, and modulus on integers and Evaluating regular expressions, string operations like substring, length of strings etc.

Syntax

expr expression

Few Examples:

1. Perform basic arithmetic operations using expr command

expr 7 + 14
expr 7 * 8

2. Comparing two expressions

x=10
y=20
res=`expr $x = $y`
echo $res

3. Match the numbers of characters in two strings

expr alphabet : alpha

4. Find the modulus value

expr 20 % 30  

5. Extract the substring

a=HelloWorld
b=`expr substr $a 6 10`
echo $b

Additional Flags and their Functionalities

Flag	Description
--version	output version information and exit
--help	Display this help and exit

For more details: Expr on Wikipedia