**In Previous Years Questions**

The expr command is a built-in command in Linux that evaluates arithmetic expressions and string comparisons. It is a simple and versatile tool for performing calculations and comparisons on the command line.

## Some of the uses of expr command:

**Basic arithmetic operations**: Addition, subtraction, multiplication, division, and modulus.**Comparisons**: Equality, inequality, greater than, less than, greater than or equal to, less than or equal to.**String operations:**Length of a string, extracting a substring, comparing strings.**Combining expressions**: Use parentheses to combine multiple expressions.

## Example 1: Adding two numbers:

`expr 10 + 5`

This command will output the sum of 10 and 5, which is 15.

## Example 2: Subtracting two numbers:

`expr 20 - 7`

This command will output the difference of 20 and 7, which is 13.

## Example 3: Multiplying two numbers:

`expr 3 * 4`

This command will output the product of 3 and 4, which is 12.

## Example 4: Dividing two numbers:

`expr 12 / 3`

This command will output the quotient of 12 and 3, which is 4.

## Example 5: Finding the length of a string:

`expr length "Hello World"`

This command will output the length of the string “Hello World”, which is 11.

## Example 6: Extracting a substring:

`expr substr "This is a string" 5 4`

This command will output the substring starting at the 5th character (index 4) of the string “This is a string”, with a length of 4 characters. The output will be “is a”.

## Example 7: Comparing two strings:

`expr "apple" == "banana"`

This command will compare the strings “apple” and “banana”. Since they are not equal, the output will be 0.

## Example 8: Combining expressions:

`expr \( 10 + 5\ )\ * 2`

This command will first evaluate the expression inside the parentheses (10 + 5), then multiply the result by 2. The output will be 30.