In Bash, the `expr`

command is used to evaluate arithmetic expressions and perform string operations. It is another way to handle arithmetic calculations and string manipulations within scripts. Here’s an overview of the `expr`

command and its usage:

The basic syntax of the `expr`

command for arithmetic expressions is as follows:

```
expr <expression>
```

The `<expression>`

represents the arithmetic expression to be evaluated. The `expr`

command evaluates the expression and prints the result to the standard output.

Here’s an example that demonstrates the usage of the `expr`

command for arithmetic calculations:

```
#!/bin/bash
result=$(expr 2 + 3)
echo "Result: $result"
a=5
b=2
c=$(expr $a \* $b)
echo "c: $c"
```

In this example, the `expr`

command is used to evaluate arithmetic expressions and store the results in variables. The first `expr`

command calculates the sum of 2 and 3 and stores it in the `result`

variable. The second `expr`

command multiplies the values of `a`

and `b`

and stores the result in the `c`

variable.

When you run this script, it will output:

```
Result: 5
c: 10
```

Apart from arithmetic calculations, the `expr`

command can also perform string operations such as pattern matching, length calculation, and substring extraction. Here’s an example that showcases the usage of the `expr`

command for string operations:

```
#!/bin/bash
string="Hello, World!"
length=$(expr length "$string")
echo "Length: $length"
substring=$(expr substr "$string" 8 5)
echo "Substring: $substring"
```

In this example, the first `expr`

command calculates the length of the string and stores it in the `length`

variable. The second `expr`

command extracts a substring starting from position 8 and of length 5 from the original string, storing it in the `substring`

variable.

When you run this script, it will output:

```
Length: 13
Substring: World
```

The `expr`

command provides a way to evaluate arithmetic expressions and perform string operations within Bash scripts. It can be useful for simple calculations and manipulations, especially when used in conjunction with variables and command substitution.