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Variables and Constants

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In this section, we will explore how to use variables and constants in Bash scripting. Understanding these concepts is crucial for writing effective and efficient scripts.

What are Variables?

Variables in Bash are used to store data that can be referenced and manipulated throughout your script. They are essential for creating dynamic and flexible scripts.

Declaring Variables

To declare a variable in Bash, you simply assign a value to a name without any spaces around the = sign.

#!/bin/bash

# Declaring a variable
greeting="Hello, World!"

# Using the variable
echo $greeting

Explanation

  • greeting="Hello, World!": This line declares a variable named greeting and assigns it the value "Hello, World!".
  • echo $greeting: This line prints the value of the greeting variable to the terminal.

Variable Naming Rules

  1. Variable names must start with a letter or an underscore (_).
  2. Variable names can contain letters, numbers, and underscores.
  3. Variable names are case-sensitive (VAR and var are different).

Example

#!/bin/bash

# Valid variable names
name="Alice"
_age=25
city="New York"

# Invalid variable names
# 1name="Bob"  # Starts with a number
# user-name="Charlie"  # Contains a hyphen

echo "Name: $name"
echo "Age: $_age"
echo "City: $city"

Constants

In Bash, there is no built-in way to declare a constant (a variable whose value cannot be changed). However, you can achieve a similar effect by using the readonly command.

Declaring Constants

#!/bin/bash

# Declaring a constant
readonly PI=3.14159

# Trying to change the value of the constant
# PI=3.14  # This will cause an error

echo "The value of PI is $PI"

Explanation

  • readonly PI=3.14159: This line declares a constant named PI and assigns it the value 3.14159. The readonly command ensures that the value of PI cannot be changed.
  • PI=3.14: This line is commented out because attempting to change the value of a constant will result in an error.

Practical Examples

Example 1: Simple Calculator

#!/bin/bash

# Declaring variables
num1=10
num2=5

# Performing arithmetic operations
sum=$((num1 + num2))
difference=$((num1 - num2))
product=$((num1 * num2))
quotient=$((num1 / num2))

# Displaying the results
echo "Sum: $sum"
echo "Difference: $difference"
echo "Product: $product"
echo "Quotient: $quotient"

Explanation

  • num1=10 and num2=5: These lines declare two variables, num1 and num2, and assign them values.
  • sum=$((num1 + num2)): This line calculates the sum of num1 and num2 and assigns the result to the sum variable.
  • The other arithmetic operations follow a similar pattern.

Example 2: Using Constants

#!/bin/bash

# Declaring constants
readonly TAX_RATE=0.07
price=100

# Calculating the total price including tax
total_price=$(echo "$price + ($price * $TAX_RATE)" | bc)

# Displaying the total price
echo "Total Price: $total_price"

Explanation

  • readonly TAX_RATE=0.07: This line declares a constant named TAX_RATE and assigns it the value 0.07.
  • total_price=$(echo "$price + ($price * $TAX_RATE)" | bc): This line calculates the total price including tax using the bc command for floating-point arithmetic.

Exercises

Exercise 1: Temperature Conversion

Write a script that converts a temperature from Celsius to Fahrenheit. The formula for conversion is F = C * 9/5 + 32.

Solution

#!/bin/bash

# Declaring a variable for Celsius temperature
celsius=25

# Converting Celsius to Fahrenheit
fahrenheit=$((celsius * 9 / 5 + 32))

# Displaying the result
echo "$celsius°C is equal to $fahrenheit°F"

Exercise 2: Area of a Circle

Write a script that calculates the area of a circle given its radius. Use the constant PI=3.14159.

Solution

#!/bin/bash

# Declaring a constant for PI
readonly PI=3.14159

# Declaring a variable for the radius
radius=5

# Calculating the area of the circle
area=$(echo "$PI * $radius * $radius" | bc)

# Displaying the result
echo "The area of a circle with radius $radius is $area"

Common Mistakes and Tips

  • Forgetting the $ sign: Always use the $ sign to reference a variable's value.
  • Using spaces around =: Do not use spaces around the = sign when assigning values to variables.
  • Changing constants: Remember that constants declared with readonly cannot be changed.

Conclusion

In this section, we covered the basics of variables and constants in Bash. You learned how to declare and use variables, the rules for naming them, and how to create constants using the readonly command. We also provided practical examples and exercises to reinforce these concepts. In the next section, we will delve into basic operators in Bash scripting.

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