Conditional Statements and Loop in Bash Scripting for BCA | MCA | BSC-IT

Mastering conditional statements and loops in Bash scripting is essential for students pursuing BCA, MCA, or BSc-IT. These fundamental concepts enable you to create powerful and efficient scripts that can automate tasks, manage system operations, and process data effectively. In this guide, we will explore the basics of conditional statements and loops in Bash scripting, providing you with the knowledge and skills needed to tackle complex problems and enhance your programming capabilities. Whether you are a beginner or looking to strengthen your scripting proficiency, this comprehensive overview will serve as a valuable resource in your academic and professional journey.

Table of Contents ▼

1. Conditional Statements
   1. If-Else Statements
   2. Case Statements
2. Loops
   1. For Loops
   2. While Loops
   3. Until Loops
   4. Summary

Conditional Statements and Loops in Bash Scripting

Conditional Statements in Bash Scripting

Conditional statements in Bash are used to make decisions in your scripts. They allow you to execute certain pieces of code only if specific conditions are met. One of the most common conditional statements  are 

1. If-Else Statement
2. Case Statements

If-Else Statements

Conditional statements allow you to make decisions in your scripts based on certain conditions. The most common conditional statement in Bash is the if-else statement. It works by checking if a condition is true and then performing an action based on whether it is true or false.

Basic Syntax

The basic syntax of a if-else statement in Bash is as follows:

if [ condition ]; then
  # Code to execute if the condition is true
else
  # Code to execute if the condition is false
fi

• if [ condition ]: Checks if the condition is true.
• then: If the condition is true, execute the code after this keyword.
• else: If the condition is false, execute the code after this keyword.
• fi: Ends the if-else statement.

Example:

Let's say we want to check if a number is greater than 10. If it is, we print "The number is greater than 10". Otherwise, we print "The number is not greater than 10".

#!/bin/bash

read -p "Enter a number: " number

if [ $number -gt 10 ]; then
  echo "The number is greater than 10"
else
  echo "The number is not greater than 10"
fi

• read -p "Enter a number: " number: Prompts the user to enter a number.
• [ $number -gt 10 ]: Checks if the number is greater than 10.
• -gt: Stands for "greater than".

Using elif

Sometimes, you might need to check multiple conditions. This is where elif (short for "else if") comes in handy. The elif statement allows you to check another condition if the previous if condition was false.

Basic Syntax:

if [ condition1 ]; then
  # Code to execute if condition1 is true
elif [ condition2 ]; then
  # Code to execute if condition2 is true
else
  # Code to execute if none of the conditions are true
fi

• elif [ condition2 ]: Checks the second condition if the first condition is false.

Example:

#!/bin/bash

read -p "Enter a number: " number

if [ $number -gt 10 ]; then
  echo "The number is greater than 10"
elif [ $number -eq 10 ]; then
  echo "The number is equal to 10"
else
  echo "The number is less than 10"
fi

Let's extend our previous example to check if the number is greater than 10, equal to 10, or less than 10.

• [ $number -eq 10 ]: Checks if the number is equal to 10.
• -eq: Stands for "equal to".

In this script:

• If the number is greater than 10, it prints "The number is greater than 10".
• If the number is equal to 10, it prints "The number is equal to 10".
• If neither condition is true, it prints "The number is less than 10".

Case Statements in Bash Scripting: Syntax and Examples

What is a Case Statement?

A case statement in Bash is like a switch statement in other programming languages. It allows you to execute different blocks of code based on the value of a variable or an expression. It's a way to simplify complex if-else statements.

Syntax of a Case Statement

Here's the basic syntax of a case statement in Bash:

case variable in
  pattern1)
    # code to execute if variable matches pattern1
    ;;
  pattern2)
    # code to execute if variable matches pattern2
    ;;
  *)
    # code to execute if variable doesn't match any pattern
    ;;
esac

• case variable in - This starts the case statement and checks the value of the variable.
• pattern1) - This checks if the variable matches pattern1.
• ;; - This marks the end of the code block for pattern1.
• *) - This is a default case that runs if none of the patterns match.
• esac - This ends the case statement (it's "case" spelled backward).

Example: Simple Menu

Let's look at a simple example. Suppose you want to create a menu script that performs different actions based on user input.

#!/bin/bash

echo "Choose an option:"
echo "1) Display date and time"
echo "2) List files in current directory"
echo "3) Show current directory"
echo "4) Exit"

read -p "Enter your choice [1-4]: " choice

case $choice in
  1)
    echo "The current date and time is: $(date)"
    ;;
  2)
    echo "Files in current directory are: "
    ls
    ;;
  3)
    echo "You are in directory: $(pwd)"
    ;;
  4)
    echo "Exiting the script. Goodbye!"
    exit 0
    ;;
  *)
    echo "Invalid choice, please enter a number between 1 and 4."
    ;;
esac

Breakdown of the Example:

1. Prompt the User: The script displays a menu with four options.
2. Read User Input: The read command takes the user's choice and stores it in the variable choice.
3. Case Statement:
• case $choice in - This checks the value of choice.
• 1) - If choice is 1, it displays the current date and time using the date command.
• 2) - If choice is 2, it lists the files in the current directory using the ls command.
• 3) - If choice is 3, it shows the current directory using the pwd command.
• 4) - If choice is 4, it exits the script with a goodbye message.
• *) - If choice is anything other than 1, 2, 3, or 4, it displays an error message.

Practical Tips:

• Patterns Can Be More Complex: Patterns can include wildcards (*, ?) to match more complex conditions.
• Multiple Patterns: You can have multiple patterns leading to the same code block by separating them with | (e.g., pattern1|pattern2)).

Example: Matching File Extensions

Here's another example that prints a message based on the file extension:

#!/bin/bash

read -p "Enter a filename: " filename

case $filename in
  *.jpg|*.jpeg)
    echo "This is a JPEG image file."
    ;;
  *.png)
    echo "This is a PNG image file."
    ;;
  *.txt)
    echo "This is a text file."
    ;;
  *)
    echo "File type is unknown."
    ;;
esac

Breakdown of the Example:

1. Prompt the User: Asks the user to enter a filename.
2. Case Statement:
   • case $filename in - Checks the value of filename.
   • *.jpg|*.jpeg) - If the filename ends with .jpg or .jpeg, it prints a message saying it's a JPEG image file.
   • *.png) - If the filename ends with .png, it prints a message saying it's a PNG image file.
   • *.txt) - If the filename ends with .txt, it prints a message saying it's a text file.
   • *) - If the filename doesn't match any of the patterns, it prints a message saying the file type is unknown.

Loop in Bash Scripting

Loops are a fundamental part of Bash scripting, allowing you to execute a set of commands repeatedly. There are three main types of loops in Bash: for loops, while loops, and until loops. Let's break them down with simple examples.

1. For Loops

What They Do: For loops are used to iterate over a list of items or a range of numbers, executing commands for each item or number.

Basic Syntax:

for item in list
do
  commands
done

Example:

#!/bin/bash
# A simple for loop to print numbers from 1 to 5

for number in 1 2 3 4 5
do
  echo "Number: $number"
done

Iterating Over a Range:

#!/bin/bash
# A for loop to print numbers from 1 to 5 using a range

for number in {1..5}
do
  echo "Number: $number"
done

2. While Loops

What They Do: While loops execute a set of commands as long as a given condition is true.

Basic Syntax:

while condition
do
  commands
done

Example:

#!/bin/bash
# A while loop to print numbers from 1 to 5

number=1
while [ $number -le 5 ]
do
  echo "Number: $number"
  number=$((number + 1))
done

3. Until Loops

What They Do: Until loops are similar to while loops, but they execute the commands as long as the given condition is false.

Basic Syntax:

until condition
do
  commands
done

Example:

#!/bin/bash
# An until loop to print numbers from 1 to 5

number=1
until [ $number -gt 5 ]
do
  echo "Number: $number"
  number=$((number + 1))
done

Detailed Explanation  of Loop with Simple Examples

1. For Loops: Iterating Over Lists and Ranges

Lists:

• Imagine you have a list of fruits and you want to print each fruit's name.

Example:

#!/bin/bash
# A for loop to print names of fruits

for fruit in apple banana cherry
do
  echo "Fruit: $fruit"
done

Explanation: This loop goes through each item in the list (apple, banana, cherry) and prints it.

Ranges:

• Suppose you want to print numbers from 1 to 5.

Example:

#!/bin/bash
# A for loop to print numbers from 1 to 5 using a range

for number in {1..5}
do
  echo "Number: $number"
done

Explanation: This loop is used {1..5} to create a range of numbers from 1 to 5 and prints each number.

2. While Loops: Basic Syntax and Examples

Counting Up:

• You want to print numbers from 1 to 5 using a while loop.

Example:

#!/bin/bash
# A while loop to print numbers from 1 to 5

number=1
while [ $number -le 5 ]
do
  echo "Number: $number"
  number=$((number + 1))
done

Explanation: The loop runs as long as the condition [ $number -le 5 ] is true. It starts with number 1 and adds 1 each time until it reaches 5.

Counting Down:

• You want to print numbers from 5 to 1 using a while loop.

Example:

#!/bin/bash
# A while loop to print numbers from 5 to 1

number=5
while [ $number -ge 1 ]
do
  echo "Number: $number"
  number=$((number - 1))
done

Explanation: The loop runs as long as the condition [ $number -ge 1 ] is true. It starts with number 5 and subtracts 1 each time until it reaches 1.

3. Until Loops: Syntax and Use Cases

Basic Example:

#!/bin/bash
# An until loop to print numbers from 1 to 5

number=1
until [ $number -gt 5 ]
do
  echo "Number: $number"
  number=$((number + 1))
done

• You want to print numbers from 1 to 5 using an until loop.

Example:

Explanation: The loop runs until the condition [ $number -gt 5 ] becomes true. It starts with the number 1 and adds 1 each time until the number is greater than 5.

Practical Use Case:

• Suppose you want to keep prompting a user for a correct password until they get it right.

Example:

#!/bin/bash
# An until loop to prompt for a correct password

correct_password="secret"
input_password=""

until [ "$input_password" == "$correct_password" ]
do
  read -p "Enter the password: " input_password
done

echo "Correct password entered!"

Explanation: The loop continues to prompt the user for a password until the entered password matches the correct one.

Summary

• For Loops: Best for iterating over a list of items or a range of numbers.
• While Loops: Best for running commands as long as a condition is true.
• Until Loops: Best for running commands until a condition becomes true.

Using these loops, you can automate repetitive tasks and make your Bash scripts more powerful and flexible.