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Loops in Bash

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Loops are fundamental constructs in programming that allow you to execute a block of code repeatedly. In Bash, loops are essential for automating repetitive tasks. This section will cover the different types of loops available in Bash and provide practical examples and exercises to help you understand and use them effectively.

Types of Loops in Bash

  1. for Loop: Iterates over a list of items.
  2. while Loop: Repeats as long as a condition is true.
  3. until Loop: Repeats until a condition becomes true.
  4. select Loop: Provides a menu for user input.

for Loop

The for loop iterates over a list of items and executes a block of code for each item.

Syntax

for variable in list
do
    commands
done

Example

#!/bin/bash

# A simple for loop
for i in 1 2 3 4 5
do
    echo "Iteration number $i"
done

Explanation

  • for i in 1 2 3 4 5: The loop will iterate over the numbers 1 to 5.
  • do ... done: The block of code between do and done will be executed for each value of i.
  • echo "Iteration number $i": Prints the current iteration number.

while Loop

The while loop repeats a block of code as long as a specified condition is true.

Syntax

while [ condition ]
do
    commands
done

Example

#!/bin/bash

# A simple while loop
counter=1
while [ $counter -le 5 ]
do
    echo "Counter is $counter"
    ((counter++))
done

Explanation

  • counter=1: Initializes the counter variable.
  • while [ $counter -le 5 ]: The loop will continue as long as counter is less than or equal to 5.
  • ((counter++)): Increments the counter by 1 in each iteration.

until Loop

The until loop is similar to the while loop but repeats until a specified condition becomes true.

Syntax

until [ condition ]
do
    commands
done

Example

#!/bin/bash

# A simple until loop
counter=1
until [ $counter -gt 5 ]
do
    echo "Counter is $counter"
    ((counter++))
done

Explanation

  • until [ $counter -gt 5 ]: The loop will continue until counter is greater than 5.
  • The rest of the code is similar to the while loop example.

select Loop

The select loop is used to create a simple menu system.

Syntax

select variable in list
do
    commands
done

Example

#!/bin/bash

# A simple select loop
PS3="Please select an option: "
select option in "Option 1" "Option 2" "Option 3" "Quit"
do
    case $option in
        "Option 1")
            echo "You selected Option 1"
            ;;
        "Option 2")
            echo "You selected Option 2"
            ;;
        "Option 3")
            echo "You selected Option 3"
            ;;
        "Quit")
            break
            ;;
        *)
            echo "Invalid option"
            ;;
    esac
done

Explanation

  • PS3="Please select an option: ": Sets the prompt for the select menu.
  • select option in "Option 1" "Option 2" "Option 3" "Quit": Creates a menu with four options.
  • case $option in ... esac: Executes different commands based on the selected option.
  • break: Exits the loop when "Quit" is selected.

Practical Exercises

Exercise 1: Print Even Numbers

Write a script that prints even numbers from 1 to 10 using a for loop.

Solution

#!/bin/bash

for i in {1..10}
do
    if [ $((i % 2)) -eq 0 ]
    then
        echo "$i is even"
    fi
done

Exercise 2: Countdown Timer

Write a script that acts as a countdown timer from 10 to 1 using a while loop.

Solution

#!/bin/bash

counter=10
while [ $counter -gt 0 ]
do
    echo "Countdown: $counter"
    ((counter--))
    sleep 1
done
echo "Time's up!"

Exercise 3: User Menu

Create a script that provides a menu with three options and performs different actions based on the user's choice using a select loop.

Solution

#!/bin/bash

PS3="Choose an option: "
select option in "Say Hello" "Show Date" "Exit"
do
    case $option in
        "Say Hello")
            echo "Hello, User!"
            ;;
        "Show Date")
            date
            ;;
        "Exit")
            break
            ;;
        *)
            echo "Invalid option"
            ;;
    esac
done

Common Mistakes and Tips

  • Forgetting to increment the counter: In while and until loops, ensure you increment or modify the counter variable to avoid infinite loops.
  • Incorrect condition syntax: Ensure the condition in while and until loops is correctly formatted and uses the appropriate comparison operators.
  • Using break and continue: Use break to exit a loop prematurely and continue to skip the current iteration and proceed to the next one.

Conclusion

In this section, you learned about the different types of loops in Bash and how to use them to automate repetitive tasks. You practiced with practical examples and exercises to reinforce your understanding. Loops are powerful tools in scripting, and mastering them will significantly enhance your ability to write efficient and effective Bash scripts. In the next section, we will delve into functions in Bash, which will further expand your scripting capabilities.

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