Different Types of Loops in Bash

Bash provides several types of loops that allow you to execute a block of code multiple times. The most commonly used loops in Bash are for, while, and until. Below, we will explore each type of loop in detail, along with sample code.

1. For Loop

The for loop is used to iterate over a list of items or a range of numbers. It is particularly useful when you know the number of iterations in advance.

Basic Syntax:

for variable in list
do
    # commands
done

Example of a For Loop:

#!/bin/bash
# Using a for loop to iterate over a list of fruits
for fruit in apple banana cherry
do
    echo "I like $fruit"
done

In this example:

• The loop iterates over the list of fruits: apple, banana, and cherry.
• For each fruit, it prints a message indicating that the user likes that fruit.

For Loop with a Range:

#!/bin/bash
# Using a for loop with a range
for i in {1..5}
do
    echo "Number $i"
done

This example prints numbers from 1 to 5 using a range.

2. While Loop

The while loop continues to execute as long as a specified condition is true. It is useful when the number of iterations is not known in advance.

Basic Syntax:

while [ condition ]
do
    # commands
done

Example of a While Loop:

#!/bin/bash
# Using a while loop to count down from 5
count=5
while [ $count -gt 0 ]
do
    echo "Countdown: $count"
    ((count--))  # Decrement the count
done

In this example:

• The loop continues as long as count is greater than 0.
• It prints the current count and decrements it by 1 in each iteration.

3. Until Loop

The until loop is similar to the while loop, but it continues to execute as long as the specified condition is false. It is useful for scenarios where you want to wait for a condition to become true.

Basic Syntax:

until [ condition ]
do
    # commands
done

Example of an Until Loop:

#!/bin/bash
# Using an until loop to count up to 5
count=1
until [ $count -gt 5 ]
do
    echo "Count: $count"
    ((count++))  # Increment the count
done

In this example:

• The loop continues until count is greater than 5.
• It prints the current count and increments it by 1 in each iteration.

4. Nested Loops

Bash also allows you to nest loops, meaning you can place one loop inside another. This is useful for working with multi-dimensional data structures.

#!/bin/bash
# Using nested loops to create a multiplication table
for i in {1..3}
do
    for j in {1..3}
    do
        echo "$i * $j = $((i * j))"
    done
done

In this example:

• The outer loop iterates over the numbers 1 to 3.
• The inner loop also iterates over the numbers 1 to 3, creating a multiplication table for the numbers 1 through 3.

Conclusion

Loops are a fundamental part of programming in Bash, allowing you to execute commands repeatedly based on conditions or lists. Understanding how to use for, while, and until loops will enable you to write more efficient and powerful scripts. By mastering these loop constructs, you can handle a wide range of tasks in your Bash scripts effectively.