Chapter 5: Loops in Bash

Learn about loops in Bash, including for, while, and until loops, to automate repetitive tasks and iterate over data effectively.

In this chapter, we’ll explore the different types of loops available in Bash scripting, allowing you to automate tasks that require repetitive actions or processing data in a structured way.

The `for` Loop

The for loop is used to iterate over a list of items, executing the same code block for each item. This loop is ideal for tasks where the number of iterations is known beforehand.

# Example: Basic for loop
for i in {1..5}; do
    echo "Iteration $i"
done

This example prints "Iteration 1" through "Iteration 5" by iterating over the sequence from 1 to 5.

Iterating Over Arrays with `for`

You can also use the for loop to iterate over arrays, making it useful for handling lists of items in Bash:

# Example: for loop with an array
fruits=("apple" "banana" "cherry")
for fruit in "${fruits[@]}"; do
    echo "Fruit: $fruit"
done

In this example, the loop iterates over each element in the fruits array and prints it.

The `while` Loop

The while loop executes as long as a specified condition is true. This loop is useful when you don’t know the exact number of iterations ahead of time.

# Example: Basic while loop
count=1
while [ $count -le 5 ]; do
    echo "Count: $count"
    count=$((count + 1))
done

Here, the loop continues to execute as long as count is less than or equal to 5, incrementing count with each iteration.

Infinite Loops with `while`

A while loop can create an infinite loop if the condition never becomes false. Use the break statement to exit the loop when needed:

# Example: Infinite loop with break
while true; do
    echo "Press CTRL+C to stop"
    sleep 1
done

The `until` Loop

The until loop is similar to the while loop, but it executes until a specified condition becomes true. This loop is helpful when you want the loop to continue until a certain condition is met.

# Example: Basic until loop
count=1
until [ $count -gt 5 ]; do
    echo "Count: $count"
    count=$((count + 1))
done

This example continues looping until count is greater than 5, incrementing count with each iteration.

Controlling Loop Execution

Bash provides several commands to control loop behavior:

break: Exits the loop immediately, skipping any remaining iterations.
continue: Skips the current iteration and moves to the next one.

# Example: Using break and continue
for i in {1..5}; do
    if [ $i -eq 3 ]; then
        echo "Skipping 3"
        continue
    fi
    if [ $i -eq 5 ]; then
        echo "Stopping at 5"
        break
    fi
    echo "Number: $i"
done

In this example, the loop skips printing "3" and stops altogether when it reaches "5" due to the continue and break statements.

Summary and Next Steps

In this chapter, you learned how to use for, while, and until loops to control repetitive tasks in Bash. In the next chapter, we’ll look at functions in Bash scripting, which allow you to create reusable code blocks for better script organization and efficiency.