Introduction

This tutorial aims to provide a comprehensive guide on how to work with nested loops in shell scripting. Nested loops are a fundamental concept in programming, allowing for complex iteration over multi-dimensional data structures.

By the end of this tutorial, you will:

• Understand the concept of nested loops
• Know how to implement nested loops in shell scripting
• Be able to use nested loops to solve real-world problems

Prerequisites:
- Familiarity with basic shell scripting
- Basic understanding of loop constructs

Step-by-Step Guide

In shell scripts, a loop is a way to repeat a set of commands until a certain condition is met. A nested loop is a loop within a loop. Understanding nested loops is important for dealing with multi-dimensional data structures like arrays and matrices.

The syntax of a nested loop is:

for outerVar in outerSet
do
    for innerVar in innerSet
    do
        command
    done
done

In this structure, the outer loop runs once, and then the inner loop runs completely. The inner loop will run its complete cycle for each iteration of the outer loop.

Code Examples

Let's look at some practical examples:

1. Printing numbers in a matrix format

# Outer loop will run 5 times
for i in {1..5}
do
    # Inner loop will run 5 times
    for j in {1..5}
    do
        echo -n "$i$j "
    done
    echo "" # This will create a new line after each outer loop iteration
done

In this script, the outer loop runs five times, and for each iteration, the inner loop also runs five times. The -n option in the echo command prevents a new line after each echo. The output will be a 5x5 matrix.

1. Finding and printing all files in a directory and its subdirectories

# Outer loop will iterate over all directories
for dir in */
do
    echo "Directory: $dir"
    # Inner loop will iterate over all files in the current directory
    for file in "$dir"*
    do
        echo "File: $file"
    done
done

This script will first iterate over all directories. For each directory, it will then iterate over all files within that directory, printing the directory name and the filename.

Summary

In this tutorial, we've covered nested loops in shell scripting, including their syntax, usage, and practical examples.

Next steps for learning could include exploring other shell scripting concepts, such as functions, conditional statements, and arrays.

For more on shell scripting, check out the Bash Academy.

Practice Exercises

1. Exercise: Write a script that prints a 3x3 matrix with each cell containing its row and column number.

Solution:

```bash
for i in {1..3}
do
    for j in {1..3}
    do
        echo -n "$i$j "
    done
    echo "" 
done
```

This solution is similar to the first example, but modified to print a 3x3 matrix.

1. Exercise: Write a script that iterates over all directories in the current directory and prints the number of files in each directory.

   Solution:

   bash for dir in */ do echo "Directory: $dir" count=0 for file in "$dir"* do let count++ done echo "Number of files: $count" done
   This solution is similar to the second example, but with an added counter for each file in a directory.

Remember, practice is key in mastering any programming concept, so try to solve more problems using nested loops. Happy scripting!