Control Structures: Loops – in Shell Scripting

What You’ll Learn 📚

• Understanding the purpose and types of loops in shell scripting
• How to implement and use different loops effectively
• Common pitfalls and how to avoid them
• Practical examples to reinforce learning

Introduction to Loops

Loops are a fundamental concept in programming that allow you to repeat a set of commands multiple times. They are incredibly useful for automating repetitive tasks, processing data, and much more. In shell scripting, loops help you efficiently manage tasks without writing redundant code.

Key Terminology

• Loop: A control structure that repeats a block of code.
• Iteration: Each time a loop executes its block of code.
• Condition: A statement that determines whether the loop continues or stops.

Simple Example: The ‘for’ Loop

#!/bin/bash
# A simple for loop example
for i in 1 2 3 4 5
do
  echo "Iteration number: $i"
done

Progressively Complex Examples

Example 1: Using a ‘while’ Loop

#!/bin/bash
# A simple while loop example
count=1
while [ $count -le 5 ]
do
  echo "Count is: $count"
  ((count++))
done

Example 2: Nested Loops

#!/bin/bash
# Nested loops example
for i in 1 2 3
do
  for j in a b
  do
    echo "Outer loop: $i, Inner loop: $j"
  done
done

Example 3: Looping Through Files

#!/bin/bash
# Loop through files in a directory
for file in /path/to/directory/*
do
  echo "Processing $file"
done

Common Questions and Answers

1. What is the difference between ‘for’ and ‘while’ loops?

   ‘For’ loops are used when you know the number of iterations in advance. ‘While’ loops are used when the number of iterations depends on a condition.

2. How do I break out of a loop early?

   Use the break statement to exit a loop before it completes all iterations.

3. Can I skip an iteration?

   Yes, use the continue statement to skip the current iteration and move to the next one.

4. Why is my loop running indefinitely?

   Check your loop condition to ensure it will eventually become false. Infinite loops occur when the condition is always true.

5. How do I loop through command output?

   Use backticks or $(command) to capture command output and loop through it.

Troubleshooting Common Issues

Ensure your loop conditions are correct to avoid infinite loops!

• Infinite Loops: Double-check your loop conditions and increment statements.
• Syntax Errors: Ensure all keywords like do and done are correctly placed.
• File Not Found: Verify file paths and permissions when looping through files.

Practice Exercises

1. Write a script using a for loop to print numbers 1 to 10.
2. Create a while loop that prints even numbers between 1 and 20.
3. Modify the nested loop example to include a third loop.

Remember, practice makes perfect! Keep experimenting with loops to solidify your understanding. 💪

Additional Resources

• Bash Looping Constructs
• Shell Scripting Loops