7-Modern-Languages-in-7-Weeks---Systems--Security---Algorithms-Track/week 1/day 5/readme.md

📘 Week 1 · Day 5 — Bash Exercises

This README documents all exercises for Week 1 Day 5, focused on Bash scripting for Linux system administration and security workflows.

The exercises mirror the logic implemented previously in C, Python, SQL, and PowerShell, reinforcing algorithmic thinking while introducing Unix shell syntax, conditionals, loops, and defensive scripting.

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🎯 Learning Objectives

By completing today’s exercises, you will:

• Use Bash variables and user input (read)
• Apply conditional logic with if / elif / else
• Loop over strings and arrays using for
• Perform character validation using regex
• Implement secure input validation logic
• Analyze algorithm runtime and scalability
• Gain confidence writing Linux automation scripts

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🛠 Exercises Overview

🧪 Exercise 1 — Even / Odd Number Checker

Task:

• Prompt the user for a number
• Determine whether the number is Even or Odd

Concepts practiced:

• Numeric input handling
• Arithmetic expressions: (( ))
• Conditional branching

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🧪 Exercise 2 — Count Letters in a String

Task:

• Prompt the user for a string
• Count letters only (A–Z, a–z)
• Ignore spaces, digits, and special characters

Concepts practiced:

• Looping over string characters
• Regex matching using [[ =~ ]]
• Counter variables

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🧪 Exercise 3 — Username Validator (Single User)

Task: Validate a username using these rules:

• Length ≤ 20 characters
• Starts with a letter
• Contains letters only
• No digits
• No spaces

Output:

• Accepted or Refused

Concepts practiced:

• Defensive input validation
• Early exit with break
• String length checking
• First-character validation

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🧪 Exercise 4 — Validate Multiple Usernames

Input:

users=("admin" "root1" "John_Doe" "Alice" "Bob42")

Task:

• Loop through each username
• Apply the same validation rules as Exercise 3
• Print whether each username is valid or invalid

Concepts practiced:

• Array iteration
• Nested loops
• Reusable validation logic

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🧪 Exercise 5 — Algorithm Analysis (Written)

Task: Explain what happens to execution time if the number of usernames doubles.

Use:

• n = number of usernames
• m = average username length

Expected result:

T(n, m) = n × m
T(2n, m) = 2 × n × m
Big-O: O(n·m)

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🔐 Security & Sysadmin Perspective

These exercises reflect real-world Bash usage in:

• Linux user management
• Input validation scripts
• SSH and access control automation
• Log parsing and monitoring
• Security audits and recon scripts

Strong validation and predictable runtime are critical for secure and reliable system scripts.

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✅ Completion Criteria

Day 5 is complete when:

• Exercises 1–4 run correctly in Bash
• Exercise 5 clearly explains runtime scaling
• Scripts follow safe Bash practices ([[ ]], quoting, defensive checks)

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🔜 Next Step

After Day 5, Week 1 concludes.

Upcoming:

• Week 2: Data Structures (arrays, maps, sets)
• Stronger algorithm focus
• More security-oriented scripting examples

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📌 This README serves as a reference and checklist for Week 1 Day 5 Bash exercises.