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+# 📘 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**.
+
+---
+
+## 🎯 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
+
+---
+
+## 🛠 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
+
+---
+
+### 🧪 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
+
+---
+
+### 🧪 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
+
+---
+
+### 🧪 Exercise 4 — Validate Multiple Usernames
+
+**Input:**
+
+```bash
+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
+
+---
+
+### 🧪 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)
+```
+
+---
+
+## 🔐 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**.
+
+---
+
+## ✅ 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)
+
+---
+
+## 🔜 Next Step
+
+After Day 5, Week 1 concludes.
+
+Upcoming:
+
+* **Week 2:** Data Structures (arrays, maps, sets)
+* Stronger algorithm focus
+* More security-oriented scripting examples
+
+---
+
+📌 *This README serves as a reference and checklist for Week 1 Day 5 Bash exercises.*