# 📘 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:**

```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

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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.*