Tuesday, April 21, 2026

Cohort 3 | Project CloudIgnite Topics: System Logs, Bash Scripting, Intro to Networking & OSI Model Duration: ~3 hours

📌 CLF-C02 Relevance sections are marked throughout. This lecture covers foundational Linux sysadmin skills and networking concepts that underpin the AWS Certified Cloud Practitioner exam.

Key Takeaways

• System logs stored in /var/log/; severity levels from Emergency (0) to Debug (7)
• Key log files: syslog, secure (Red Hat), auth.log (Ubuntu), kern.log, httpd/
• Bash challenge: Auto-incrementing file creator using wc -l and $(ls | wc -l)
• OSI Model: 7 layers — focus on Layer 7 (Application), Layer 4 (Transport), Layer 3 (Network)
• Network basics: Node, client, server, LAN, switch, router, modem, MAC address
• Submarine cables carry data as light; IoT and satellite as alternatives

Table of Contents

1. System Logs
2. Bash Scripting — Challenge Lab Walkthrough
3. Introduction to Networking
4. OSI Model
5. CLF-C02 Exam Relevance Summary

1. System Logs

Why Logs Matter

Logs are the primary tool for understanding what is happening inside a system. Key use cases:

• Troubleshooting — server unreachable, app crashes, DB connection failures, high CPU
• Security audits — tracking who did what and when
• Compliance — many industries require logging all application activity
• Event tracking — user logins, logouts, and system events

Log Severity Levels (Lowest → Highest Priority)

⚠️ If a log shows Emergency, Alert, Critical, or Error — it requires immediate attention. Warning and below are less urgent.

Where Logs Are Stored (Linux)

All logs are stored in /var/log/

💡 Amazon Linux is derived from Red Hat and uses yum/dnf as the package manager. Debian/Ubuntu uses apt/dpkg.

Useful Log Commands

# View full log file
cat /var/log/syslog

# View last N lines (most recent)
tail -n 50 /var/log/syslog

# View first N lines
head -n 50 /var/log/syslog

# Scroll through large files
less /var/log/syslog

# Search for errors in a log
grep "error" /var/log/syslog
grep -i "critical\|error\|alert\|emergency" /var/log/syslog

# Write matching lines to a new file for investigation
grep "error" /var/log/syslog > errors_only.txt

# Check last login per user
lastlog

# Check sudo usage
grep "sudo" /var/log/secure

💡 Use grep or awk for quick keyword searches — log files can be thousands of lines long. For servers without a GUI, these are your only options.

Log Rotation

• Logs can grow very rapidly, especially on high-traffic servers (every HTTP request is logged).
• Log rotation = automated cleanup process to keep log sizes manageable.
• You can write a cron job (bash script scheduled to run daily/weekly) to delete logs older than a set period (e.g., 7 days).
• Logs do not delete themselves automatically by default on most systems.

Security Investigation with Logs

• If an EC2 instance is compromised, check login logs (lastlog, /var/log/secure) to trace attacker activity.
• Use su <username> to switch users; sudo passwd <username> to set a password.
• Root access is needed to read sensitive logs — regular users cannot access root's home or some system log folders.

2. Bash Scripting — Challenge Lab Walkthrough

Challenge: Auto-Incrementing File Creator

Goal: Write a bash script that:

• Creates 25 new empty files every time it runs
• File names follow the pattern <yourname><number> (e.g., Ahmad1, Ahmad2, …)
• Numbering continues from where it left off — running twice creates files 1–25, then 26–50, etc.

Key Concepts Used

Counting Existing Files

counter=$(ls <yourname>* 2>/dev/null | wc -l)
# wc -l  = counts lines (= number of files)
# 2>/dev/null  = suppresses "no such file" error messages

For Loop Approach

name="Ahmad"
for i in {1..25}; do
  touch "${name}${i}"
done

While Loop Approach

name="Ahmad"
counter=$(ls ${name}* 2>/dev/null | wc -l)
last_count=$((counter + 25))

while [ $counter -lt $last_count ]; do
  counter=$((counter + 1))
  touch "${name}${counter}"
done

Important Syntax Notes

Running the Script

chmod +x create_files.sh   # give execute permission
./create_files.sh           # run the script
ls -v                       # list files in natural sort order

Tips & Lessons

• Always review AI-generated code before running it, especially for system admin tasks — AI can suggest destructive operations (e.g., dropping a database).
• Test edge cases: what happens if you delete some files mid-sequence?
• For more practice: try CMD Challenge or LeetCode's bash challenges.

3. Introduction to Networking

What Is the Internet?

• Internet = interconnected network of networks
• Brief history:
  • 1960s — Packet switching theory developed
  • 1970s — TCP (Transmission Control Protocol) developed
  • 1980s — Dial-up introduced; email becomes possible
  • 1990s — WWW (World Wide Web) introduced — beginning of websites
  • 2000s+ — Explosion of devices and internet adoption

Core Networking Concepts

Physical Infrastructure

• Computers on the same LAN communicate via a switch (often built into your router).
• Data travels between continents via submarine cables — thin glass fibres carrying light signals.
• Optical fibre transmits data as light (different frequencies/colours).
• Alternatives: satellite (e.g., Tesla Starlink) uses satellite links instead of cables.
• Data for computers is ultimately just 0s and 1s transmitted as light or electrical signals.

Simple Network Diagram

[Computer A] ─┐
[Computer B] ─┤── [Switch] ── [Router] ── Internet
[Computer C] ─┘

MAC Address

• Every network device has a MAC (Media Access Control) address — a unique hardware identifier.
• Used to identify devices within a local network.

4. OSI Model

What Is the OSI Model?

The OSI (Open Systems Interconnection) model is a conceptual framework that standardises how data is sent across a network. It has 7 layers.

📬 Analogy — Sending a Letter: You write a letter (application), choose a language (presentation), drop it at the post office (session), the postal service routes it (transport/network), a courier carries it (data link/physical). Your recipient reverses the process to read it.

The 7 Layers

Most Important Layers to Know

• Layer 7 (Application) — HTTP/HTTPS lives here. HTTPS = HTTP + TLS encryption (the "S" = Secure).
• Layer 4 (Transport) — TCP establishes logical connections between two devices.
• Layer 3 (Network) — Handles routing and IP addresses; decides the path data takes.
• Layer 2 (Data Link) — Uses MAC addresses to identify devices on a local network.

How Data Is Broken Down

When you send a large file or email, it is not sent as one chunk:

1. Data is split into packets (Layer 3)
2. Packets travel independently across the network
3. Packets are reassembled in the correct order at the destination

Each packet contains: packet number, protocol, encryption type, bit order (endianness), and flags.

💡 Next class will continue with more OSI model detail and networking components (switches, routers in depth).

CLF-C02 Exam Relevance Summary

The AWS Certified Cloud Practitioner (CLF-C02) exam tests foundational cloud and IT knowledge. Here is how today's topics map to exam domains:

✅ Key CLF-C02 AWS Services to connect these concepts to:

• Amazon CloudWatch — log collection, monitoring, and alerting (maps to log management)
• AWS CloudTrail — API and user activity auditing (maps to security/auth logs)
• Amazon VPC — virtual networking (maps to OSI model, routing, subnets)
• AWS IAM — identity and access management (maps to authentication logging)
• Amazon EC2 — Linux instances where all these log concepts apply directly

Notes compiled from April 21, 2026 lecture. Next session: OSI Model deep-dive + Networking Components.