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DevOps Learning Repo

This repository is a hands-on collection of small examples and notes you can use to upskill your DevOps knowledge. The content is organized by folder name; most folders contain ready-to-run examples or short documentation. Use this README as a quick map and a suggested learning path.

How to use this repo

  1. Clone the repo locally and open it in your editor of choice.
  2. Read the Readme.md inside each folder when present (some folders have a single README). If a folder has no README but contains files, open those files — their names usually describe the content.
  3. Try the examples locally (many are small Docker/Python demos). Run one example end-to-end, then read and tweak the code or Dockerfile to learn how changes affect behavior and the image.
  4. Repeat: pick a different folder and try a new exercise. Track what you learned in a personal notes file or a PR-style change in the repo.

Folder overview

  • Docker/ — Practical Docker examples and notes. This is the most content-rich area right now.

    • Portforwarding.md — A focused document that explains port forwarding with Docker containers (read this to understand host<->container networking basics).
    • subfolders include container examples such as bind_mount, Container_with_localDB, demo, interactive-mode, multiple_container, userinfo, and working_with_api — each contains a Dockerfile and small demo scripts like myapp.py or sql_demo.py.
    • Suggested exercises: build and run each Docker example, inspect image layers, experiment with --no-cache-dir for pip installs, and try Docker BuildKit cache mounts for faster iterative builds.

  • Git&github/ — A folder with Readme.md containing Git and GitHub notes/exercises. Start here if you want to strengthen your source-control workflows (branches, PRs, rebasing, tags, hooks).

  • yaml/ — Various YAML and related config examples. There is no Readme.md here yet, but the filenames are descriptive:

    • datatypes.yml, advanceDatatype.yml, hello.yaml, school.yml and also school.json / school.xml for format comparison.
    • Suggested exercises: validate YAML with yamllint/yq, convert JSON↔YAML, write a small Kubernetes manifest based on these examples, and try templating with Helm or Kustomize.

Quick learning path (suggested)

  1. Git fundamentals: open Git&github/Readme.md and follow the exercises there. Practice branching, committing, pushing, and making PRs.
  2. Docker basics: read Docker/Portforwarding.md then build and run Docker/demo and Docker/userinfo examples to get comfortable with Dockerfiles, volumes, and ports.
  3. Intermediate Docker: try Container_with_localDB and multiple_container to learn about multi-container setups, bind mounts, and networking between containers.
  4. Configuration and manifests: use yaml/ to practice writing and validating configuration files and transform them into simple Kubernetes manifests.
  5. Automation & CI: once comfortable, add a GitHub Actions workflow to build and test one Docker example on push.

Commands to try (zsh / Linux)

Open a terminal and run these from the repo root:

# list top-level folders
ls -la

# explore Docker examples
cd Docker
ls -la

# build a specific example (adjust the path to the folder you want)
docker build -f Container_with_localDB/Dockerfile -t my-sql-demo ./Container_with_localDB

# run the image interactively (if the demo supports it)
docker run --rm -it my-sql-demo

# validate a YAML file (install yamllint or yq first)
yamllint yaml/hello.yaml

Note: some examples use Python. If a Dockerfile installs Python packages, you may see --no-cache-dir used with pip to avoid leaving pip's download cache in the final image (this keeps images smaller).

Prerequisites

  • Docker installed and running
  • (Optional) Python 3.11+ for running local scripts outside containers
  • (Optional) yamllint / yq if you want to validate and manipulate YAML locally

Next steps and ideas

  • Add Readme.md files for the yaml/ folder and for other Docker subfolders that don't have one, describing how to run each demo.
  • Add simple GitHub Actions workflows that build Docker images for Docker/demo and run any included tests.
  • Convert a few yaml/ examples into Kubernetes manifests and add kubectl --dry-run=client -f checks.

If you'd like, I can:

  • create missing Readme.md files inside yaml/ and other folders with run instructions, or
  • add a minimal GitHub Actions workflow that builds one of the Docker examples.

Tell me which follow-up you'd prefer and I'll implement it.

Port managing(in linux): Once you understand ports properly, debugging servers becomes 10x easier.

Let’s make this crystal clear.

🧠 What Is A Port?

Think of your system like:

IP Address = Building Ports = Room numbers

Example:

localhost:3000

means:

your computer → room 3000

A process/app binds itself to a port.

🚨 Why Port Conflicts Happen

Suppose:

Node app uses 3000

Then another app also tries:

listen on 3000

Linux says:

Port already in use

because:

only ONE process can bind to a port at a time.

🔥 MOST IMPORTANT COMMANDS

These are the REAL commands you must know.

1️⃣ See Which Process Uses A Port Modern command: sudo lsof -i :3000

Example output:

node 12345 user 23u IPv4 ...

Meaning:

Node app PID = 12345 using port 3000 2️⃣ Kill Process Using Port kill -9 12345

Now port becomes free.

🚀 Faster One-Liner sudo kill -9 $(sudo lsof -t -i:3000)

Kills whatever uses port 3000.

VERY useful.

3️⃣ See All Listening Ports sudo ss -tulnp

Modern replacement for netstat.

Shows:

TCP UDP listening ports process names 4️⃣ Filter Specific Port sudo ss -tulnp | grep 8080

Useful for:

Jenkins Node Docker nginx 🧠 Common Dev Ports Service Port React/Vite 5173 Node/Express 3000 MongoDB 27017 MySQL 3306 Jenkins 8080 nginx 80 HTTPS 443 SSH 22 Docker Registry 5000 🚀 Changing Ports Node/Express app.listen(5000) React/Vite npm run dev -- --port 4000 Jenkins

Edit:

/etc/default/jenkins

Find:

HTTP_PORT=8080

Change to:

HTTP_PORT=9090

Restart:

sudo systemctl restart jenkins Docker Port Mapping

VERY important.

Example:

docker run -p 8081:8080 image

Meaning:

Host Container 8081 8080

So browser uses:

localhost:8081

while app inside container still uses:

8080 🚨 VERY Common Problem

You close terminal but app still runs.

Example:

Node server still alive Vite still alive Docker container still alive

Then next run:

EADDRINUSE

means:

address already in use.

🔥 How To Debug Properly Step 1

Check port:

sudo lsof -i :3000 Step 2

See process:

PID Step 3

Kill:

kill -9 PID Step 4

Restart app.

🧠 Docker Port Understanding Container Port vs Host Port

Example:

docker run -p 9000:8080 jenkins

means:

Machine Container localhost:9000 container:8080

Browser:

localhost:9000

Container internally:

8080

VERY important concept.

🔥 How To See Docker Containers Using Ports docker ps

Example:

0.0.0.0:8080->8080/tcp

means:

host 8080 mapped to container 8080 🚀 Stop Docker Container docker stop container_id

or:

docker stop container_name 🧠 Best Practices Development

Use:

3000 5173 8080 Multiple Apps

Use:

3001 3002 8081 8082 Production

Usually:

nginx → 80/443 backend internal ports hidden 🚀 MOST IMPORTANT DEVOPS UNDERSTANDING

Port conflict means:

two processes trying to occupy same network socket.

That’s all.

🏆 Commands You ABSOLUTELY Must Remember Find process using port sudo lsof -i :PORT Kill process kill -9 PID See listening ports sudo ss -tulnp Docker running containers docker ps