Linux interview questions

Dokcer Interview Questions: ============================= 🔥 Advanced Practical Docker Interview Questions 🧱 1. Docker Architecture & Internals Q: Explain how Docker works internally when you run docker run nginx. 👉 Expected: Docker CLI → Docker Daemon → containerd → runc Image pull from registry Creation of container using namespaces & cgroups 🐳 2. Write a Dockerfile (Real Scenario) Q: Create a Dockerfile for a Node.js app. FROM node:18-alpine WORKDIR /app COPY package*.json ./ RUN npm install COPY . . EXPOSE 3000 CMD ["npm", "start"] 👉 Follow-up: How to reduce image size? Why use alpine? 📦 3. Multi-Stage Build (Very Important) Q: Optimize this Dockerfile for production. # Build stage FROM node:18 as builder WORKDIR /app COPY . . RUN npm install && npm run build # Production stage FROM nginx:alpine COPY --from=builder /app/build /usr/share/nginx/html 👉 Key concept: Reduce final image size Separate build & runtime 🔐 4. Secrets Handling Q: How do you pass secrets (DB password) securely? 👉 Expected answers: Docker secrets (Swarm) Environment variables (not best for sensitive) Use AWS Secrets Manager / Vault 🌐 5. Networking (Practical) Q: Two containers need to communicate. How? docker network create mynet docker run -d --name app --network mynet myapp docker run -d --name db --network mynet mysql 👉 Follow-up: Default network vs bridge vs host vs overlay 📁 6. Volumes & Persistence Q: My container data is lost after restart. Fix it. docker volume create mydata docker run -v mydata:/var/lib/mysql mysql 👉 Concept: Volumes vs bind mounts ⚙️ 7. Debugging Container Issues Q: Container is crashing. What will you do? 👉 Commands: docker logs docker inspect docker exec -it sh docker top 🚀 8. Performance Optimization Q: How to reduce Docker image size? 👉 Expected: Use alpine images Multi-stage builds Remove unnecessary packages Use .dockerignore 🔄 9. Container Lifecycle Q: Difference between: CMD vs ENTRYPOINT 👉 Example: ENTRYPOINT ["nginx"] CMD ["-g", "daemon off;"] 🔒 10. Security (Very Important) Q: How to secure Docker containers? 👉 Expected: Run as non-root user Use minimal base images Scan images (Trivy) Limit capabilities 📊 11. Resource Limits Q: Limit CPU and memory of a container. docker run -d --memory="512m" --cpus="1.0" nginx 🔁 12. Restart Policies docker run -d --restart=always nginx 👉 Types: no on-failure always unless-stopped 🧪 13. Health Checks HEALTHCHECK CMD curl --fail http://localhost || exit 1 📦 14. Image Management Q: Difference between: docker save vs docker export 👉 Expected: save → image export → container filesystem ☁️ 15. Registry & CI/CD Q: How do you push image to Docker Hub / ECR? docker login docker tag myapp username/myapp:latest docker push username/myapp:latest 🔧 16. Docker Compose (Real Use) version: '3' services: app: build: . ports: - "3000:3000" db: image: mysql environment: MYSQL_ROOT_PASSWORD: root 👉 Follow-up: How to scale services? docker-compose up --scale app=3 ⚠️ 17. Troubleshooting Scenario Q: Container works locally but not in server. Why? 👉 Possible reasons: Port not exposed Firewall issue Env variables missing Different architecture 🧠 18. Real DevOps Scenario Q: How do you deploy Docker in production? 👉 Expected: Use Kubernetes / ECS CI/CD pipeline (GitHub Actions, Jenkins) Monitoring (Prometheus, Grafana) 🎯 19. Logging Strategy Q: Where logs are stored? /var/lib/docker/containers//-json.log 🧩 20. Difference Questions (Very Common) Container vs VM Image vs Container Volume vs Bind Mount CMD vs ENTRYPOINT