Docker Lesson 6 – Docker CLI Basics | Dataplexa

Docker Course

I. Docker Fundamentals
1. What is Docker 2. Problems with Traditional Deployment 3. Virtual Machines vs Containers 4. Docker Architecture 5. Installing Docker 6. Docker CLI Basics 7. Docker Images Explained 8. Docker Containers Explained 9. Docker Lifecycle 10. First Docker Hands-On
II. Images, Storage & Networking
11. Dockerfile Introduction 12. Writing Your First Dockerfile 13. Building Docker Images 14. Docker Image Layers 15. Docker Volumes 16. Bind Mounts vs Volumes 17. Docker Networking Basics 18. Bridge Network 19. Host & Overlay Networks 20. Port Mapping 21. Environment Variables 22. Docker Best Practices
III. Compose, Registry & Security
23. Docker Compose Introduction 24. Docker Compose YAML Deep Dive 25. Multi-Container Applications 26. Docker Compose Networking 27. Docker Compose Volumes 28. Docker Registry Concepts 29. Docker Hub 30. Private Docker Registry 31. Image Versioning & Tagging 32. Docker Security Basics 33. Secrets Management 34. Resource Limits 35. Logging & Monitoring
IV. Production, Cloud & CI/CD
36. Docker in Dev vs Prod 37. Dockerizing Backend Applications 38. Dockerizing Databases 39. Docker for Microservices 40. Docker Performance Optimization 41. Docker Security Hardening 42. Docker Troubleshooting 43. Docker with CI/CD 44. Docker on AWS 45. Mini Project
Docker · Lesson 6

Docker CLI Basics

Master the essential Docker commands that every developer uses daily to manage containers and debug production issues.

This lesson covers

Essential Docker commands · Container lifecycle management · Debugging running containers · Command structure and flags

Your Docker CLI Toolbox

The docker command is your control center. Every container you'll ever create, stop, or debug starts here.
Docker CLI — Daily Commands
$ docker run nginx
Unable to find image 'nginx:latest' locally
latest: Pulling from library/nginx
$ docker ps
CONTAINER ID IMAGE COMMAND STATUS
$ docker stop 7f8a9b2c
7f8a9b2c
Every Docker command follows the same pattern: docker [command] [options] [image/container]. The CLI is consistent — learn one command structure, and you know them all.

Docker Command Anatomy

docker
run
-p 80:80
nginx
Base command
Action
Flags
Target

Container Lifecycle Commands

The scenario: You're a backend developer at a fintech startup. Your team lead asks you to spin up a PostgreSQL database for testing a new payment feature. You need it running in 5 minutes.

Tools used:

  • docker run — Creates and starts a new container from an image
  • docker ps — Lists running containers (like ps for processes)
  • docker stop — Gracefully shuts down a running container
# Start a PostgreSQL container for development
docker run --name dev-postgres -e POSTGRES_PASSWORD=mypassword -p 5432:5432 postgres:13

# Check if the container is running
docker ps

# Stop the database when done testing
docker stop dev-postgres

What just happened?

0.0.0.0:5432->5432/tcp — Docker mapped your computer's port 5432 to the container's port 5432. Your apps can now connect to localhost:5432 and reach the PostgreSQL database running inside the container.

--name flag

Gives your container a memorable name. Without it, Docker generates random names like angry_tesla

-e flag

Sets environment variables inside the container. PostgreSQL requires POSTGRES_PASSWORD

-p flag

Port mapping. Format: host:container. Makes the container accessible from your machine

postgres:13

Image name and tag. Always specify versions in production — latest changes unpredictably

Common Mistake: Forgetting -d flag

Without -d (detached), the container runs in foreground and locks your terminal. Add -d to run in background: docker run -d --name dev-postgres ...

Inspecting and Debugging Containers

The scenario: It's 2 AM. Your company's API is returning 500 errors. Your team suspects the Redis cache container is having issues. You need to check what's happening inside the container — fast. 
# Show all containers (running and stopped)
docker ps -a

# View container logs to see what went wrong
docker logs redis-cache

# Get inside a running container to debug
docker exec -it redis-cache /bin/bash

What just happened?

Exited (1) shows the container crashed with error code 1. The logs reveal Out of memory — Redis ran out of RAM. The root@a1b2c3d4e5f6 prompt means you're now inside the container's shell, ready to investigate further.

🔍 For Investigation

docker logs

Shows container output. Add -f to follow logs in real-time, --tail 50 for recent lines only.

🛠️ For Debugging

docker exec

Runs commands inside running containers. -it gives you an interactive terminal session.

Essential Debugging Commands

# Check container resource usage
docker stats redis-cache

# See detailed container information
docker inspect redis-cache

# Restart a crashed container
docker restart redis-cache

What just happened?

MEM %: 95.98% confirms the memory issue — Redis was using almost all available RAM. "ExitCode": 1 from inspect shows it crashed with an error. The restart command will attempt to start it again with the same configuration.

Managing Container Data and Files

The scenario: You're deploying a Node.js application. The container works locally, but you need to copy the latest configuration file into the production container before the evening launch. 
# Copy a file from host into running container
docker cp ./config.json web-app:/app/config.json

# Copy a file from container to host (for backup/debugging)
docker cp web-app:/app/logs/error.log ./error-backup.log

# Mount a directory for real-time development
docker run -d --name web-app -v $(pwd):/app -p 3000:3000 node:16

What just happened?

docker cp works even with running containers — no need to stop them. The -v $(pwd):/app mount creates a live connection: changes to files on your computer instantly appear inside the container at /app.

Path Format Matters

Docker cp syntax is container:path for container files, just path for host files. Wrong: /container/file. Right: container:/path/file.

Cleanup and Management

Docker containers accumulate quickly during development. Your disk fills up. Your teammates complain about slow Docker builds. Time to clean house. 
# Remove a stopped container
docker rm web-app

# Remove a running container (force)
docker rm -f redis-cache

# Remove all stopped containers at once
docker container prune

# Nuclear option: remove everything unused
docker system prune -a