the_information_nexus/tech_docs/docker_primer.md

Here’s a no-nonsense Docker CLI guide focused on the 20% of commands that deliver 80% of the value for senior engineers, with real-world examples and troubleshooting tips:

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Docker CLI: The Senior Engineer’s Cheat Sheet

Goal: Master Docker’s command-line interface for debugging, optimization, and orchestration.

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1. Core Commands (The Essentials)

Command	What It Does	Key Flags
docker run	Start a container	-d (detached), -p 80:80 (port map)
docker ps	List running containers	-a (show stopped)
docker exec -it	Run a command in a running container	-it (interactive TTY)
docker logs	View container logs	-f (follow), --tail 100
docker stop / docker rm	Stop or remove a container	-f (force remove running)

Pro Tip:

• Use docker run --rm to auto-remove containers after they exit (great for testing).

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2. Image Management

Command	What It Does	Key Flags
docker build	Build an image from a Dockerfile	-t my-image:tag (tag image)
docker images	List local images	-q (quiet, just IDs)
docker pull	Download an image from a registry	--platform linux/amd64
docker push	Upload an image to a registry
docker image prune	Delete unused images	-a (remove all dangling)

Critical Knowledge:

• Image Layers: Each RUN, COPY, etc., in a Dockerfile creates a layer (cacheable).
• Multi-Platform Builds:

  docker buildx build --platform linux/amd64,linux/arm64 -t my-image:multiarch .
  

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3. Networking & Ports

Command	What It Does	Example
docker network ls	List networks
docker network inspect	Show network details (IPs, gateways)	docker network inspect bridge
docker port	List port mappings	docker port my-container
docker run --network	Attach to a specific network	--network host (host mode)

Key Concepts:

• Bridge Network: Default (NAT’d containers).
• Host Network: Bypasses Docker networking (faster, less secure).
• Overlay Network: For multi-host Swarm/Kubernetes.

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4. Storage & Volumes

Command	What It Does	Example
docker volume ls	List volumes
docker volume create	Create a named volume	docker volume create my-vol
docker run -v	Mount a volume or bind mount	-v /data:/app/data
docker cp	Copy files between host/container	docker cp my-container:/file .

Pro Tips:

• Named Volumes: Managed by Docker (best for databases).
• Bind Mounts: Link to host dir (great for development):

  docker run -v $(pwd):/app my-image
  

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5. Debugging & Troubleshooting

Command	What It Does	When to Use
docker stats	Live resource usage (CPU/mem/IO)	Identify resource hogs
docker top	View processes inside a container	Debug hung processes
docker inspect	Low-level container/image details	Find IPs, volumes, configs
docker events	Real-time Docker daemon events	Audit container lifecycles
docker system df	Show disk usage (images, containers, volumes)	Cleanup decisions

Critical Flags for docker inspect:

docker inspect -f '{{.NetworkSettings.IPAddress}}' my-container  # Get container IP
docker inspect -f '{{.LogPath}}' my-container                   # Find log file path

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6. Cleanup & Maintenance

Command	What It Does	Nuclear Option
docker container prune	Remove stopped containers
docker image prune	Remove dangling images	-a (remove all unused)
docker system prune	Remove everything unused	--volumes (include volumes)

Warning:

• docker system prune --all --volumes deletes all unused images, containers, and volumes (use with caution!).

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7. Docker Compose (Bonus)

Command	What It Does
docker-compose up	Start services defined in docker-compose.yml
docker-compose down	Stop and remove services
docker-compose logs	View aggregated logs
docker-compose exec	Run a command in a service container

Pro Tip:

• Use docker-compose --profile to enable/disable services (e.g., dev vs. prod):

  services:
    redis:
      profiles: ["prod"]
  

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8. Real-World Scenarios

1. Debug a Crashing Container

docker logs -f my-container       # Check logs
docker exec -it my-container sh  # Shell into it
docker inspect my-container      # Check exit code

2. Simulate Out-of-Memory (OOM) Killer

docker run -m 100m --rm alpine tail /dev/zero  # Trigger OOM
dmesg | grep -i kill                           # Find OOM event

3. Optimize Builds

# Bad (creates huge layers):
RUN apt-get update && apt-get install -y python
RUN pip install -r requirements.txt

# Good (single layer):
RUN apt-get update && apt-get install -y python && \
    pip install -r requirements.txt && \
    apt-get clean

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9. Interview Questions

1. How do you reduce Docker image size?

   • Use multi-stage builds, Alpine-based images, and clean up temp files in the same RUN layer.

2. What’s the difference between CMD and ENTRYPOINT?

   • ENTRYPOINT defines the executable; CMD provides default args (can be overridden by docker run).

3. How do you persist data in Docker?

   • Volumes (-v my-vol:/data) or bind mounts (-v /host/path:/data).

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10. Cheat Sheet

# Start a container with a shell
docker run -it --rm alpine sh

# Build and tag an image
docker build -t my-app:1.0 .

# Clean up everything unused
docker system prune -a --volumes

Next Steps:

• Learn Docker Swarm (docker swarm init).
• Dive into Kubernetes (kubectl).
• Master eBPF for container tracing (bpftrace).

Need a deep dive on Docker security or multi-host networking? Let me know! 🐳