A simple cheatsheet for docker

docker_cheatsheet.md

Docker CLI Overview & Cheatsheet

Getting Started

Downloading Docker

• Head on to https://docs.docker.com/get-docker and select the appropriate OS
• Since I'll be doing it on Ubuntu 20.04 - https://docs.docker.com/engine/install/ubuntu/
  • Now there are 3 options to install -
    1. Install using the repository
    2. Install from a package
    3. Install using the convenience script
• I'll be using the 3rd option, so only 3 commands here -
  • curl -fsSL https://get.docker.com -o get-docker.sh
  • chmod +x get-docker.sh
  • sudo sh get-docker.sh
• For running docker in rootless mode, refer to this link

Testing

• Check the docker version: sudo docker version
• Do a simple test - head on to https://hub.docker.com/r/docker/whalesay
• We'll use this image for a test run
  • sudo docker pull docker/whalesay Save the whales! - pull the image and run it
• Next time it will run from the local image

Docker basic commands

Command	Description
docker version	to check the version of docker installed
docker run nginx	to start a container of nginx from image (if already exist, else it will pull from hub)
docker run --name myserver nginx	to start a container of nginx with a name
docker ps	list all running containers
docker ps -a	to list all containers - currently running as well stopped ones
docker stop containerID/name	to stop a running container provide either name or ID (first few letter works as long as it unique)
docker rm containerID/name	to remove a container permanently
docker rm -f containerID/name	to remove a running container permanently
docker images	to see a list of available images
docker rmi name	to delete a docker image
docker pull nginx	to only pull the docker image and not run the container
docker exec ubuntu cat /etc/passwd	to execute command in a container
docker run -d ubuntu:latest	to run and detach a container
docker attach ID/name	to attach back to the running container
docker cp nginx:file.txt data/	to copy a file from container to host [cp container:source target] (just like scp)
docker cp data.txt nginx:file.txt	to copy a file from host to container [cp target container:source]

Docker run

Command	Description
docker run redis:4.0	to run a container with a different version, specify with name:tag format (will take latest tag by default)
docker run -i name	to run in interactive mode (by default won't take input from STDIN)
docker run -it name	to attach a psuedo terminal to the container
docker run -p 69:3306 mysql	to map the port 69 of localhost/dockerhost to port 3306 of the container running mysql (-p HOST:CONTAINER)
docker run -P mysql	to map all ports
docker run -v /opt/datadir:/var/lib/mysql	to persist the data : map a dir outside the container on docker host to a dir inside the container (-v HOST:CONTAINER)
docker inspect name/ID	to inspect a container (output JSON)
docker logs name/ID	to view logs of a container
docker stats	to view stats of running containers
docker top name/ID	to view processes of a container

Docker environment variables

Command	Description
docker run -e BG_COLOR=blue simple-webapp-color	to set the environment variable for our container
docker inspect name/ID	to find the env var in a running container, refer to config section in output

Docker Images

Command	Description
docker build Dockerfile -t webserver/release-one	to build a docker image with a Dockerfile and give it a tag
docker tag Ubuntu Ubuntu:20.04	to tag an image
docker build .	build an image from Dockerfile in current directory
docker save nginx > nginx.tar	save an image to tar file
docker load -i nginx.tar	load an image from tar file
docker push webserver/release-one	to host it on docker hub community (docker registry)

Docker CMD vs ENTRYPOINT

• In case of CMD, when we pass a cmd line argument, docker run ubuntu sleep 10, it will replace the CMD in the Dockerfile
• In case of ENTRYPOINT, when we pass a cmd line argument, docker run ubuntu 10, it will get appended to ENTRYPOINT in the Dockerfile
• If user doesn't pass an arg, use both of them instead (the cmds in Dockerfile should be in Json format)-

FROM Ubuntu

ENTRYPOINT ["sleep"]

CMD ["5"]

The CMD will get appended to ENTRYPOINT, else with the cmd line arg, if specified.

• docker run --entrypoint blah ubuntu 10 : to override default entrypoint

Docker networking

• When installed docker, it creates 3 networks automatically -

  1. Bridge: the default network a container gets attached to (private + internal). Usually in range of 172.17.x.y
  2. None: container not attached to any network; no connection to external host or other containers, they'll run in completely isolated network
  3. Host: connect directly to the host network

• docker run Ubuntu --network=none : Can specify a different network

To access the containers externally -

1. Either port map from container to host
2. OR run the container on host network: docker run Ubuntu --network=host
   • No network isolation left
   • Now we can access it on same port directly
   • BUT can't use the same port for same another container - unlike before

• By default docker only creates 1 internal bridge network. We can create more and group containers -

  Command	Description
  docker network create --driver bridge --subnet 182.18.0.0/16 custom-iso-net	to create a new bridge network
  docker network ls	to list all networks
  docker inspect name/ID	to check a container's IP, mac, subnet, etc.

• Docker has a built-in DNS server that helps the containers to resolve each other using the container name

  • This DNS server always runs at - 127.0.0.11

• Docker uses network namespaces that creates a separate namespace for each container. It then uses virtual ethernet to connect containers together.

Docker Storage

Command	Description
/var/lib/docker	location in file system where docker store its data
docker volume create data_vol	create a volume for persistence storage in /var/lib/docker/data_vol
docker run -v data_vol:/var/lib/mysql mysql	to mount the vol in the container
docker run --mount type=bind,source=~/Desktop/data,target=/var/lib/mysql mysql	Using -v is old style, use this instead

• volume mount mounts from volume dir in /var/lib/docker while bind mount mounts from any dir

Topics Uncovered

Topic name	Cheatsheet Link
Docker-Compose	link
Docker-Swarm	link

Other Docker Cheatsheets

• Link 1
• Link 2
• Link 3
• Link 4