Docker: facts and CLI commands for a beginner

I first realized that I would like to and need to learn about containers when at one of the interviews I had I was asked if I had worked with containerized applications (I hadn’t). In my current team, there has been some conversation about deploying applications using Docker in the near future. So I have started to familiarize myself with Docker containers and images, starting from the basics.

Key points from the first week of learning:

  • a container has its own virtual network
  • you can interact with the container by name or ID
  • Docker images provide a filesystem called a union file system; it consists of OS/Framework files and additions/modifications to original files. The user operates in the topmost layer (writeable layer), so if a change is made to a file, that file is copied into the writeable layer and a record of difference between those two files is saved too. This is called “the copy-on-write” (CoW) strategy (easy to remember – moooooooo!)
  • Docker images are layered
  • Docker images are downloaded into an image cache – the system can detect if an image has changed and then rebuild the layers
  • not all Docker repositories support “latest” tag (which is used by default when you pull an image). Example:

In this case you need to use a specific tag

docker pull

Useful commands to learn:

  1.  choco install docker-cli -y </code installs docker cli
  2. docker pull IMAGE_NAME or REPO_NAME gets an image by name/repo name in Dockerhub
  3. docker ps (-a) shows active containers ; (-a) shows all active and stopped containers
  4. docker images shows available images
  5. docker run [OPTIONS] IMAGE_NAME spins up a container based on an image. Some options are:
  6. -d detached mode, i.e. console won’t be attached to the I/O of the process

    -p PORT_ON_HOST:PORT_IN_CONTAINER maps a port on host to a port on container

    -v PATH_TO_MAP_ON_HOST:/PATH_TO_MAP_IN_CONTAINER – mount a volume and map it to a directory on host

    –rm clean up flag (removes container after you are done; also removes volume)

    –it starts a shell inside the containerdocker stop myContainer

  7. docker inspect network CONTAINER_ID_OR_NAME shows container network info:
  8. docker cp SOURCE_PATH:/DESTINATION_PATH copies files from a host to a path in the container
  9. docker commit CONTAINER_ID_OR_NAME REPO_NAME:TAG creates image from an existing container
  10. docker build -t myimage:v1 . builds an image from a Dockerfile. Full stop at the end means “build context”, i.e. files located in a specific path. -t tags the new image
  11. docker diff IMAGE_INAME shows changes in files for an image
  12. docker tag myimage:v1 oxiehorlock/ (correct tagging to be published on Docker hub) (oxiesImage:v1 is a source image and oxiehorlock/ is a target image)
  13. docker push oxiehorlock/ pushes an image to a repository (in this case the repository name is oxiehorlock, an image is oxiesImage with a tag; you need to do docker login before pushing
  14. docker stop CONTAINER_ID_OR_NAME
  15. docker rm CONTAINER_ID_OR_NAME removes a container (can also chain names/ids to remove multiple containers)
  16. docker rmi IMAGE_NAME removes an image