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README.md

Exercise 9: troubleshooting

Try to practise these commands with any of the previous containers. If you have executed all the prune commands in the previous exercise, you may have to first build some new images.

9.1 Docker built-in tools

  • Show low-level information on any Docker object 
    docker inspect my-alpine-cat:v1
  • Show low-level information for a specific object type. This is useful when there is more than one resource with the same name. E.g. for volumes: 
    docker volume inspect exercise-08_registry_data
  • Display a live stream of container(s) resource usage statistics 
    docker stats
  • Display the running processes for the docker compose services 
    docker compose top
  • Fetch the logs of a container 
    docker logs [your-container-id-or-name]

Getting inside the container

  • Only for troubleshooting or exploratory reasons, sometimes it is convenient to get into the container: 
    docker exec -ti [your-container-id-or-name] [command]
    where the command could be something like sh or bash (it depends on what is available inside the container)
  • In the context of a docker compose, you could run: 
    docker compose exec [your-service] [command]

9.2 Using external tools

  • Top-like interface for container metrics: ctop
  • A simple terminal UI for both docker and docker compose: lazydocker
  • With both previous tools you can even execute a shell (if the image allows it)
  • Some IDEs, like VS Code or IntelliJ, offer extensions that you can use to interact and inspect your Docker resources

9.3 Common errors

  • running out of disk space
    • You could run docker system df to check what resources are taking space.
  • failed: port is already allocated
    • This would happen when trying to run a container in a port already in use. Either use another port or you can find out what process is using it in order to free it.
  • If you intended to pass a host directory, use absolute path
    • You would get that error message when trying to mount a volume using a relative path, e.g. docker run --rm -d -v ./FOOBAR:/foobar nginx
    • You need to use an absolute path, e.g. docker run --rm -d -v ${PWD}/FOOBAR:/foobar nginx

Bonus track

Fix a failing container

From the folder my-nano-app:

  1. Build the image from the Dockerfile (name it my-nano-app)
  2. Run: 
    docker run my-nano-app nano --help
  3. Fix everything needed until the previous command works

Fix another failing container

From the folder my-node-app:

  1. Build the image from the Dockerfile (name it my-node-app)
  2. Run: 
    docker run --rm my-node-app
  3. Fix everything needed until the previous commands works

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