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Inspecting and Debugging

These exercises cover how to inspect, execute a command within and attach to a running container. They are often used when building your container application and troubleshooting once it has moved beyond local development.

Use the below command references to complete the exercises.

Inspect

Command Description
docker inspect View low-level information on a container.
docker image inspect View low-level information on an image.
docker network inspect View low-level information on a network.
docker volume inspect View low-level information on a volume.

Attach and Exec

Command Description
docker attach <container name> Attaches to console of container.
docker exec <container name> <command> Executes a command within a container.

Exercise 1

Inspect the different resources available and format their output.

  • Create a daemonized container with the following command: docker run -d -p 127.0.0.1:8080:80 --name webserver nginx:stable-alpine
  • Inspect the webserver container and locate it's IP Address.
    • Format it's output so it only prints that field.
  • Inspect an image, network and volume and explore what is available.
  • If moving onto the next exercise, do not stop or remove the webserver container.

Often when troubleshooting or working on a script you will be required to view and work with specific information from a container. In this exercise, you will inspect, view and format output. Think about the situations in which the information would be useful or needed when chaining together with something else.

[Solution]

Exercise 2

Execute a command and spawn a shell within a container. Note the process IDs and how many there are.

  • Execute the command: ps aux in the webserver container from the previous exercise.
  • Spawn an interactive shell (/bin/sh) within the webserver container.
    • From within the container list the processes again using the ps aux command. Note the process IDs and how many processes there are.
  • Exit then stop and delete the container.

When debugging or troubleshooting an application, you will frequently find yourself needing to work "within" the context of the container. It is possible to both execute commands or spawn a new shell within the container.

[Solution]

Exercise 3

Attach and detach to a running container. Note the process IDs and how many there are.

  • Create an interactive container with the following command: docker run -it --name myalpine alpine /bin/sh
  • While in the container, execute the command: ps aux to list all the current processes. Note the process IDs and how many processes there are.
  • Press control+p+q to detach from the console
  • Reattach to the container.
  • type ps aux again.
  • Exit then stop and delete the container.

Attaching and Detaching is generally less useful than executing within the context of a container, but it can be used when you want to leave something up and running to monitor or debug a process and come back to it later.

[Solution]