Docker Container Management and Interaction

Building and Running Docker Containers

TLDR

Use docker build to create an image from a Dockerfile. Use docker run to create and start a new container from an image. Use docker start with -a to restart a container and view its output. Use docker start and docker logs to restart a container and view its logs separately.

Building a Docker Image

To build a Docker image from a Dockerfile, use the following command. This will create an image tagged as cowsayapp from the Dockerfile in the current directory.

docker build -t cowsayapp .

Running a Container from an Image

Once the image is built, you can run a container from this image. This command will start a new container instance from the cowsayapp image.

docker run --name my-cowsay-container cowsayapp

This creates and starts a container named my-cowsay-container. If your container is set up to display output (like using the cowsay command), this output will be shown in the terminal.

Restarting a Stopped Container

After running a container, it will eventually stop (especially if its main task completes). To restart the same container without creating a new one, you can use the docker start command.

Option 1: Attach to the Container's Output To restart the container and view its output, use:

docker start -a my-cowsay-container

This restarts the container and attaches your terminal to its output, allowing you to see the output from the container's main process.

Option 2: Viewing Container's Logs Alternatively, you can start the container and view its logs:

docker logs my-cowsay-container

This will start the container in the background and docker logs will display the output of the container.

Running

To start a container and not have its output, or if the container runs and you don't want it to hanf in the shell. Just start it without the -a flag.

docker start container-name

To stop the container if it is running do.

docker stop container-name

Additional Notes on Flags

The -a or --attach flag attaches your terminal to the container's output, useful for viewing output or interacting with the container.

The -d or --detach flag starts the container in detached mode, where it runs in the background and your terminal is free for other tasks.

Flags

The -a and -d flags in Docker commands serve different purposes, and understanding them will clarify how your containers behave when you run them:

-a Flag (Attach)

Flag: -a or --attach

Usage: This flag is used with docker start or docker run commands.

Function: It attaches your terminal's standard input, output, and error (STDIN, STDOUT, STDERR) to the container.

Effect: When used with docker start, it allows you to see the output of the container in your terminal. For docker run, it can be used to keep the terminal attached to the container even after the command to start the container has been issued.

Scenario: You would use -a when you want to interact with the container or view its output directly in your terminal.

-d Flag (Detached)

Flag: -d or --detach

Usage: This flag is used with the docker run command.

Function: It starts the container in detached mode.

Effect: The container runs in the background and does not occupy the current terminal session. You won't see any immediate output from the container in your terminal.

Scenario: You would use -d when you don’t need to interact with the container immediately and want your terminal free for other tasks.

Example

Running a Container in Attached Mode:

docker run -a my-container

This command will show you the output of the container in your terminal.

Running a Container in Detached Mode:

docker run -d my-container

This command will start the container in the background, and you'll get the command prompt back immediately for other uses.

In summary, use -a when you want to see or interact with the container's output directly in your terminal, and use -d when you want the container to run in the background without tying up your terminal.

Seeing the Name of the Created Container

List All Running Containers

docker ps

This command shows all currently running containers, including their names, IDs, and other details, note if a container is one off and starts and exits after a CMD was run, it won't show as running.

List All Containers (Running and Stopped)

docker ps -a

This command lists all containers, whether they are running or have been stopped. You can find the names in the output.

Inspect a Specific Container

docker inspect <container-id>

Replace with the actual container ID.

Understanding Container Lifecycle

Run and Exit

If the Docker container's main process (specified in the CMD or ENTRYPOINT of the Dockerfile) completes, the container stops. For your cowsayapp, this happens immediately after displaying the message.

Running State

While a container's main process is active, the container is considered "running", and you can interact with it.

Accessing Container's Filesystem and Running Different Commands

Interactive Mode

Run the container in interactive mode with a shell. This allows you to execute commands inside the container. For example, to start a Bash shell in an Ubuntu-based container:

Check where your local shell that you want to use is installed, for example if using Git Bash on a windows, do which bash here the ouput is usr/bin/bash

docker run -it --name my-temp-container cowsayapp usr/bin/bash

This opens a Bash shell inside the container. You can run commands, explore the filesystem, or perform other tasks. The -it flags are for interactive (-i) and terminal (-t) mode. Remember run, is for the first time you run the container. This will give you a interactive shell everytime the container starts. So whenever you do docker start containerappname

Use a Temporary Container for Interaction

If you just need temporary access to the container's environment, you can run a one-off new container with an interactive shell using the same image.

docker run -it --rm cowsayapp usr/bin/bash

Note, you run the image name, not the container. Essentialy you run an image and start a container. This command starts a new container with an interactive Bash shell. The --rm flag ensures that the container is removed after you exit the shell, which is useful for temporary or one-off use.

Execute Commands in a Running Container

If your container is not running and it is set to run a command and stop and you did not run it with a shell like above, this will NOT work. However, if the container is already running and you have a long-running container, you can execute commands in it using docker exec. For example:

docker exec -it my-running-container usr/bin/bash

Replace my-running-container with the name or ID of your running container. This command gives you access to a Bash shell inside the container

Best Practices

One-Off Commands

Containers are often used for single, specific tasks (like running a web server, executing a script, etc.). In these cases, the container stops after completing its task.

Persistent/Interactive Work

For exploration, debugging, or development, running a container in interactive mode with access to a shell is common.

Long-Running Services

For services (like databases, web servers, etc.), containers are run in the background and managed as needed.

Docker Usage Guide: Bind Mounts, Volumes, and File Transfers

Understanding Bind Mounts in Docker

Using a bind mount in Docker allows you to mount a specific file or directory from the host machine into the container. This is particularly useful for development purposes, as it allows you to work on your code in the host environment and see the changes reflected in the container in real-time.

Using Bind Mounts

1. Choose the Directory to Mount:

   • Identify the directory on your host machine that you want to mount into the container.

2. Run the Container with a Bind Mount:

   • Use the docker run command with the -v or --mount flag to specify the bind mount.

   • The general format is:

     docker run -v <host_directory>:<container_directory> ...

     or

     docker run --mount type=bind,source=<host_directory>,target=<container_directory> ...

   • For Windows Git Bash:

     docker run -v /c/Users/Username/path:/containerPath -it imageName

   • For Windows WSL:

     docker run -v /mnt/c/Users/Username/path:/containerPath -it imageName

   • This command mounts the directory from your host machine to the specified path in the container.

3. Use the -it Flag for Interactive Mode:

   • The -it flags are combined to make the Docker container start in interactive mode with a TTY, so you can interact with the container through the command line.

   • -i or --interactive keeps the standard input open even if not attached.

   • -t or --tty allocates a pseudo-TTY.

   • Example Command:

     docker run -it -v /path/on/host:/path/in/container imageName

Important Considerations for Bind Mounts

• Overwriting Data: The directory you mount into the container will overwrite the contents of the directory in the container at the specified path. Ensure that you are not overwriting any important data within the container.
• Path Differences: Be aware of the path differences in Git Bash and WSL. Use the appropriate path format for your environment.
• Permissions: Ensure that you have the necessary read/write permissions for the directories you are mounting.

Docker Volumes for Data Persistence

Docker volumes are used to persist data and share it between the host and containers. Volumes are managed by Docker and are more robust than bind mounts.

Creating and Using Volumes

1. Create a Docker Volume:

   docker volume create my_volume
   

2. Attach the Volume to a Container:

   docker run -v my_volume:/path/in/container -it containerName
   

Transferring Files Between Host and Container

Copying Files to a Container

docker cp /path/on/host/file host_container:/path/in/container

Copying Files from a Container

docker cp container:/path/in/container/file /path/on/host

Network Access for File Transfers

If your container has network access and the necessary tools (like curl, wget, or scp), you can download or upload files directly inside the container.

Security and Persistence

• Isolation and Security: Be cautious about what you mount into a container, as it can potentially compromise the security and isolation provided by Docker.
• Data Persistence: Data inside a container's filesystem is ephemeral and will be lost when the container is removed unless it is stored in a volume or a bind mount.

Best Practices

• Use bind mounts for development and real-time code testing.
• Use Docker volumes for data that needs to persist beyond the life of a container.
• Be mindful of the paths and permissions when working with file mounts in Docker.

Personal Efforts

• This works in windows git bash, to bind a file path.

docker run -v C:/Users/Victor/hello/:/victor -it ubuntu usr/bin/bash

• This works in windows wsl Ubuntu, to bind a file path.

docker run -v /mnt/c/Users/Victor/hello:/victor -it ubuntu usr/bin/bash

It was haning and not working, thus I had to reset WSL with command wsl --shutdown executed on powershell. I also closed and reopend the docker service.

Create and test image on the fly

docker run -it ubuntu usr/bin/bash

This will download an ubuntu image and create a container and drop you into the shell of that container. Note the name of the container is random. If you want to name it, add the --name flag.