How to check Docker login status using the info command

Introduction

Docker is a powerful containerization platform that allows you to package and run applications in isolated environments. When working with Docker, you often need to interact with container registries, which require authentication. Knowing your current login status is essential for managing your Docker workflow efficiently.

In this lab, you will learn how to check your Docker login status using the docker info command. This skill is fundamental for Docker users as it helps ensure you have the proper access to pull and push images from private registries.

Verify Docker Installation

Before we check the login status, let us first verify that Docker is correctly installed and running on our system. This step ensures we have a working Docker environment for the subsequent operations.

Checking Docker Status

Open a terminal in the LabEx environment. You can do this by clicking on the terminal icon in the desktop environment.

Once the terminal is open, run the following command to check if Docker is installed and running:

docker --version

This command should display the version of Docker installed on your system. You should see output similar to this:

Docker version 20.10.21, build 20.10.21-0ubuntu1~22.04.3

Next, let us check if the Docker daemon is running properly. Execute the following command:

docker run hello-world

If Docker is working correctly, it will download a test image and run it in a container. You should see output that includes:

Hello from Docker!
This message shows that your installation appears to be working correctly.
...

This confirms that Docker is properly installed and running on your system. If you encounter any errors, please check if the Docker service is running with:

sudo systemctl status docker

The output should show that the Docker service is active (running).

Now that we have confirmed Docker is working correctly, we can proceed to learn about the docker info command in the next step.

Understanding the Docker Info Command

In this step, we will explore the docker info command and understand how it provides valuable information about your Docker environment.

Running the Docker Info Command

The docker info command provides a comprehensive overview of your Docker installation. It displays system-wide information such as the number of containers and images, server version, storage driver, and much more.

Let us run the docker info command in the terminal:

docker info

You should see a detailed output with various sections. The output will be similar to this:

Client:
 Context:    default
 Debug Mode: false
 Plugins:
  buildx: Docker Buildx (Docker Inc.)
    Version:  v0.9.1
    Path:     /usr/libexec/docker/cli-plugins/docker-buildx
  ...

Server:
 Containers: 1
  Running: 0
  Paused: 0
  Stopped: 1
 Images: 1
 Server Version: 20.10.21
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Native Overlay Diff: true
  userxattr: false
 ...

 Registry: https://index.docker.io/v1/
 ...

The output contains several sections with information about both the Docker client and the Docker server.

Key Sections in Docker Info Output

Let us understand some key sections of the docker info output:

1. Client: Information about the Docker client configuration.
2. Server: Information about the Docker server (daemon).
3. Containers: The number of containers on your system, including running, paused, and stopped containers.
4. Images: The number of Docker images on your system.
5. Server Version: The version of the Docker server.
6. Storage Driver: The storage driver used by Docker.
7. Registry: Information about configured Docker registries.

For checking login status, we are particularly interested in the Registry section, which we will explore in the next step.

Checking Docker Login Status

Now that we understand the docker info command, let us focus on how to use it to check our Docker login status.

Interpreting Login Status in Docker Info

When you run the docker info command, the output contains information about your Docker Hub login status. Specifically, you should look for the Registry section.

Run the command again:

docker info

Now, look for the Registry section in the output.

If you are not logged in to any Docker registry, the output will simply show the default registry URL:

Registry: https://index.docker.io/v1/

If you are logged in, you will see additional information, including your username:

Registry: https://index.docker.io/v1/
Username: yourusername

Currently, you are most likely not logged in to Docker Hub or any other registry.

Logging in to Docker Hub

Let us try logging in to Docker Hub to see how the login status information changes. To log in to Docker Hub, use the docker login command:

docker login

This command will prompt you for your Docker Hub username and password:

Login with your Docker ID to push and pull images from Docker Hub. If you don't have a Docker ID, head over to https://hub.docker.com to create one.
Username:
Password:

For the purpose of this lab, you do not need to enter actual credentials. Instead, press Ctrl+C to cancel the login process.

Alternative Method to Check Login Status

Besides using the docker info command, you can also check if you are logged in to Docker Hub by examining the Docker configuration file:

ls -la ~/.docker/

If you are logged in, you should see a file named config.json in this directory. This file contains your authentication information. You can view its contents with:

cat ~/.docker/config.json

If you are not logged in, the file might not exist or might not contain authentication information.

Logging Out from Docker Registry

Now that we understand how to check login status and attempt to log in, let us learn how to log out from a Docker registry.

Using the Docker Logout Command

To log out from a Docker registry, you can use the docker logout command. This command removes the stored credentials for the specified registry, or the default registry if none is specified.

Run the following command:

docker logout

If you were not logged in, you will see a message like:

Not logged in to https://index.docker.io/v1/

If you were logged in and the logout was successful, you will see:

Removing login credentials for https://index.docker.io/v1/

Verifying Logout Status

After logging out, let us verify our login status using the docker info command:

docker info

Look for the Registry section again. It should now only show the registry URL without any username information:

Registry: https://index.docker.io/v1/

You can also check the Docker configuration file again:

cat ~/.docker/config.json

If you were previously logged in, the file would have been updated to remove the authentication information for the registry you logged out from.

Understanding Docker Registry Authentication

Docker uses token-based authentication to manage access to registries. When you log in to a registry, Docker stores the authentication token in the configuration file (~/.docker/config.json). This token is used for subsequent interactions with the registry until you log out or the token expires.

Understanding how to check your login status and manage your authentication is essential for secure Docker operations, especially when working with private registries.

Summary

In this lab, you have learned essential skills for managing Docker registry authentication:

1. Verifying that Docker is properly installed and running
2. Using the docker info command to view detailed information about your Docker environment
3. Checking your Docker login status using the docker info command
4. Understanding the process of logging in to Docker Hub
5. Logging out from a Docker registry and verifying the logout was successful

These skills are fundamental for working with Docker, especially when you need to interact with private registries or protect your authentication credentials.

You now have the knowledge to confidently manage your Docker login status, ensuring secure and efficient Docker operations in your future projects.