Docker login and authentication - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the command to log in to Docker Hub.

Docker

docker [1]

Drag options to blanks, or click blank then click option'

Abuild

Bpush

Crun

Dlogin

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'docker push' instead of 'docker login'.

Using 'docker run' which starts containers, not login.

The docker login command is used to authenticate your Docker client to a Docker registry like Docker Hub.

2fill in blank

medium

Complete the command to log in to a private Docker registry at example.com.

Docker

docker login [1]

Drag options to blanks, or click blank then click option'

Alocalhost

Bdocker.io

Cexample.com

Dregistry.hub.docker.com

Attempts:

3 left

💡 Hint

Common Mistakes

Using the default Docker Hub URL instead of the private registry URL.

Leaving the registry URL blank when logging into a private registry.

To log in to a private registry, specify its URL after docker login. Here, example.com is the private registry address.

3fill in blank

hard

Fix the error in the command to log in with a username.

Docker

docker login -u [1]

Drag options to blanks, or click blank then click option'

Amyusername

B-p

Cpassword

D--password

Attempts:

3 left

💡 Hint

Common Mistakes

Putting password or flags instead of username after -u.

Confusing -u with -p.

The -u option expects the username. Here, myusername is the correct value to provide.

4fill in blank

hard

Fill both blanks to log in with username and password in one command.

Docker

docker login -u [1] -p [2]

Drag options to blanks, or click blank then click option'

Amyuser

Bmypassword

Cpassword123

Dadmin

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping username and password positions.

Using flags instead of actual username or password.

The -u flag is for username and -p is for password. Here, myuser and password123 are example credentials.

5fill in blank

hard

Fill all three blanks to create a Docker config file with authentication for a registry.

Docker

echo '{"auths": {"[1]": {"auth": "[2]"}}}' > [3]

Drag options to blanks, or click blank then click option'

Aexample.com

BdXNlcjpwYXNz

C~/.docker/config.json

Ddocker.io

Attempts:

3 left

💡 Hint

Common Mistakes

Using plain text credentials instead of base64 encoded string.

Writing to wrong file path.

Using wrong registry URL.

This command writes a JSON config file for Docker authentication. example.com is the registry, dXNlcjpwYXNz is the base64 encoded 'user:pass', and ~/.docker/config.json is the config file path.

Practice

1. What is the main purpose of the docker login command?

easy

A. To authenticate your computer with a Docker registry

B. To start a Docker container

C. To build a Docker image

D. To remove unused Docker images

Solution

1. Step 1: Understand the role of docker login

   The docker login command is used to provide your credentials to a Docker registry so you can access private images.

2. Step 2: Differentiate from other Docker commands

   Starting containers, building images, or removing images are done by other commands like docker run, docker build, and docker image prune.

3. Final Answer:

   To authenticate your computer with a Docker registry -> Option A

4. Quick Check:

   docker login = authenticate registry [OK]

Hint: Login connects you to registry for private image access [OK]

Common Mistakes:

• Confusing login with image build or container start
• Thinking login removes images
• Assuming login runs containers

2. Which of the following is the correct syntax to login to Docker Hub using the command line?

easy

A. docker login username password

B. docker login -u username

C. docker login --user username

D. docker login --user=username password

Solution

1. Step 1: Recall the docker login syntax

   The correct syntax to login is docker login -u username or docker login --username username. The password is prompted securely after running the command.

2. Step 2: Identify the correct option

   docker login -u username uses the short flag -u correctly without exposing the password inline, which is the recommended way.

3. Final Answer:

   docker login -u username -> Option B

4. Quick Check:

   docker login -u username = correct syntax [OK]

Hint: Use -u or --username, password entered after prompt [OK]

Common Mistakes:

• Passing password directly in command
• Using incorrect flags like --user
• Omitting username flag

3. What will be the output of the following command if the login is successful?

docker login -u user123

medium

A. Error: Invalid username or password

B. Cannot connect to the Docker daemon

C. Usage: docker login [OPTIONS]

D. Login Succeeded

Solution

1. Step 1: Understand the expected output of successful login

   When docker login succeeds, it prints Login Succeeded to confirm authentication.

2. Step 2: Differentiate from error messages

   Other options represent errors or usage info, which appear only if login fails or command is incorrect.

3. Final Answer:

   Login Succeeded -> Option D

4. Quick Check:

   Successful login message = Login Succeeded [OK]

Hint: Success message is 'Login Succeeded' exactly [OK]

Common Mistakes:

• Confusing error messages with success
• Expecting no output on success
• Misreading usage info as success

4. You run docker login -u user123 but get the error: Cannot connect to the Docker daemon. What is the most likely cause?

medium

A. Docker daemon is not running on your machine

B. Incorrect username or password

C. You forgot to specify the registry URL

D. Docker CLI version is outdated

Solution

1. Step 1: Analyze the error message

   The error Cannot connect to the Docker daemon means the Docker service (daemon) is not running or accessible.

2. Step 2: Rule out other causes

   Incorrect credentials or missing registry URL cause authentication errors, not daemon connection errors. CLI version issues usually show different messages.

3. Final Answer:

   Docker daemon is not running on your machine -> Option A

4. Quick Check:

   Daemon connection error = daemon not running [OK]

Hint: Daemon errors mean Docker service is off or unreachable [OK]

Common Mistakes:

• Assuming wrong password causes daemon error
• Thinking registry URL is mandatory for Docker Hub login
• Blaming CLI version for connection issues

5. You want to automate Docker login in a CI/CD pipeline without exposing your password in the command line. Which approach is best?

hard

A. Use docker login -u user -p password directly in the script

B. Write your password in a plain text file and read it in the script

C. Store credentials in environment variables and use echo $PASSWORD | docker login -u user --password-stdin

D. Skip login and pull images anonymously

Solution

1. Step 1: Understand secure login methods for automation

   Using --password-stdin with environment variables avoids exposing passwords in command history or process lists.

2. Step 2: Evaluate other options for security

   Passing password directly in command or storing in plain text files risks exposure. Skipping login may fail for private images.

3. Final Answer:

   Store credentials in environment variables and use echo $PASSWORD | docker login -u user --password-stdin -> Option C

4. Quick Check:

   Use --password-stdin with env vars for secure automation [OK]

Hint: Use --password-stdin and env vars to keep passwords safe [OK]

Common Mistakes:

• Passing password directly in command line
• Storing passwords in plain text files
• Skipping login for private image pulls