What is Docker - Practice Questions & Exercises

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the primary purpose of Docker?

Docker is a popular tool in DevOps. What is its main purpose?

A To create isolated environments called containers to run applications consistently.

B To manage virtual machines with heavy resource usage.

C To monitor network traffic between servers.

D To write code in multiple programming languages.

💡 Hint

Think about how Docker helps developers run apps the same way everywhere.

Docker creates containers that package an application and its dependencies, ensuring it runs the same on any system.

💻 Command Output

intermediate

What is the output of 'docker ps' command?

After running docker ps on a system with two running containers, what will you see?

Docker

docker ps

A A list showing details of all running containers including container ID, image, status, and ports.

B A list of all images downloaded on the system.

C A list of all stopped containers only.

D An error saying 'command not found'.

💡 Hint

Remember, docker ps shows running containers, not images or stopped containers.

The docker ps command lists all currently running containers with their details.

❓ Configuration

advanced

Which Dockerfile instruction sets the base image?

In a Dockerfile, which instruction is used to specify the base image for your container?

Docker

Dockerfile snippet

A CMD python app.py

B RUN apt-get update

C COPY . /app

D FROM ubuntu:20.04

💡 Hint

The base image is the starting point for your container's environment.

The FROM instruction sets the base image in a Dockerfile, defining the starting environment.

🔀 Workflow

advanced

What is the correct order to build and run a Docker container?

Arrange these steps in the correct order to build and run a Docker container from a Dockerfile.

A 3,2,1,4

B 3,1,2,4

C 1,3,2,4

D 2,1,3,4

💡 Hint

First create the Dockerfile, then build the image, run the container, and finally check it.

You must write the Dockerfile first, then build the image, run the container, and check its status.

❓ Troubleshoot

expert

What error occurs if Docker daemon is not running when executing 'docker ps'?

You run docker ps but get an error. What is the most likely error message if the Docker daemon is not running?

A Error: No such container found.

B Syntax error near unexpected token 'ps'.

C Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?

D Permission denied while accessing Dockerfile.

💡 Hint

Think about what happens if the Docker service is stopped but you try to use Docker commands.

If the Docker daemon is not running, commands like docker ps cannot connect and show this error.

Practice

1. What is the main purpose of Docker?

easy

A. To write code faster using a new programming language

B. To package applications into containers that run consistently everywhere

C. To replace operating systems on servers

D. To create virtual machines for running multiple OS

Solution

1. Step 1: Understand Docker's core function

   Docker packages applications and their dependencies into containers.

2. Step 2: Compare with other options

   The other options describe unrelated technologies or incorrect uses.

3. Final Answer:

   To package applications into containers that run consistently everywhere -> Option B

4. Quick Check:

   Docker = container packaging [OK]

Hint: Docker = app in container for consistent runs [OK]

Common Mistakes:

• Confusing Docker with a programming language
• Thinking Docker replaces the OS
• Mixing Docker with virtual machines

2. Which of the following is the correct command to check the installed Docker version?

easy

A. docker version

B. docker check

C. docker --install

D. docker status

Solution

1. Step 1: Recall Docker commands

   The command to check Docker version is docker version.

2. Step 2: Validate other options

   Commands like docker check, docker --install, and docker status are invalid or do not show version.

3. Final Answer:

   docker version -> Option A

4. Quick Check:

   Check version = docker version [OK]

Hint: Use 'docker version' to see Docker's installed version [OK]

Common Mistakes:

• Using 'docker check' which is not a valid command
• Trying 'docker status' to check version
• Confusing install commands with version check

3. What will be the output of the command docker ps if no containers are running?

medium

A. Displays an empty table with headers but no container entries

B. Shows an error message 'No containers found'

C. Lists all running containers

D. Starts a new container automatically

Solution

1. Step 1: Understand 'docker ps' behavior

   The command docker ps lists running containers. If none are running, it shows headers but no container rows.

2. Step 2: Check other options

   It does not show an error or start containers automatically.

3. Final Answer:

   Displays an empty table with headers but no container entries -> Option A

4. Quick Check:

   docker ps empty = headers only [OK]

Hint: 'docker ps' shows running containers or empty headers if none [OK]

Common Mistakes:

• Expecting an error message when no containers run
• Thinking it starts containers automatically
• Confusing 'docker ps' with 'docker images'

4. You run docker run hello-world but get an error saying the image is not found locally. What should you do to fix this?

medium

A. Delete all containers and try again

B. Reinstall Docker completely

C. Run docker pull hello-world to download the image first

D. Change the command to docker start hello-world

Solution

1. Step 1: Understand the error cause

   The error means the image 'hello-world' is not on your machine.

2. Step 2: Fix by downloading the image

   Use docker pull hello-world to get the image from Docker Hub.

3. Final Answer:

   Run docker pull hello-world to download the image first -> Option C

4. Quick Check:

   Image missing? Use docker pull [OK]

Hint: Pull missing images before running containers [OK]

Common Mistakes:

• Trying to start a container without the image
• Reinstalling Docker unnecessarily
• Deleting containers unrelated to the image issue

5. Which of the following best explains why Docker containers help teams work faster and more reliably?

hard

A. Containers automatically fix bugs in the application code

B. Containers run only on the developer's local machine

C. Containers replace the need for any testing before deployment

D. Containers isolate apps and dependencies, avoiding setup conflicts across environments

Solution

1. Step 1: Understand container benefits

   Docker containers package apps with all needed parts, so they run the same everywhere.

2. Step 2: Evaluate options

   Containers isolate apps and dependencies, avoiding setup conflicts across environments correctly states isolation and consistency. The other options are false or misleading.

3. Final Answer:

   Containers isolate apps and dependencies, avoiding setup conflicts across environments -> Option D

4. Quick Check:

   Isolation + consistency = faster, reliable work [OK]

Hint: Containers isolate apps to avoid environment issues [OK]

Common Mistakes:

• Believing containers fix code bugs automatically
• Skipping testing because of containers
• Thinking containers only run locally