What is Docker - CLI Guide

Introduction

Docker helps you package your app and everything it needs into a small container. This container runs the same way on any computer, so you avoid problems caused by different setups.

When you want to share your app with others and be sure it works the same on their computers.

When you need to run multiple apps on one server without them interfering with each other.

When you want to test your app in a clean environment that matches production.

When you want to deploy your app quickly and easily to cloud servers.

When you want to keep your development environment consistent across your team.

Commands

Check if Docker is installed and see its version.

Terminal

docker --version

Expected Output

Docker version 24.0.2, build cb74dfc

Run a simple test container to verify Docker is working correctly on your machine.

Terminal

docker run hello-world

Expected Output

Unable to find image 'hello-world:latest' locally

latest: Pulling from library/hello-world

Digest: sha256:7e4a7f3a5a3a4e5a3a4e5a3a4e5a3a4e5a3a4e5a3a4e5a3a4e5a3a4e5a3a4e5a3

Status: Downloaded newer image for hello-world:latest



Hello from Docker!

This message shows that your installation appears to be working correctly.

Key Concept

If you remember nothing else from Docker, remember: it packages your app and its environment into a container that runs the same everywhere.

Common Mistakes

Trying to run Docker commands without Docker installed or running.

The commands will fail because the Docker engine is not available to process them.

Install Docker and start the Docker service before running commands.

Expecting Docker containers to behave exactly like virtual machines.

Docker containers share the host OS kernel and are lighter, so some VM features are not available.

Understand Docker containers are lightweight and designed for app isolation, not full OS virtualization.

Summary

Docker packages your app and its environment into a container.

Containers run the same way on any computer with Docker installed.

Use 'docker run hello-world' to test your Docker setup.

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