Containers vs virtual machines in Docker - CLI Comparison

Introduction

Containers and virtual machines both help run software in isolated environments, but they do it differently. Containers share the host system's resources efficiently, while virtual machines run full operating systems, which can be heavier.

When you want to run multiple apps on the same server without them interfering with each other.

When you need fast startup times for your applications.

When you want to save system resources like CPU and memory.

When you need full isolation with separate operating systems for security or compatibility.

When you want to test software in different operating systems on the same hardware.

Commands

This command runs a lightweight container using the Alpine Linux image. It starts a shell inside the container so you can see how containers share the host OS kernel but run isolated processes.

Terminal

docker run --rm -it alpine sh

Expected Output

/ # 

→

--rm - Automatically removes the container after you exit.

→

-it - Runs the container interactively with a terminal.

Shows running containers. After exiting the previous container, this confirms no containers are running, demonstrating how containers are temporary and lightweight.

Terminal

docker ps

Expected Output

CONTAINER ID   IMAGE     COMMAND   CREATED   STATUS    PORTS     NAMES

Displays the host operating system information to highlight that containers share the host OS kernel.

Terminal

docker info | grep 'Operating System'

Expected Output

Operating System: Docker Desktop

Key Concept

Containers share the host operating system kernel and isolate applications, making them lightweight and fast compared to virtual machines that run full guest operating systems.

Common Mistakes

Trying to run a container with a different OS kernel than the host.

Containers cannot run a different kernel; they share the host's kernel, so OS-level differences can cause issues.

Use virtual machines if you need a different OS kernel or full OS isolation.

Leaving containers running without cleanup.

This wastes system resources and can clutter your environment.

Use the --rm flag or manually stop and remove containers when done.

Summary

Run a lightweight container with 'docker run' to see how containers share the host OS kernel.

Use 'docker ps' to check running containers and understand their temporary nature.

Containers are faster and use fewer resources than virtual machines because they do not run full operating systems.

Practice

1. Which statement best describes the main difference between containers and virtual machines?

easy

A. Containers require more disk space than virtual machines.

B. Virtual machines are faster to start than containers.

C. Virtual machines share the host OS kernel, containers do not.

D. Containers share the host OS, while virtual machines run separate full OS instances.

Solution

1. Step 1: Understand container architecture

   Containers share the host operating system kernel, making them lightweight and fast.

2. Step 2: Understand virtual machine architecture

   Virtual machines run a full guest OS on top of a hypervisor, making them heavier and fully isolated.

3. Final Answer:

   Containers share the host OS, while virtual machines run separate full OS instances. -> Option D

4. Quick Check:

   Containers share OS, VMs run full OS [OK]

Hint: Containers share OS; VMs run full OS instances [OK]

Common Mistakes:

• Thinking containers run full OS instances
• Believing VMs share the host OS kernel
• Assuming containers use more disk space

2. Which of the following is the correct command to list running Docker containers?

easy

A. docker ps

B. docker images

C. docker run

D. docker start

Solution

1. Step 1: Identify command to list running containers

   The command docker ps shows all running containers.

2. Step 2: Differentiate from other commands

   docker images lists images, docker run starts a container, and docker start restarts stopped containers.

3. Final Answer:

   docker ps -> Option A

4. Quick Check:

   List running containers = docker ps [OK]

Hint: Use 'docker ps' to see running containers [OK]

Common Mistakes:

• Using 'docker images' to list containers
• Confusing 'docker run' with listing
• Using 'docker start' instead of listing

3. Given the following scenario: You start a Docker container and run uname -a inside it. What will this command output compared to running uname -a on a virtual machine?

medium

A. Both outputs will be identical because containers and VMs have the same kernel.

B. Both outputs will be completely unrelated to the host system.

C. The container's output shows the host kernel, VM shows its own kernel version.

D. The container shows a different kernel version than the host, VM shows host kernel.

Solution

1. Step 1: Understand container kernel sharing

   Containers share the host OS kernel, so uname -a inside a container shows the host kernel version.

2. Step 2: Understand VM kernel isolation

   Virtual machines run their own OS kernel, so uname -a inside a VM shows the VM's kernel version, which can differ from the host.

3. Final Answer:

   The container's output shows the host kernel, VM shows its own kernel version. -> Option C

4. Quick Check:

   Container shares kernel, VM has own kernel [OK]

Hint: Containers share kernel; VMs have separate kernels [OK]

Common Mistakes:

• Assuming containers have separate kernels
• Thinking VM shares host kernel
• Believing outputs are identical always

4. You tried to run a Docker container but got an error saying the container cannot start due to kernel incompatibility. What is the most likely cause?

medium

A. The Docker daemon is not installed.

B. The container image requires a different kernel feature not supported by the host OS.

C. The container image is corrupted and cannot be loaded.

D. The container is trying to run a full OS which is not allowed.

Solution

1. Step 1: Analyze kernel incompatibility error

   Containers rely on the host kernel. If the container needs kernel features missing on the host, it cannot start.

2. Step 2: Rule out other causes

   Docker daemon missing causes different errors; corrupted images cause load errors; containers can run full OS but share kernel.

3. Final Answer:

   The container image requires a different kernel feature not supported by the host OS. -> Option B

4. Quick Check:

   Kernel features mismatch causes container start failure [OK]

Hint: Kernel mismatch blocks container start, check host features [OK]

Common Mistakes:

• Assuming Docker daemon missing causes kernel errors
• Blaming image corruption for kernel issues
• Thinking containers run full OS independently

5. You want to deploy an application that requires full OS isolation for security but also needs to start quickly and use minimal resources. Which approach is best?

hard

A. Use containers with additional security layers like SELinux or AppArmor.

B. Use containers for speed and efficiency but accept less isolation.

C. Use virtual machines for full isolation despite slower startup and higher resource use.

D. Use virtual machines with lightweight OS images to speed up startup.

Solution

1. Step 1: Understand isolation vs speed tradeoff

   Virtual machines provide full OS isolation but are slower and heavier; containers are fast but share kernel.

2. Step 2: Consider enhanced container security

   Adding security layers like SELinux/AppArmor to containers improves isolation while keeping speed and resource benefits.

3. Step 3: Evaluate other options

   Using VMs sacrifices speed; lightweight VMs help but still slower than containers; plain containers lack full isolation.

4. Final Answer:

   Use containers with additional security layers like SELinux or AppArmor. -> Option A

5. Quick Check:

   Secure containers balance isolation and speed [OK]

Hint: Secure containers with extra layers for isolation and speed [OK]

Common Mistakes:

• Choosing VMs ignoring startup speed
• Using plain containers without security layers
• Assuming lightweight VMs match container speed