1. What is Docker?
Docker is a container management platform.
In simple words, Docker helps us create, start, stop, restart, pause, unpause, kill, and remove containers.
Just like VMware or AWS helps us manage virtual machines, Docker helps us manage containers.
VMware / AWS → manage VMs
Docker → manage containers
Docker is mainly used to run applications in a lightweight and fast way.
2. Why Do We Use Docker?
Docker helps us in three major ways:
1. Save cost
A container uses fewer resources than a virtual machine.
A VM normally needs:
Kernel + Operating System + Root Filesystem + Application
A Docker container uses:
Kernel from host + Root filesystem + Application
So containers consume less:
CPU
RAM
Disk
Operating system overhead
2. Save time
A virtual machine can take several minutes to start.
VM start time → around 5 minutes
Container start time → around 1 second
Containers are much faster because they do not boot a full operating system.
3. Improve quality of work
Docker makes application environments consistent.
For example, if Jenkins runs inside a Docker container, the same Jenkins container can run on:
Developer laptop
Testing server
Production server
Cloud server
This reduces environment-related issues.
3. What is a Container?
A container is a lightweight application runtime environment.
Example:
Jenkins application
↓
JRE + Jenkins files
↓
Root filesystem
↓
Linux kernel from host
Container structure:
Host Kernel
↓
RootFS
↓
AppFS: JRE + Jenkins
↓
Running Container
The main goal of a container is:
Run applications with less CPU, RAM, disk, and OS cost.
4. Docker vs Virtual Machine
| Feature | Virtual Machine | Docker Container |
|---|---|---|
| Boot time | Slow | Very fast |
| OS required | Full OS required | Uses host kernel |
| Resource usage | High | Low |
| Size | Large | Small |
| Use case | Full machine virtualization | Application runtime |
Example:
VM Image:
Kernel + RootFS + Apps
Docker Image:
RootFS + Apps
Docker images do not contain a full operating system kernel.
5. Docker Architecture
Docker has multiple components.
Human
↓
Docker Client
↓
Docker Daemon / Docker Server
↓
containerd
↓
Linux Kernel
↓
Containers
Main components
| Component | Meaning |
|---|---|
| Docker Client | Command-line tool used by humans |
| Docker Daemon | Docker server that manages containers |
| containerd | Container runtime used internally by Docker |
| Kernel | Linux kernel that provides container features |
| Container | Running application environment |
When you run a command like:
docker ps
You are using the Docker client.
The Docker client talks to the Docker daemon, and the Docker daemon manages containers.
6. Docker Workflow
Basic Docker workflow:
Step 1: Install Docker Engine
Step 2: Download Docker image
Step 3: Pull image from Docker registry
Step 4: Create container from image
Step 5: Start, stop, restart, pause, unpause, kill, or remove container
Docker registry example:
Docker Hub
Docker Hub stores Docker images such as:
httpd
nginx
ubuntu
mysql
jenkins
tomcat
7. Verify Docker Installation
First check whether Docker is installed.
docker version
Expected result:
Client: Docker Engine
Server: Docker Engine
You can also check detailed Docker information:
docker info
This shows details such as:
Number of containers
Number of images
Docker version
Storage driver
Operating system
CPU
Memory
8. Check Existing Docker Images
To list Docker images available on your system:
docker images
Example output:
REPOSITORY TAG IMAGE ID CREATED SIZE
If no image is available, the list may be empty.
9. Download Docker Image
Now download the Apache HTTP Server image.
Docker image name:
httpd
Command:
docker pull httpd
This downloads the image from Docker Hub.
Now check images again:
docker images
Example output:
REPOSITORY TAG IMAGE ID CREATED SIZE
httpd latest xxxxxxxxxxxx few days ago xxxMB
10. Create a Container
A container is created from an image.
Command:
docker create httpd
This creates a container but does not start it.
To see running containers:
docker ps
You may not see the container because it is not running yet.
To see all containers, including stopped containers:
docker ps -a
Example output:
CONTAINER ID IMAGE COMMAND STATUS PORTS NAMES
943e7fcd9ce9 httpd "httpd-foreground" Created nice_name
11. Start a Container
Use the container ID from docker ps -a.
Example:
docker start 943e7fcd9ce9
Now check again:
docker ps
Example output:
CONTAINER ID IMAGE STATUS PORTS NAMES
943e7fcd9ce9 httpd Up few seconds 80/tcp nice_name
12. Stop a Container
To stop a running container:
docker stop 943e7fcd9ce9
Check container status:
docker ps -a
Example:
STATUS
Exited
13. Restart a Container
To restart a container:
docker restart 943e7fcd9ce9
Check status:
docker ps -a
14. Pause a Container
Pause temporarily freezes the running container.
docker pause 943e7fcd9ce9
Check status:
docker ps -a
Example:
STATUS
Up ... Paused
15. Unpause a Container
To resume a paused container:
docker unpause 943e7fcd9ce9
Check again:
docker ps -a
16. Check Container Resource Usage
To see live CPU, memory, network, and disk usage:
docker stats
Example output:
CONTAINER ID NAME CPU % MEM USAGE / LIMIT NET I/O
943e7fcd9ce9 web_app 0.10% 20MiB / 1GiB 1kB / 1kB
To exit from docker stats, press:
CTRL + C
17. Kill a Container
docker stop gracefully stops a container.
docker kill forcefully stops it.
docker kill 943e7fcd9ce9
Check status:
docker ps -a
18. Remove a Container
A container must be stopped before removing it.
docker rm 943e7fcd9ce9
Check all containers:
docker ps -a
The removed container should not appear anymore.
19. Docker Container Lifecycle Commands
Here is the full lifecycle:
docker create httpd
docker ps
docker ps -a
docker start <container_id>
docker stop <container_id>
docker restart <container_id>
docker pause <container_id>
docker unpause <container_id>
docker stats
docker kill <container_id>
docker rm <container_id>
Example using container ID:
docker create httpd
docker ps -a
docker start 943e7fcd9ce9
docker stop 943e7fcd9ce9
docker start 943e7fcd9ce9
docker restart 943e7fcd9ce9
docker pause 943e7fcd9ce9
docker unpause 943e7fcd9ce9
docker kill 943e7fcd9ce9
docker rm 943e7fcd9ce9
20. Shortcut Command: docker run
Instead of doing this:
docker pull httpd
docker create httpd
docker start <container_id>
You can use:
docker run httpd
docker run performs:
Pull image if not available
Create container
Start container
Attach to container
So this command:
docker run httpd
means:
pull + create + start + attach
This runs the container in the foreground.
21. Run Container in Detached Mode
Detached mode means the container runs in the background.
Command:
docker run -d httpd
This means:
pull + create + start + do not attach
Check running containers:
docker ps
Example output:
CONTAINER ID IMAGE STATUS PORTS NAMES
6e8eb2b841ba httpd Up few seconds 80/tcp random_name
22. Give a Name to a Container
It is easier to manage containers by name instead of ID.
Example:
docker run -d --name web1 httpd
Now check:
docker ps
You can stop it by name:
docker stop web1
Start it again:
docker start web1
Remove it:
docker rm web1
If it is running, stop it first:
docker stop web1
docker rm web1
23. Go Inside a Running Container
To enter a running container:
docker exec -it <container_id> /bin/bash
Example:
docker exec -it 6e8eb2b841ba /bin/bash
Now you are inside the container.
Example prompt:
root@6e8eb2b841ba:/usr/local/apache2#
You can run Linux commands inside the container:
ls
pwd
whoami
hostname
To exit from the container:
exit
Important:
docker exec does not create a new container.
It opens a shell inside an already running container.
24. Inspect a Container
To see full details of a container:
docker inspect <container_id>
Example:
docker inspect 6e8eb2b841ba
This shows details like:
Container ID
Image name
Container IP address
Network settings
Mounts
Environment variables
Port configuration
25. Find Container IP Address
You can inspect the container:
docker inspect 6e8eb2b841ba
Look for:
IPAddress
Example container IP:
172.17.0.5
To test the Apache web server from the Docker host:
curl http://172.17.0.5
Expected output:
<html><body><h1>It works!</h1></body></html>
This confirms that the Apache container is running.
26. Access Container from Outside
A container has its own internal IP.
Example:
Container IP: 172.17.0.5
But this IP is usually accessible only from the Docker host.
To access the container from outside, you should use port mapping.
27. Docker Port Mapping
Apache inside the container runs on port 80.
To access it from the host machine, map a host port to the container port.
Format:
docker run -d -p <host_port>:<container_port> <image_name>
Example:
docker run -d -p 80:80 httpd
Meaning:
Host port 80 → Container port 80
Now access from browser:
http://server-ip
Or using curl:
curl http://localhost
28. Run Apache on Host Port 81
If port 80 is already busy, use another host port.
Example:
docker run -d -p 81:80 httpd
Meaning:
Host port 81 → Container port 80
Now access:
curl http://localhost:81
From browser:
http://server-ip:81
Expected output:
It works!
29. Foreground vs Detached Port Mapping
Foreground mode:
docker run -p 80:80 httpd
This starts Apache and attaches your terminal to the container.
Detached mode:
docker run -d -p 80:80 httpd
This starts Apache in the background.
For servers like Apache, detached mode is usually better.
30. Complete Practical Lab
Lab Goal
Run Apache HTTP Server using Docker and access it from browser.
Step 1: Check Docker
docker version
docker info
Step 2: Check images
docker images
Step 3: Pull Apache image
docker pull httpd
Step 4: Verify image
docker images
Step 5: Create container
docker create httpd
Step 6: Check all containers
docker ps -a
Copy the container ID.
Example:
943e7fcd9ce9
Step 7: Start container
docker start 943e7fcd9ce9
Step 8: Check running container
docker ps
Step 9: Inspect container
docker inspect 943e7fcd9ce9
Find the container IP address.
Example:
172.17.0.5
Step 10: Test Apache using container IP
curl http://172.17.0.5
Expected output:
It works!
Step 11: Stop container
docker stop 943e7fcd9ce9
Step 12: Remove container
docker rm 943e7fcd9ce9
31. Practical Lab Using docker run
Now do the same thing using shortcut command.
docker run -d --name apache1 -p 81:80 httpd
Check container:
docker ps
Test:
curl http://localhost:81
Enter container:
docker exec -it apache1 /bin/bash
Inside container:
pwd
ls
hostname
Exit:
exit
Stop container:
docker stop apache1
Remove container:
docker rm apache1
32. Useful Docker Commands Summary
Docker information
docker version
docker info
Image commands
docker images
docker pull httpd
Container list commands
docker ps
docker ps -a
Container lifecycle commands
docker create httpd
docker start <container_id>
docker stop <container_id>
docker restart <container_id>
docker pause <container_id>
docker unpause <container_id>
docker kill <container_id>
docker rm <container_id>
Shortcut run commands
docker run httpd
docker run -d httpd
docker run -d --name web1 httpd
docker run -d -p 80:80 httpd
docker run -d -p 81:80 httpd
Access container shell
docker exec -it <container_id> /bin/bash
Inspect container
docker inspect <container_id>
Monitor container
docker stats
33. Common Mistakes and Fixes
Mistake 1: Using wrong command
Wrong:
docker versions
Correct:
docker version
Mistake 2: Container not showing in docker ps
docker ps only shows running containers.
Use:
docker ps -a
This shows all containers.
Mistake 3: Port already in use
If this fails:
docker run -d -p 80:80 httpd
Use another port:
docker run -d -p 81:80 httpd
Mistake 4: Removing running container
If this fails:
docker rm apache1
Stop it first:
docker stop apache1
docker rm apache1
Or force remove:
docker rm -f apache1
34. Final Practice Exercise
Run these commands yourself:
docker version
docker info
docker images
docker pull httpd
docker run -d --name myweb -p 8080:80 httpd
docker ps
curl http://localhost:8080
docker exec -it myweb /bin/bash
exit
docker inspect myweb
docker stats
docker stop myweb
docker rm myweb
docker ps -a
Expected result:
Apache HTTP Server container starts successfully.
Application is accessible on host port 8080.
Container can be stopped and removed successfully.
35. Key Takeaways
Docker is a container management platform.
Docker containers are lightweight application runtime environments.
Docker helps save:
CPU
RAM
Disk
Time
Cost
Docker image contains:
Root filesystem + Application
Docker container is:
Running instance of Docker image
Important Docker commands are:
docker pull
docker images
docker create
docker start
docker stop
docker restart
docker pause
docker unpause
docker kill
docker rm
docker run
docker exec
docker inspect
docker stats
docker ps
The most common shortcut command is:
docker run -d -p 81:80 httpd
This command downloads Apache if needed, creates a container, starts it in background, and maps host port 81 to container port 80.
Yes — add this References section at the end of the tutorial.
References
Docker Learning References
- Docker internal/container concepts
https://www.devopsschool.xyz/course/section.php?id=767 - Docker command-line complete reference
https://www.devopsschool.com/blog/the-docker-command-line-complete-referenece/ - Docker installation and configuration
https://www.devopsschool.com/blog/docker-installation-and-configurations/ - How to install Docker in Linux
https://www.devopsschool.com/blog/how-to-install-docker-in-linux/ - Lifecycle of Docker containers
https://www.devopsschool.com/blog/lifecycle-of-docker-containers/ - Docker Hub registry
https://hub.docker.com/
Related SSH References
These are useful when accessing Docker servers remotely.
- Secure Shell SSH tools
https://www.devopsschool.com/blog/secure-shell-ssh-tools/ - Top SSH clients: features, pros, cons, comparison
https://www.devopsschool.com/blog/top-10-ssh-clients-features-pros-cons-comparison/
Optional Official Docker References
You can also include these for students:
- Official Docker documentation
https://docs.docker.com/ - Docker Engine installation guide
https://docs.docker.com/engine/install/ - Docker CLI reference
https://docs.docker.com/reference/cli/docker/ - Apache HTTPD image on Docker Hub
https://hub.docker.com/_/httpd
