Docker Essentials: A Beginner's Guide to Containerization Commands

Docker has become a cornerstone in the modern software development landscape. It empowers developers to package and run applications in a consistent and portable manner, irrespective of the underlying infrastructure. This guide serves as a gentle introduction to Docker, delving into the essential commands that will kickstart your journey into the world of containers.

1. Verifying Docker Installation:#

The first step is to verify that Docker is installed on your system. Run the following command:

sudo docker --version

This command displays the installed Docker version.

2. Running the “Hello World” Application:#

To experience the magic of Docker firsthand, let’s run the “hello-world” application:

sudo docker run hello-world

This command fetches the “hello-world” image from Docker Hub and executes it, displaying a simple message on your console.

3. Listing Running Containers:#

Docker allows you to manage multiple containers simultaneously. To view a list of all running containers, use the following command:

sudo docker ps

This command displays a table with information about each running container, including its ID, image name, status, and ports.

4. Launching an Interactive Container:#

One of Docker’s key features is the ability to run applications in an isolated environment. To experience this, let’s launch an interactive container based on the Ubuntu image:

sudo docker run -it ubuntu bash

This command not only starts the container but also provides you with a terminal inside the container, allowing you to execute commands and interact with the system as if it were a regular machine.

5. Searching for Images:#

Docker Hub, a public registry, hosts a vast collection of images. You can search for images by name, description, or other criteria using the following command:

sudo docker search ubuntu

This command searches for images containing the keyword “ubuntu” and displays a list of relevant results.

6. Running a Command Inside a Container:#

Often, you may want to run specific commands within a container. To achieve this, you can combine the run command with the -it flag and specify the desired command:

sudo docker run -it ubuntu bash
sudo apt-get update

This example updates the package list inside the Ubuntu container.

7. Diving Deeper with docker info:#

For a comprehensive overview of your Docker environment, use the info command:

sudo docker info

This command provides detailed information about the Docker daemon, including its configuration, network settings, and storage options.

8. Exploring All Containers:#

By default, docker ps only lists running containers. To view all containers, including those that have stopped, use the -a flag:

sudo docker ps
sudo docker ps -a

The first command displays only running containers, while the second command shows both running and stopped containers.

9. Alternative Listing Formats:#

Docker provides various options for customizing the output of the docker ps command. The -l flag allows you to specify the desired format, such as JSON or table:

sudo docker container ls
sudo docker container ls -a

The container ls command is an alias for docker ps. The first command lists all running containers in a table format, while the second command, with the -a flag, lists all containers (running and stopped) in a table format.

10. Mastering Container Management:#

Now that you’ve gained familiarity with basic commands let’s delve into container management, covering operations such as starting, stopping, removing, and restarting containers.

11. Initiating a Container:#

To start a container, use the run command with the -it flag for an interactive session:

sudo docker run -it centos bash

This command starts a container based on the CentOS image and provides you with a terminal inside the container.

12. Halting a Container:#

To stop a running container, use the stop command:

sudo docker stop <container_id>

Replace <container_id> with the ID of the container you want to stop.

13. Removing a Container:#

Once you’re done with a container, you can remove it using the rm command:

sudo docker rm <container_id>

This command removes the specified container from your system.

14. Restarting a Container:#

To restart a stopped container, use the restart command:

sudo docker restart <container_id>

This command restarts.

15. Running a CentOS Container:#

sudo docker container run -it centos:7 bash

sudo: Grants root privileges for container operations. Docker container run: Creates a new Docker container. -it: Allocates a pseudo-terminal (interactive shell) for interacting with the container. centos:7: Specifies the Docker image to use (CentOS version 7). bash: Launches a Bash shell within the container.

16. System Update:#

sudo yum -y update

sudo: Again, for root privileges (package management often requires them). Yum: I am the package manager for RPM-based systems like CentOS. -y: Automatically confirms any prompts during the update process (use with caution in production environments). Update: Updates system packages to their latest versions.

17. Listing Directory Contents:#

ls -l

ls: Lists files and directories. -l: Provides detailed information in a long format (including permissions, owner, group, size, and timestamps).

18. Creating a File:#

echo "we are here" > example.org

echo: Prints text to the terminal or a file. “we are here”: The text to be written.

: Redirects the output to a file (creates example.org in this case)

19. Verifying File Creation:#

ls -l

Lists the directory contents again, showing the newly created example.org.

20. Viewing File Contents:#

cat example.org

cat: Displays the contents of a file. example.org: The file to display.

21. Exiting the Container:#

exit

Terminates the interactive shell session within the container.

22. Listing Running Containers:#

sudo docker ps

sudo: For root privileges. docker ps: Lists currently running Docker containers.

23. Listing All Containers (Including Stopped Ones):#

sudo docker ps -a

-a: Shows all containers, regardless of their running state.

24. Running Another Container:#

sudo docker container run -it centos:7 bash

Re-runs the container creation command from step 1.

25. Exiting the Second Container:#

exit

Terminates the shell session within the second container

26. Listing All Containers Again:#

sudo docker ps -a

Verifies both containers are listed.

27. Starting a Stopped Container:#

sudo docker container start [containerid]

start: Attempts to start a stopped container. [containerid]: Replace this with the actual ID of the container you want to start (obtainable from docker ps -a).

28. Verifying the Started Container:#

sudo docker ps

Checks if the previously stopped container is now listed as running.

29. Inspecting Processes Within a Container:#

sudo docker container exec [containerid] ps -ef

exec: Executes a command within a running container. [containerid]: The ID of the container to access. ps -ef: Lists all processes running inside the container (similar to ps -aux on Linux).

30. Interactive Access to the Container:#

sudo docker container exec -it [containerid] bash

-it: Allocates a pseudo-terminal for interactive interaction. Provides access to the Bash shell within the specified container, allowing you to execute further commands.

31. Verifying File Creation (Inside the Container):#

ls -l

Lists directory contents, showing example.org

32. Viewing File Contents (Inside the Container):#

cat example.org

Displays the contents of example.org created earlier.

33. Remember:#

Replace [containerid] with the actual container ID in steps 13, 15, and 16. Use docker ps -a to view all containers, including stopped ones.

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