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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