Docker Notes 1 - Beginner
Basic Concept
Docker can be seen as a simplified VM.
Since it will not install the whole operating system,
it comes with a smaller size and faster speed.
Image
Image contains a lightweight runtime environment,
including some libraries and executables within it.
Images can be seen as the .iso file on VM for docker.
It can only be read but not to be executed.
If anyone ever wants to modify the image,
he or she can only create a new image based on the old one.
Container
When the image is used and becomes a running environment,
it is the container.
Like VM,
the docker container is also isolated from the host environment.
Whatever is done in the container has nothing to do with the host environment,
unless we have done some special settings.
For example,
we can open a port of a docker container,
but the host port remains closed.
But if we can also expose the port of the host if we want.
Repository
A repository is where we keep the images.
It is like the repository of Git:
There can be lots of repositories,
and each of them is where to place the code of a project.
Likewise,
The repositories of docker are where to place images.
Every image in the same repository has the same name but different tags.
And also,
there are many different repositories for different images.
Registry
A registry is also somewhere to place images.
The difference between registry and repository is,
the registry is a service where people can push their images from or pull images back to their local machine;
like GitHub for Git.
The most famous one is Docker Hub.
While the repository is somewhere to keep images with the same name and different tags.
Basic Usage of Docker
Install
It is really simple in Ubuntu,
sudo apt-get install docker.io
Pull Image
There are lots of images on Docker Hub that can be used.
If I want a clean Ubuntu environment,
I can pull a Ubuntu image back to my local machine:
docker pull ubuntu
Or if I want to specify a tag:
docker pull ubuntu:14.04
Run the Image
We can echo a Hello world using the image we just get:
docker run ubuntu /bin/echo 'Hello world'
Should print Hello world on the terminal.
What just happened is the docker run command creates a temporary container,
terminates itself after the echo command.
List Images at Local
docker images
It should list the image we just pulled to our local machine.
Create a Container
Once we have a container,
we have a running environment that can be changed by what we do.
Creating a container with an image is just like using .iso to create a virtual machine.
We can create a container running Ubuntu with the image we pulled:
docker create -it ubuntu
We can also create a container with a name:
docker create -it --name CONTAINER_NAME ubuntu
i refers to ‘input’ (open stdin of the container).t refers to ’tty’ (so we can access it with terminal).
Or if we want to create a container and run it:
docker run -itd ubuntu
or
docker run -itd --name CONTAINER_NAME ubuntu
d refers to ‘detach’ (let the container runs in the background).
List Containers
docker ps -a
Should be able to list all the containers in the host machine.
And we can see there is a difference in status between container created with docker create and docker run:
Containers created with docker create is only created and have not been running,
so the status is created;
while containers created with docker run is not only created but also run,
so the status is up.
There is a container id that can be used at running the container or terminate it.
Run Containers
So,
if the container is created with docker create then it has to be run before we can access it:
We can use container ID to run the container:
docker start "CONTAINER_ID"
Or,
if the container creates with a name,
we can use the name to run it:
docker start "CONTAINER_NAME"
If the status of the container is exit,
it also has to be started to run before we can access it.
Use
docker ps -a
to check the status first.
As for containers created with docker run,
or the containers have already started to run by docker start,
we can access them with docker exec:
docker exec -it "CONTAINER_ID" bash
bash is the command we want it to run,
it can be replaced with other commands like echo or something else.
We can also use the container’s name to access the container:
docker exec -it "CONTAINER_NAME" bash
If the command used is bash,
we should find ourselves in the container.
The user has become root,
and we can start to do some settings or install something on the container.
If we want to leave the container:
exit
The container is left running in the background after we exit it.
Stop Containers
It is pretty similar to turning off the virtual machine.
Stops a container only changes the status of the container to be exit,
it will not remove the container entirely.
docker stop "CONTAINER_ID"
Or,
docker stop "CONTAINER_NAME"
If we inspect with,
docker ps -a
We will find that the container remains to exist,
only the status has changed to exit.
Export Container
Once a container is exported,
it can be moved to another host machine.
We can export a container to be a .tar file.
For example,
export a container to be exported.tar:
docker export "CONTAINER_ID" > exported.tar
Or,
docker export "CONTAINER_NAME" > exported.tar
Then we can move the .tar file to another machine.