What is the difference between images and containers in docker

The difference between images and containers in docker: 1. An image is a template that contains various environments or services, while a container is an instance of an image; 2. An image cannot be run and is static, while Containers are runnable and dynamic.

The operating environment of this tutorial: linux7.3 system, docker-1.13.1 version, Dell G3 computer.

What is the difference between an image and a container in docker?

1. Docker image

To understand the difference between a Docker image and a Docker container, indeed not easy.

Assuming that the Linux kernel is layer 0, then no matter how you run Docker, it will run on top of the kernel layer. This Docker image is a read-only image, located at layer 1. It cannot be modified or the state cannot be saved.

A Docker image can be built on top of another Docker image, and this cascading relationship can be multi-layered. The image layer of the first layer is called the base image (Base Image), and the images of other layers (except the top layer) are called the parent image (Parent Image). These images inherit all properties and settings from their parent image and add their own configuration in the Dockerfile.

Docker images are identified by image ID. The image ID is a 64-character hexadecimal string. But when we run an image, we usually don't use the image ID to refer to the image, but the image name. To list all valid local images, you can use the command

# docker images

Mirrors can be published as different versions. This mechanism is called Tag.

As shown in the figure above, the neo4j image has two versions: the lastest version and the 2.1.5 version.

You can use the pull command to add the specified label:

# docker pull ubuntu:14.04
# docker pull ubuntu:12.04

2. Docker container

Docker container can be created using the command:

# docker run imagename

It will add a writable layer on top of all image layers. This writable layer has processes running on the CPU and has two different states: Running and Exited. This is the Docker container. When we use docker run to start the container, the Docker container enters the running state. When we stop the Docker container, it enters the exit state.

When we have a running Docker container, all changes we make to it from the running state to the stopped state will be permanently written to the container's file system. Remember that changes to a container are written to the container's file system, not to the Docker image.

We can use the same image to start multiple Docker containers. These containers are all active after startup and are isolated from each other. Changes we make to one of the containers will only be limited to that container itself.

If the underlying image of the container is modified, the currently running container will not be affected and automatic updates will not occur.

If we want to update a container to a new version of its image, we must be careful to ensure that we build the data structure in the correct way, otherwise we may end up losing all the data in the container.

A 64-character hexadecimal string that defines the container ID, which is the unique identifier of the container. The interaction between containers is identified by the container ID. Since the characters of the container ID are too long, we usually only need to type the first 4 characters of the container ID. Of course, we can also use the container name, but it is obviously easier to use a 4-character container ID.

Recommended learning: "docker video tutorial"

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