Working with Docker images
Docker images are used to create Docker containers.
Docker images are the build component of Docker.
Picture credit : Understand the architecture
We create Docker containers using [base] images. An image can be basic, with nothing but the operating-system fundamentals, or it can consist of a sophisticated pre-built application stack ready for launch.
When we build images with docker, each action taken (i.e. a command executed such as apt-get install) forms a new layer on top of the previous one. These base images then can be used to create new containers.
Docker registries hold images.
These are public or private stores from which we can upload or download images.
The public Docker registry is provided with the Docker Hub.
We use "docker run" command from client to tell the Docker daemon to run a container, for example:
$ docker run -it ubuntu:latest /bin/bash
Basically, it's a container "launch" command.
At docker, a Docker client is launched and at run subcommand, a new container will be launched.
The new container will be built from ubuntu base image with "latest" tag.
Here i: interactive, : terminal.
As described in the official document, here are the things happening under the hood:
- Pulls the ubuntu image:
Docker checks for the presence of the ubuntu image and, if it doesn't exist locally on the host, then Docker downloads it from Docker Hub.
If the image already exists, then Docker uses it for the new container.
- Creates a new container:
Once Docker has the image, it uses it to create a container.
- Allocates a filesystem and mounts a read-write layer:
The container is created in the file system and a read-write layer is added to the image.
- Allocates a network and sets up an IP address:
Creates a network interface that allows the Docker container to talk to the local host.
- Executes a process that we specify:
Runs our application.
$ docker search ubuntu NAME DESCRIPTION STARS OFFICIAL AUTOMATED ubuntu Ubuntu is a Debian-based Linux operating s... 4416 [OK] ubuntu-upstart Upstart is an event-based replacement for ... 65 [OK] ...
docker pull pulls an image from registry to local machine.
$ docker pull ubuntu latest: Pulling from ubuntu 20ee58809289: Pull complete f905badeb558: Pull complete 119df6bf2a3a: Pull complete 94d6eea646bc: Pull complete bb4eabee84bf: Pull complete Digest: sha256:85af8b61adffea165e84e47e0034923ec237754a208501fce5dbeecbb197062c Status: Downloaded newer image for ubuntu:latest
Docker images can consist of multiple layers.
In the example above, the image consists of five layers (20ee58809289,...,bb4eabee84bf).
We can use a tag to specify what to download. For example, 'latest' for tag:
$ docker pull ubuntu:latest
To list the images on the host:
docker images REPOSITORY TAG IMAGE ID CREATED VIRTUAL SIZE ubuntu latest bb4eabee84bf 2 weeks ago 124.8 MB ubuntu 16.04 bb4eabee84bf 2 weeks ago 124.8 MB centos 7 2a332da70fd1 9 weeks ago 196.8 MB ubuntu trusty 9bc953763843 10 weeks ago 188 MB debian latest cea663c8c811 10 weeks ago 125.1 MB centos latest ce20c473cd8a 9 months ago 172.3 MB
Now we want to launch a container:
$ docker run -it ubuntu:latest /bin/bash
We're logged in as a root:
root@859d4a27d4c8:/# whoami root
Check the OS:
root@859d4a27d4c8:/# cat /etc/*release DISTRIB_ID=Ubuntu DISTRIB_RELEASE=16.04 ...
Let's check what processes are currently running:
root@859d4a27d4c8:/# top ... PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 1 root 20 0 18232 2032 1556 S 0.0 0.1 0:00.09 bash 14 root 20 0 36628 1704 1272 R 0.0 0.0 0:00.02 top
As we can see there are only two processes, and they are isolated ones from the host processes.
"Ctrl + P + Q" will do the trick:
From the PID, we can check the "top" on the host is using different space from the "top" on the docker container:
k@laptop:~$ pa aux | grep top root 14512 0.0 0.0 36628 1704 pts/17 S+ 22:01 0:00 top
Now, we may want to get back to our container again via "attach". To do that, we need to know the "CONTAINER ID":
k@laptop:~$ docker ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 859d4a27d4c8 ubuntu:latest "/bin/bash" 5 minutes ago Up 5 minutes cranky_swartz
Let's do attach:
k@laptop:~$ docker attach 859d4a27d4c8 root@859d4a27d4c8:/#
We can stop the container and get out of it via "Ctrl + D":
root@859d4a27d4c8:/# exit k@laptop:~$
If we issue "docker ps" command again, we see the container is not running anymore:
k@laptop:~$ docker ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES k@laptop:~$
We can list all containers including old ones that stopped running using "docker ps -a":
k@laptop:~$ docker ps -a CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 859d4a27d4c8 ubuntu:latest "/bin/bash" 31 minutes ago Exited (0) 4 minutes ago cranky_swartz ...
To run a container in background, we use "docker start" command:
k@laptop:~$ docker start cranky_swartz cranky_swartz k@laptop:~$ docker ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 859d4a27d4c8 ubuntu:latest "/bin/bash" 36 minutes ago Up 4 seconds cranky_swartz
We can check what processes are running inside a container using "docker top":
k@laptop:~$ docker top cranky_swartz UID PID PPID C STIME TTY TIME CMD root 15680 1559 0 22:33 pts/17 00:00:00 /bin/bash
To stop a container running in background, we use "docker stop" command:
k@laptop:~$ docker stop cranky_swartz
What's in our system related to the Docker containers?
root@laptop:/# tree /var/lib/docker/containers |-- 859d4a27d4c883db39e68590f1d1d2c340f8775a94fe721eba85111d3b79c1fe │ |-- 859d4a27d4c883db39e68590f1d1d2c340f8775a94fe721eba85111d3b79c1fe-json.log │ |-- config.json │ |-- hostconfig.json │ |-- hostname │ |-- hosts │ |-- resolv.conf │ |-- resolv.conf.hash
Note that there are no binary images for the container which makes docker consumes much less space compare to the other virtual tools.
We can run a container in a "detach" mode, and later we can attach to it:
Note we need to use "-it" so that we can do something with the container after attaching. Also, we used "--name" to give our own name to the container.
k@laptop:~$ docker run -d -it --name=yaong ubuntu:16.04 /bin/bash 6bbba8e00d68a9b9c38bd7fdbd807dae01b9329d3b5ecd7ad2918305743bf5ea
Ok, it's started, and we can check it:
k@laptop:~$ docker ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 6bbba8e00d68 ubuntu:16.04 "/bin/bash" About a minute ago Up About a minute yaong
To stop it:
k@laptop:~$ docker stop 6bbba8e00d68 6bbba8e00d68 k@laptop:~$ docker ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES k@laptop:~$
Ph.D. / Golden Gate Ave, San Francisco / Seoul National Univ / Carnegie Mellon / UC Berkeley / DevOps / Deep Learning / Visualization