Here is a short and sweet guide to Docker
for absolute beginners. I have added a few FAQs as well.
Q. What is a container?
A. Container is an isolated Linux system running on a Linux machine itself. They are lightweight and consume less resources than a virtual machine. They rely on kernels cgroups and namespace features to create isolation for CPU, memory etc..
Q. What is Docker?
A. Docker is a container based platform to build and ship applications. Docker makes containers easy to use by providing a lot of automation and tools for container management.
Q. Why would I use Docker?
A. If you have any of the following concerns then you should use Docker:
- My production needs to be homogeneous
- I need to ship entire environment to my colleague
- My hypervisor ate all the CPU (or RAM)
- .. it works on my machine, but not in production ..
How to play with Docker
Step1: Let us install and run the Docker first:
# yum install docker-io
# systemctl start docker
Step2: Docker has something called registries. A registry stores container images from which we can download and run containers. These registries can be public or private. Docker.io maintains a public registry which is the default if we want to download an image. The command below will download an image with name fedora-busybox, contributed by user adimania:
# docker pull adimania/fedora-busybox
Pulling repository adimania/fedora-busybox
605bfcc0af5d: Download complete
Step3: Let us check out the image that we just downloaded.
# docker images
REPOSITORY TAG IMAGE ID CREATED VIRTUAL SIZE
adimania/fedora-busybox latest 605bfcc0af5d 7 minutes ago 1.309 MB
Step4: Once we have the image, we would want to run a container off it. The command below will take care of that and drop us in the container’s shell:
# docker run -i -t adimania/fedora-busybox /sbin/sh
The run command takes certain parameters and run the image provided as argument. The arguments “-i” and “-t” tells run command to open STDIN and allocate a pseudo-TTY. Last argument is the command that is runs inside the container in foreground. One thing to note here is that docker always need a process to run in foreground. As soon as this process exits, the docker container shuts down. For certain containers, this foreground process is implicit and we may not need to tell docker what to run. However for certain other containers, like the one which we are using, we specify “/sbin/sh” to run as foreground process. docker run command supports several other arguments and flags. It is advisable to fire docker run –help to check out all the options.
Step5: We can see more information about this containers that are currently running by using docker ps command:
# docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
3af04d663b3d adimania/fedora-busybox:latest "/sbin/sh" 25 seconds ago Up 24 seconds furious_leakey
docker ps commands shows all the containers that are running along with other useful info like uptime, foreground command etc.. This command takes an optional argument “-a” which shows all the containers, including the stopped ones.
Step6: Let us stop and start the container again. We’ll need the container id obtained from the docker ps command
# docker stop 3af04d663b3d
# docker start 3af04d663b3d
Above commands are a part of workshop which I have conducted before at Flock and CentOS Dojo. Check out the slides here