A method of isolating multiple user-space environments using the system kernel instead of a hypervisor
Changes the apparent root directory for the current running process and its children
Uses integrated features included in the Linux Kernel. Basically chroot on steroids.
Two components are required in Linux to make containers work:
veth).Open source project that automates deployments of applications inside of Linux containers
$ yum install docker
$ systemctl start docker
# Hello world
$ docker run centos /bin/echo 'Hello world'
# Interactive container
$ docker run -t -i ubuntu /bin/bash
root@f9a237bdde9f:/#
# Daemonized Hello World
$ docker run -d ubuntu /bin/sh -c "while true; do echo hello world; sleep 1; done"
45637cb38ddcfe4b9693fecd956e865167798dab435c55aae712cf6b83f62ecd
# Show running containers
$ docker ps
# See output from container
$ docker logs <container name>
# Stop container
$ docker stop <container name>
Dockerfiles are configuration files for Docker and have a fairly simple
syntax of the form:
# This is a comment!
INSTRUCTION arguments
If FROM is the first instruction, it will use another container
as a base:
FROM centos:latest
# do more stuff
| Instruction | Explanation |
|---|---|
FROM |
The container to build from the Docker Hub. |
MAINTAINER |
Lets you set the author metadata. |
RUN |
Runs command inside the docker image that is being built. |
CMD |
The command to run for docker run after container is built.
Only one allowed. |
EXPOSE |
Ports to expose for when docker links are being used. Does not expose ports to the host. |
ENV |
Sets environment variables in the container |
| Instruction | Explanation |
|---|---|
ADD |
Copies new files into the container. Allows input to be compressed or urls |
COPY |
Like ADD. No use of urls or compressed archives |
ENTRYPOINT |
Command for docker run to default to; CMD is
appended. |
USER |
User to run all subsequent commands as |
VOLUME |
Creates a mount point with the specified name and marks it as holding externally mounted volumes from native host or other containers |
WORKDIR |
Default working dir for other commands |
ONBUILD |
Trigger when container is used as a base for other containers. |
Lets build an example Dockerfile that serves a simple python-based echo
server.
FROM centos
MAINTAINER cs312@osuosl.org # Change your email here
ADD http://ilab.cs.byu.edu/python/code/echoserver-simple.py /echoserver-simple.py
This is a good start, but we should also:
FROM centos
MAINTAINER cs312@osuosl.org # Change your email here
ADD http://ilab.cs.byu.edu/python/code/echoserver-simple.py /echoserver-simple.py
EXPOSE 50000
CMD ["python", "/echoserver-simple.py"]
$ docker build -t cs312/echo .
$ docker run -d -p 50000:50000 cs312/echo
$ yum install nc
$ nc localhost 50000
foo
foo
What happens when our server reboots? We lose our container! Lets fix this by adding a systemd unit file:
[Unit]
Description=echo service
BindsTo=echo.service
[Service]
ExecStartPre=-/bin/docker kill echo
ExecStartPre=-/bin/docker rm echo
ExecStart=/bin/docker run --name echo -p 50000:50000 cs312/echo
ExecStop=/bin/docker stop echo