moby--moby/docs/sources/introduction/working-with-docker.md

page_title: Working with Docker and the Dockerfile page_description: Working with Docker and The Dockerfile explained in depth page_keywords: docker, introduction, documentation, about, technology, understanding, Dockerfile

Working with Docker and the Dockerfile

How to use and work with Docker?

Warning! Don't let this long page bore you. If you prefer a summary and would like to see how a specific command works, check out the glossary of all available client commands on our User's Manual: Commands Reference.

Introduction

On the last page, Understanding the Technology, we covered the components that make up Docker and learnt about the underlying technology and how everything works.

Now, it is time to get practical and see how to work with the Docker client, Docker containers and images and the Dockerfile.

Note: You are encouraged to take a good look at the container, image and Dockerfile explanations here to have a better understanding on what exactly they are and to get an overall idea on how to work with them. On the next page (i.e., Get Docker), you will be able to find links for platform-centric installation instructions.

Elements of Docker

As we mentioned on the, Understanding the Technology page, the main elements of Docker are:

• Containers;
• Images, and;
• The Dockerfile.

Note: This page is more practical than technical. If you are interested in understanding how these tools work behind the scenes and do their job, you can always read more on Understanding the Technology.

Working with the Docker client

In order to work with the Docker client, you need to have a host with the Docker daemon installed and running.

How to use the client

The client provides you a command-line interface to Docker. It is accessed by running the docker binary.

Tip: The below instructions can be considered a summary of our interactive tutorial. If you prefer a more hands-on approach without installing anything, why not give that a shot and check out the Docker Interactive Tutorial.

The docker client usage consists of passing a chain of arguments:

# Usage:  [sudo] docker [option] [command] [arguments] ..
# Example:
docker run -i -t ubuntu /bin/bash

Our first Docker command

Let's get started with our first Docker command by checking the version of the currently installed Docker client using the docker version command.

# Usage: [sudo] docker version
# Example:
docker version

This command will not only provide you the version of Docker client you are using, but also the version of Go (the programming language powering Docker).

Client version: 0.8.0
Go version (client): go1.2

Git commit (client): cc3a8c8
Server version: 0.8.0

Git commit (server): cc3a8c8
Go version (server): go1.2

Last stable version: 0.8.0

Finding out all available commands

The user-centric nature of Docker means providing you a constant stream of helpful instructions. This begins with the client itself.

In order to get a full list of available commands run the docker binary:

# Usage: [sudo] docker
# Example:
docker

You will get an output with all currently available commands.

Commands:
     attach    Attach to a running container
     build     Build a container from a Dockerfile
     commit    Create a new image from a container's changes
. . .

Command usage instructions

The same way used to learn all available commands can be repeated to find out usage instructions for a specific command.

Try typing Docker followed with a [command] to see the instructions:

# Usage: [sudo] docker [command] [--help]
# Example:
docker attach
Help outputs . . .

Or you can pass the --help flag to the docker binary.

docker images --help

You will get an output with all available options:

Usage: docker attach [OPTIONS] CONTAINER

Attach to a running container

  --no-stdin=false: Do not attach stdin
  --sig-proxy=true: Proxify all received signal to the process (even in non-tty mode)

Working with images

Docker Images

As we've discovered a Docker image is a read-only template that we build containers from. Every Docker container is launched from an image and you can use both images provided by others, for example we've discovered the base ubuntu image provided by Docker, as well as images built by others. For example we can build an image that runs Apache and our own web application as a starting point to launch containers.

Searching for images

To search for Docker image we use the docker search command. The docker search command returns a list of all images that match your search criteria together with additional, useful information about that image. This includes information such as social metrics like how many other people like the image - we call these "likes" stars. We also tell you if an image is trusted. A trusted image is built from a known source and allows you to introspect in greater detail how the image is constructed.

# Usage: [sudo] docker search [image name]
# Example:
docker search nginx

NAME                                     DESCRIPTION                                     STARS     OFFICIAL   TRUSTED
dockerfile/nginx                         Trusted Nginx (http://nginx.org/) Build         6                    [OK]
paintedfox/nginx-php5                    A docker image for running Nginx with PHP5.     3                    [OK]
dockerfiles/django-uwsgi-nginx           Dockerfile and configuration files to buil...   2                    [OK]
. . .

Note: To learn more about trusted builds, check out this blog post.

Downloading an image

Downloading a Docker image is called pulling. To do this we hence use the docker pull command.

# Usage: [sudo] docker pull [image name]
# Example:
docker pull dockerfile/nginx

Pulling repository dockerfile/nginx
0ade68db1d05: Pulling dependent layers
27cf78414709: Download complete
b750fe79269d: Download complete
. . .

As you can see, Docker will download, one by one, all the layers forming the final image. This demonstrates the building block philosophy of Docker.

Listing available images

In order to get a full list of available images, you can use the docker images command.

# Usage: [sudo] docker images
# Example:
docker images

REPOSITORY          TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
myUserName/nginx    latest              a0d6c70867d2        41 seconds ago      578.8 MB
nginx               latest              173c2dd28ab2        3 minutes ago       578.8 MB
dockerfile/nginx    latest              0ade68db1d05        3 weeks ago         578.8 MB

Working with containers

Docker Containers

Docker containers are directories on your Docker host that are built from Docker images. In order to create or start a container, you need an image. This could be the base ubuntu image or an image built and shared with you or an image you've built yourself.

Running a new container from an image

The easiest way to create a new container is to run one from an image.

# Usage: [sudo] docker run [arguments] ..
# Example:
docker run -d --name nginx_web nginx /usr/sbin/nginx

This will create a new container from an image called nginx which will launch the command /usr/sbin/nginx when the container is run. We've also given our container a name, nginx_web.

Containers can be run in two modes:

• Interactive;
• Daemonized;

An interactive container runs in the foreground and you can connect to it and interact with it. A daemonized container runs in the background.

A container will run as long as the process you have launched inside it is running, for example if the /usr/bin/nginx process stops running the container will also stop.

Listing containers

We can see a list of all the containers on our host using the docker ps command. By default the docker ps commands only shows running containers. But we can also add the -a flag to show all containers - both running and stopped.

# Usage: [sudo] docker ps [-a]
# Example:
docker ps

CONTAINER ID        IMAGE                     COMMAND             CREATED             STATUS              PORTS                NAMES
842a50a13032        dockerfile/nginx:latest   nginx               35 minutes ago      Up 30 minutes       0.0.0.0:80->80/tcp   nginx_web

Stopping a container

You can use the docker stop command to stop an active container. This will gracefully end the active process.

# Usage: [sudo] docker stop [container ID]
# Example:
docker stop nginx_web
nginx_web

If the docker stop command succeeds it will return the name of the container it has stopped.

Starting a Container

Stopped containers can be started again.

# Usage: [sudo] docker start [container ID]
# Example:
docker start nginx_web
nginx_web

If the docker start command succeeds it will return the name of the freshly started container.

Working with the Dockerfile

The Dockerfile holds the set of instructions Docker uses to build a Docker image.

Tip: Below is a short summary of our full Dockerfile tutorial. In order to get a better-grasp of how to work with these automation scripts, check out the Dockerfile step-by-step tutorial.

A Dockerfile contains instructions written in the following format:

# Usage: Instruction [arguments / command] ..
# Example:
FROM ubuntu

A # sign is used to provide a comment:

# Comments ..

Tip: The Dockerfile is very flexible and provides a powerful set of instructions for building applications. To learn more about the Dockerfile and it's instructions see the Dockerfile Reference.

First steps with the Dockerfile

It's a good idea to add some comments to the start of your Dockerfile to provide explanation and exposition to any future consumers, for example:

#
# Dockerfile to install Nginx
# VERSION 2 - EDITION 1

The first instruction in any Dockerfile must be the FROM instruction. The FROM instruction specifies the image name that this new image is built from, it is often a base image like ubuntu.

# Base image used is Ubuntu:
FROM ubuntu

Next, we recommend you use the MAINTAINER instruction to tell people who manages this image.

# Maintainer: O.S. Tezer <ostezer at gmail com> (@ostezer)
MAINTAINER O.S. Tezer, ostezer@gmail.com

After this we can add additional instructions that represent the steps to build our actual image.

Our Dockerfile so far

So far our Dockerfile will look like.

# Dockerfile to install Nginx
# VERSION 2 - EDITION 1
FROM ubuntu
MAINTAINER O.S. Tezer, ostezer@gmail.com

Let's install a package and configure an application inside our image. To do this we use a new instruction: RUN. The RUN instruction executes commands inside our image, for example. The instruction is just like running a command on the command line inside a container.

RUN echo "deb http://archive.ubuntu.com/ubuntu/ raring main universe" >> /etc/apt/sources.list
RUN apt-get update
RUN apt-get install -y nginx
RUN echo "\ndaemon off;" >> /etc/nginx/nginx.conf

We can see here that we've run four instructions. Each time we run an instruction a new layer is added to our image. Here's we've added an Ubuntu package repository, updated the packages, installed the nginx package and then echo'ed some configuration to the default /etc/nginx/nginx.conf configuration file.

Let's specify another instruction, CMD, that tells Docker what command to run when a container is created from this image.

CMD /usr/sbin/nginx

We can now save this file and use it build an image.

Using a Dockerfile

Docker uses the Dockerfile to build images. The build process is initiated by the docker build command.

# Use the Dockerfile at the current location
# Usage: [sudo] docker build .
# Example:
docker build -t="my_nginx_image" .

Uploading context 25.09 kB
Uploading context
Step 0 : FROM ubuntu
  ---> 9cd978db300e
Step 1 : MAINTAINER O.S. Tezer, ostezer@gmail.com
  ---> Using cache
  ---> 467542d0cdd3
Step 2 : RUN echo "deb http://archive.ubuntu.com/ubuntu/ raring main universe" >> /etc/apt/sources.list
  ---> Using cache
  ---> 0a688bd2a48c
Step 3 : RUN apt-get update
 ---> Running in de2937e8915a
. . .
Step 10 : CMD /usr/sbin/nginx
  ---> Running in b4908b9b9868
  ---> 626e92c5fab1
Successfully built 626e92c5fab1

Here we can see that Docker has executed each instruction in turn and each instruction has created a new layer in turn and each layer identified by a new ID. The -t flag allows us to specify a name for our new image, here my_nginx_image.

We can see our new image using the docker images command.

docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
my_nginx_img        latest              626e92c5fab1        57 seconds ago      337.6 MB

Where to go from here

Understanding Docker

Visit Understanding Docker in our Getting Started manual.

Learn about parts of Docker and the underlying technology

Visit Understanding the Technology in our Getting Started manual.

Get the product and go hands-on

Visit Get Docker in our Getting Started manual.

Get the whole story

https://www.docker.io/the_whole_story/