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Run a simple application

In the Hello world in a container you launched your first containers using the docker run command. You ran an interactive container that ran in the foreground. You also ran a detached container that ran in the background. In the process you learned about several Docker commands:

  • docker ps - Lists containers.
  • docker logs - Shows us the standard output of a container.
  • docker stop - Stops running containers.

Learn about the Docker client

If you didn’t realize it yet, you’ve been using the Docker client each time you typed docker in your Bash terminal. The client is a simple command line client also known as a command-line interface (CLI). Each action you can take with the client is a command and each command can take a series of flags and arguments.

# Usage:  [sudo] docker [subcommand] [flags] [arguments] ..
# Example:
$ docker run -i -t ubuntu /bin/bash

You can see this in action by using the docker version command to return version information on the currently installed Docker client and daemon.

$ docker version

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

Client:
  Version:      1.8.1
  API version:  1.20
  Go version:   go1.4.2
  Git commit:   d12ea79
  Built:        Thu Aug 13 02:35:49 UTC 2015
  OS/Arch:      linux/amd64

Server:
  Version:      1.8.1
  API version:  1.20
  Go version:   go1.4.2
  Git commit:   d12ea79
  Built:        Thu Aug 13 02:35:49 UTC 2015
  OS/Arch:      linux/amd64

Get Docker command help

You can display the help for specific Docker commands. The help details the options and their usage. To see a list of all the possible commands, use the following:

$ docker --help

To see usage for a specific command, specify the command with the --help flag:

$ docker attach --help

Usage: docker attach [OPTIONS] CONTAINER

Attach to a running container

  --help              Print usage
  --no-stdin          Do not attach stdin
  --sig-proxy=true    Proxy all received signals to the process

Note: For further details and examples of each command, see the command reference in this guide.

Running a web application in Docker

So now you’ve learned a bit more about the docker client you can move onto the important stuff: running more containers. So far none of the containers you’ve run did anything particularly useful, so you can change that by running an example web application in Docker.

For our web application we’re going to run a Python Flask application. Start with a docker run command.

$ docker run -d -P training/webapp python app.py

Review what the command did. You’ve specified two flags: -d and -P. You’ve already seen the -d flag which tells Docker to run the container in the background. The -P flag is new and tells Docker to map any required network ports inside our container to our host. This lets us view our web application.

You’ve specified an image: training/webapp. This image is a pre-built image you’ve created that contains a simple Python Flask web application.

Lastly, you’ve specified a command for our container to run: python app.py. This launches our web application.

Note: You can see more detail on the docker run command in the command reference and the Docker Run Reference.

Viewing our web application container

Now you can see your running container using the docker ps command.

$ docker ps -l

CONTAINER ID  IMAGE                   COMMAND       CREATED        STATUS        PORTS                    NAMES
bc533791f3f5  training/webapp:latest  python app.py 5 seconds ago  Up 2 seconds  0.0.0.0:49155->5000/tcp  nostalgic_morse

You can see you’ve specified a new flag, -l, for the docker ps command. This tells the docker ps command to return the details of the last container started.

Note: By default, the docker ps command only shows information about running containers. If you want to see stopped containers too use the -a flag.

We can see the same details we saw when we first Dockerized a container with one important addition in the PORTS column.

PORTS
0.0.0.0:49155->5000/tcp

When we passed the -P flag to the docker run command Docker mapped any ports exposed in our image to our host.

Note: We’ll learn more about how to expose ports in Docker images when we learn how to build images.

In this case Docker has exposed port 5000 (the default Python Flask port) on port 49155.

Network port bindings are very configurable in Docker. In our last example the -P flag is a shortcut for -p 5000 that maps port 5000 inside the container to a high port (from ephemeral port range which typically ranges from 32768 to 61000) on the local Docker host. We can also bind Docker containers to specific ports using the -p flag, for example:

$ docker run -d -p 80:5000 training/webapp python app.py

This would map port 5000 inside our container to port 80 on our local host. You might be asking about now: why wouldn’t we just want to always use 1:1 port mappings in Docker containers rather than mapping to high ports? Well 1:1 mappings have the constraint of only being able to map one of each port on your local host.

Suppose you want to test two Python applications: both bound to port 5000 inside their own containers. Without Docker’s port mapping you could only access one at a time on the Docker host.

So you can now browse to port 49155 in a web browser to see the application.

Viewing the web application.

Our Python application is live!

Note: If you have been using a virtual machine on OS X, Windows or Linux, you’ll need to get the IP of the virtual host instead of using localhost. You can do this by running the docker-machine ip your_vm_name from your command line or terminal application, for example:

$ docker-machine ip my-docker-vm
192.168.99.100

In this case you’d browse to http://192.168.99.100:49155 for the above example.

A network port shortcut

Using the docker ps command to return the mapped port is a bit clumsy so Docker has a useful shortcut we can use: docker port. To use docker port we specify the ID or name of our container and then the port for which we need the corresponding public-facing port.

$ docker port nostalgic_morse 5000

0.0.0.0:49155

In this case you’ve looked up what port is mapped externally to port 5000 inside the container.

Viewing the web application’s logs

You can also find out a bit more about what’s happening with our application and use another of the commands you’ve learned, docker logs.

$ docker logs -f nostalgic_morse

* Running on http://0.0.0.0:5000/
10.0.2.2 - - [23/May/2014 20:16:31] "GET / HTTP/1.1" 200 -
10.0.2.2 - - [23/May/2014 20:16:31] "GET /favicon.ico HTTP/1.1" 404 -

This time though you’ve added a new flag, -f. This causes the docker logs command to act like the tail -f command and watch the container’s standard out. We can see here the logs from Flask showing the application running on port 5000 and the access log entries for it.

Looking at our web application container’s processes

In addition to the container’s logs we can also examine the processes running inside it using the docker top command.

$ docker top nostalgic_morse

PID                 USER                COMMAND
854                 root                python app.py

Here we can see our python app.py command is the only process running inside the container.

Inspecting our web application container

Lastly, we can take a low-level dive into our Docker container using the docker inspect command. It returns a JSON document containing useful configuration and status information for the specified container.

$ docker inspect nostalgic_morse

You can see a sample of that JSON output.

[{
    "ID": "bc533791f3f500b280a9626688bc79e342e3ea0d528efe3a86a51ecb28ea20",
    "Created": "2014-05-26T05:52:40.808952951Z",
    "Path": "python",
    "Args": [
       "app.py"
    ],
    "Config": {
       "Hostname": "bc533791f3f5",
       "Domainname": "",
       "User": "",
. . .

We can also narrow down the information we want to return by requesting a specific element, for example to return the container’s IP address we would:

$ docker inspect -f '{{range .NetworkSettings.Networks}}{{.IPAddress}}{{end}}' nostalgic_morse

172.17.0.5

Stopping our web application container

Okay you’ve seen web application working. Now you can stop it using the docker stop command and the name of our container: nostalgic_morse.

$ docker stop nostalgic_morse

nostalgic_morse

We can now use the docker ps command to check if the container has been stopped.

$ docker ps -l

Restarting our web application container

Oops! Just after you stopped the container you get a call to say another developer needs the container back. From here you have two choices: you can create a new container or restart the old one. Look at starting your previous container back up.

$ docker start nostalgic_morse

nostalgic_morse

Now quickly run docker ps -l again to see the running container is back up or browse to the container’s URL to see if the application responds.

Note: Also available is the docker restart command that runs a stop and then start on the container.

Removing our web application container

Your colleague has let you know that they’ve now finished with the container and won’t need it again. Now, you can remove it using the docker rm command.

$ docker rm nostalgic_morse

Error: Impossible to remove a running container, please stop it first or use -f
2014/05/24 08:12:56 Error: failed to remove one or more containers

What happened? We can’t actually remove a running container. This protects you from accidentally removing a running container you might need. You can try this again by stopping the container first.

$ docker stop nostalgic_morse

nostalgic_morse

$ docker rm nostalgic_morse

nostalgic_morse

And now our container is stopped and deleted.

Note: Always remember that removing a container is final!

Next steps

Until now you’ve only used images that you’ve downloaded from Docker Hub. Next, you can get introduced to building and sharing our own images.

Go to Working with Docker Images.