Hello world in a container
So what’s this Docker thing all about?
Docker allows you to run applications, worlds you create, inside containers. Running an application inside a container takes a single command: docker run
.
Note: Depending on your Docker system configuration, you may be required to preface each
docker
command on this page withsudo
. To avoid this behavior, your system administrator can create a Unix group calleddocker
and add users to it.
Run a Hello world
Let’s run a hello world container.
$ docker run ubuntu /bin/echo 'Hello world' Hello world
You just launched your first container!
In this example:
docker run
runs a container.ubuntu
is the image you run, for example the Ubuntu operating system image. When you specify an image, Docker looks first for the image on your Docker host. If the image does not exist locally, then the image is pulled from the public image registry Docker Hub./bin/echo
is the command to run inside the new container.
The container launches. Docker creates a new Ubuntu environment and executes the /bin/echo
command inside it and then prints out:
Hello world
So what happened to the container after that? Well, Docker containers only run as long as the command you specify is active. Therefore, in the above example, the container stops once the command is executed.
Run an interactive container
Let’s specify a new command to run in the container.
$ docker run -t -i ubuntu /bin/bash root@af8bae53bdd3:/#
In this example:
-
docker run
runs a container. -
ubuntu
is the image you would like to run. -
-t
flag assigns a pseudo-tty or terminal inside the new container. -
-i
flag allows you to make an interactive connection by grabbing the standard in (STDIN
) of the container. -
/bin/bash
launches a Bash shell inside our container.
The container launches. We can see there is a command prompt inside it:
root@af8bae53bdd3:/#
Let’s try running some commands inside the container:
root@af8bae53bdd3:/# pwd / root@af8bae53bdd3:/# ls bin boot dev etc home lib lib64 media mnt opt proc root run sbin srv sys tmp usr var
In this example:
-
pwd
displays the current directory, the/
root directory. -
ls
displays the directory listing of the root directory of a typical Linux file system.
Now, you can play around inside this container. When completed, run the exit
command or enter Ctrl-D to exit the interactive shell.
root@af8bae53bdd3:/# exit
Note: As with our previous container, once the Bash shell process has finished, the container stops.
Start a daemonized Hello world
Let’s create a container that runs as a daemon.
$ docker run -d ubuntu /bin/sh -c "while true; do echo hello world; sleep 1; done" 1e5535038e285177d5214659a068137486f96ee5c2e85a4ac52dc83f2ebe4147
In this example:
-
docker run
runs the container. -
-d
flag runs the container in the background (to daemonize it). -
ubuntu
is the image you would like to run.
Finally, we specify a command to run:
/bin/sh -c "while true; do echo hello world; sleep 1; done"
In the output, we do not see hello world
but a long string:
1e5535038e285177d5214659a068137486f96ee5c2e85a4ac52dc83f2ebe4147
This long string is called a container ID. It uniquely identifies a container so we can work with it.
Note: The container ID is a bit long and unwieldy. Later, we will cover the short ID and ways to name our containers to make working with them easier.
We can use this container ID to see what’s happening with our hello world
daemon.
First, let’s make sure our container is running. Run the docker ps
command. The docker ps
command queries the Docker daemon for information about all the containers it knows about.
$ docker ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 1e5535038e28 ubuntu /bin/sh -c 'while tr 2 minutes ago Up 1 minute insane_babbage
In this example, we can see our daemonized container. The docker ps
returns some useful information:
-
1e5535038e28
is the shorter variant of the container ID. -
ubuntu
is the used image. - the command, status, and assigned name
insane_babbage
.
Note: Docker automatically generates names for any containers started. We’ll see how to specify your own names a bit later.
Now, we know the container is running. But is it doing what we asked it to do? To see this we’re going to look inside the container using the docker logs
command.
Let’s use the container name insane_babbage
.
$ docker logs insane_babbage hello world hello world hello world . . .
In this example:
-
docker logs
looks inside the container and returnshello world
.
Awesome! The daemon is working and you have just created your first Dockerized application!
Next, run the docker stop
command to stop our detached container.
$ docker stop insane_babbage insane_babbage
The docker stop
command tells Docker to politely stop the running container and returns the name of the container it stopped.
Let’s check it worked with the docker ps
command.
$ docker ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
Excellent. Our container is stopped.
Next steps
So far, 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.
Now, you have the basis learn more about Docker and how to do some more advanced tasks. Go to “Run a simple application“ to actually build a web application with the Docker client.
Please login to continue.