Docker Tutorial: Using Containers (Simple Example)

If you’re new to Docker, take a look at some of the basic commands you should learn to start managing containers.

Docker has had a huge impact on the software development lifecycle, making large-scale software deployment simple and secure. This tutorial will cover the basics of running, starting, stopping, and deleting Docker containers.

Docker makes it easy to use different programming languages ​​on different operating systems, all on the same host.

Deploying your software behind Docker becomes much easier and you don’t have to worry about missing out on system configurations or prerequisites.

Docker and Virtual Machines

If you are using virtualization to run your software, why do you need Docker instead?

The main difference between them is that Docker is a separate process that runs within your native operating system, whereas a VM is a complete isolated operating system that runs on top of the host operating system and takes more time to load. So Docker has more advantages than virtual machines, such as:

• Loading speed is different from virtual machines and requires very few hardware resources.
• Run multiple Docker containers simultaneously on the same operating system.
• You can modify the container and deploy it, or give the Docker file definition to a friend to work in the same environment.

In fact, Docker is not a replacement for virtual machines, but solves specific problems.

Suppose your application requires 3 or more services running on different operating systems, then you can run 3 containers smoothly on the same host instead of running 3 virtual machines on the same host. Sounds great!

Run your container

Before you begin, make sure you have Docker properly installed and ready to accept commands. Type the following command in a new terminal window:

$ docker -v

The above command outputs the Docker version installed on your PC:

Docker version 17.12.0-ce-rc2, build f9cde63

It's time to start running the container:

$ docker container run alpine echo "Hello World"

When you run the above command for the first time, you should see output similar to this in your terminal window:

That's easy, isn't it? Try running the same command again:

$ docker container run alpine echo "Hello World"

The second, third, or nth time you run the above command, you should just see this output in your terminal:

Hello World

Now that you have successfully run the container, it’s time to analyze what exactly happened. See the following commands:

$ docker container run alpine echo "Hello World"

This command consists of several parts. First, you have the word "docker". This is the name of the Docker command-line interface (CLI) used to interact with the Docker Engine, which is responsible for running containers.

Next, you have the word “container,” which indicates the context you’re working with.

The next step is to actually execute the command run.

Now, you need to tell Docker which container to run. Here, the alpine container is running.

Finally, you need to define the type of process or task that should be executed within the container when the container is run. This is the last part of the command, echo "Hello World".

Running a process inside a container

Now that you understand the parts of the command to run a container, try running a different process in another container:
$ docker container run centos ping -c 5 127.0.0.1

The output is as follows:

In the preceding example, the container image used is CentOS, and the process executed inside the CentOS container is ping -c 5 127.0.0.1, which pings the loopback address five times until it stops.

The first line is as follows:

Unable to find image 'centos:latest' locally

This tells you that Docker did not find an image called centos:latest in the system’s local cache. Therefore, Docker knows that it must pull it from some image repository where the container is stored.

By default, the Docker environment is configured to pull images from the Docker Hub at hub.docker.com. This is represented by the second line as follows:

latest: Pulling from library/centos

The next three lines of output are as follows:

85432449fd0f: Pull completeDigest: sha256:3b1a65e9a05...Status:

This tells you that Docker has successfully pulled the image centos:latest from Docker Hub.

Subsequent output is generated by the process running in the container, in this case the ping tool.

You may also notice that the keyword latest appears a few times. Each image has a version (also called a tag), and if a version is not explicitly specified, Docker automatically assumes it is the latest version.

If you run this container again on your system, the previous five lines will not be output because Docker will have cached the container image locally so it doesn’t have to download it first. Try to see if this is the case.

Run a random reference container

In order to run the random statement container, an algorithm for generating random statements is required. The API for generating these random statements can be found here [1].
The goal now is to run a process in the container that generates a random statement every 5 seconds and outputs it to STDOUT:

Press Ctrl+C to stop the script. This is the output:

Each response is a JSON-formatted string containing the quote, the author, and its category.
Now, let’s run this container in the background. To do this, you need to shrink the previous script into one line and execute it using /bin/sh -c "...". The expression for Docker is as follows:

$ docker container run -d --name quotes alpine \ /bin/sh -c "while :; do wget -qO- https://talaikis.com/api/quotes/random; printf '\n'; sleep 5; done"

In the above expression, you used two command line parameters, -d and --name. -d tells Docker to run the container as a Linux daemon. The -name parameter is used to specify an explicit name for the container.

If you do not specify an explicit container name, Docker will automatically assign a random but unique name to the container. The name will consist of the name of a famous scientist and an adjective.

Such as, "boring_borg" or "angry_goldberg". Quite humorous, isn't it?

An important aspect is that container names must be unique. Make sure the quotes container is up and running:

$ docker container ls -l

The important part of the previous output is the STATUS column, which in this case shows UP 16 seconds. This means the container has been up and running for 16 seconds.

List Container

As you continue to run containers over time, your system may end up with many containers. To find the containers currently running on the host, you can use the container ls command as follows:

$ docker container ls

This will list all currently running containers.
By default, Docker outputs seven columns, with the following meanings:

If you want to list all containers defined on your system, you can use the command line parameter -a or -all, as follows:

$ docker container ls -a

This will list containers in any state, whether created, running, or exited.

Sometimes, you may just want to list the IDs of all containers. For this you have the -q parameter:

$ docker container ls -q

You might be wondering what this is useful for. Here is an example:

$ docker container rm -f $(docker container ls -a -q)

The command above removes all containers currently defined on the system, including stopped containers. The rm command stands for remove and will be explained further in this tutorial.
In the previous section, you used the -l parameter with the list command. Try using Docker help to find out what the -l parameter stands for. You can invoke help for the list command as follows:
$ docker container ls -h

Stopping and starting containers

Sometimes, you may need to temporarily stop a running container. Try this container:

$ docker container run -d --name quotes alpine \ /bin/sh -c "while :; do wget -qO- https://talaikis.com/api/quotes/random; printf '\n'; sleep 5; done"

Now you can stop this container with the following command:

$ docker container stop quotes

When you try to pause the container, you may notice that it takes a while (about 10 seconds) to complete. Why is this happening? Docker sends the Linux SIGTERM signal to the main process running inside the container.

In the above command, the name of the container is used to specify the container to be stopped. A container ID can also be used.

How do you get the container ID?

There are several ways to do this. The manual method is to list all running containers and find the container you are looking for in the list. Just copy its ID from there.
A more automated approach is to use shell scripts and environment variables. For example, if you want to get the ID of a quote container, here's an example:

$ export CONTAINER_ID = $(docker container ls | grep quotes | awk '{print $1}')

Here we use AWK to get the first field, which is the container ID. Now, instead of using the container name, you can use the $CONTAINER_ID variable in your expression:

$ docker container stop $CONTAINER_ID

Once the container is stopped, its status changes to Exited.

You can restart a stopped container using the docker container start command.

Removing a container

When you run the docker container ls -a command, you can see a lot of containers in the "Exited" state.

If you no longer need these containers, it is best to remove them from memory; otherwise, they will take up valuable resources. The command to delete the container is as follows:

$ docker container rm <container ID>

Alternatively, you can use this command:

$ docker container rm <container name>

Sometimes it is not possible to delete a running container; if you want to force the deletion, you can use the command line parameter -f or --force.
Containerization has changed the way the industry operates, reducing maintenance costs by more than 50% and shortening time to market by approximately 90%. Additionally, containers make applications more secure than running outside of containers.

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