Chapter 5. Using the docker command and service

The Docker project was responsible for popularizing container development in Linux systems. The original project defined a command and service (both named docker) and a format in which containers are structured. This chapter provides a hands-on approach to using the docker command and service to begin working with containers in Red Hat Enterprise Linux 7 and RHEL Atomic Host by getting and using container images and working with running containers.

If you are interested in more details on how the docker command works, refer to the following:

To get an environment where you can develop Docker containers, you can install a Red Hat Enterprise Linux 7 system to act as a development system as well as a container host. The docker package itself is stored in a RHEL Extras repository (see the Red Hat Enterprise Linux Extras Life Cycle article for a description of support policies and life cycle information for the Red Hat Enterprise Linux Extras channel).

Using the RHEL 7 subscription model, if you want to create container images, you must properly register and entitle the host computer on which you build them. When you use yum install within a container to add packages, the container automatically has access to entitlements available from the RHEL 7 host, so it can get RPM packages from any repository enabled on that host.

With the docker service running, you can obtain some Docker images and use the docker command to begin working with Docker images in RHEL 7.

RHEL Atomic Host is a light-weight Linux operating system distribution that was designed specifically for running containers. It contains two different versions of the docker service, as well as some services that can be used to orchestrate and manage Docker containers, such as Kubernetes. Only one version of the docker service can be running at a time.

Because RHEL Atomic Host is more like an appliance than a full-featured Linux system, it is not made for you to install RPM packages or other software on. Software is added to Atomic Host systems by running container images.

RHEL Atomic Host has a mechanism for updating existing packages, but not for allowing users to add new packages. Therefore, you should consider using a standard RHEL 7 server system to develop your applications (so you can add a full compliment of development and debugging tools), then use RHEL Atomic Host to deploy your containers into a variety of virtualization and cloud environment.

That said, you can install a RHEL Atomic Host system and use it to run, build, stop, start, and otherwise work with containers using the examples shown in this topic. To do that, use the following procedure to get and install RHEL Atomic Host.

Whether you are using the docker service in RHEL Atomic Host or on a RHEL Server, you can change the behavior of the docker service. Ways of changing the behavior of the docker service include:

When the docker daemon starts in RHEL or RHEL Atomic Host, it reads the settings in the /etc/sysconfig/docker file and adds them to the docker daemon command line. See available options by typing the following command:

$ docker daemon --help

The following are a few options you may want to consider adding to your /etc/sysonfig/docker file so that they are picked up when your docker daemon runs.

OPTIONS='--selinux-enabled --log-driver=journald'

OPTIONS='--selinux-enabled --log-driver=journald --signature-verification=false -H tcp://0.0.0.0:2375'

[registries.search]
registries = ["reg1.example.com", "reg2.example.com"]

[registries.insecure]
registries = ["reg3.example.com"]

[registries.block]
registries = ['docker.io']

A Docker registry provides a place to store and share docker containers that are saved as images that can be shared with other people. While you can build and store container images on your local system without installing a registry, or use the Docker Hub Registry to share your images with the world, installing a private registry lets you share your images with a private group of developers or users.

With the registry software available with RHEL and RHEL Atomic Host, you can pull images from the Red Hat Customer Portal and push or pull images to and from your own private registry. You see what images are available to pull from the Red Hat Customer Portal (using docker pull) by searching the Red Hat Container Images Search Page.

This section describes how to start up a local registry, load Docker images to your local registry, and use those images to start up docker containers. The version of the Docker Registry that is currently available with Red Hat Enterprise Linux is Docker Registry 2.0.

# docker pull <registry>[:<port>]/[<namespace>/]<name>:<tag>

# docker pull registry.access.redhat.com/rhel7/rhel

# docker images
REPOSITORY
             TAG        IMAGE ID      CREATED       VIRTUAL SIZE
registry.access.redhat.com/rhel7/rhel
             latest     95612a3264fc  6 weeks ago   203.3 MB
registry.access.redhat.com/rhel7/rhel-tools
             latest     3b7bd2d69242  6 weeks ago   1.219 GB
registry.access.redhat.com/rhel7/cockpit-ws
             latest     3bf463e43334  6 weeks ago   220.1 MB
registry.access.redhat.com/aep3_beta/aep-docker-registry
             latest     3c272743b20a  6 weeks ago   478.5 MB
registry.access.redhat.com/rhel7/etcd
             latest     c0a7c32e9eb9  9 weeks ago   241.7 MB

If images have been pulled or loaded into your local registry, you can use the docker command docker images to view those images. Here’s how to list the images on your local system:

# docker images
REPOSITORY                                   TAG     IMAGE ID     CREATED      VIRTUAL SIZE
registry.access.redhat.com/rhel7/rhel-tools  latest  3b7bd2d69242 6 weeks ago  1.219 GB
registry.access.redhat.com/rhel7/cockpit-ws  latest  3bf463e43334 6 weeks ago  220.1 MB
registry.access.redhat.com/rhel7/rhel        latest  95612a3264fc 6 weeks ago  203.3 MB

Now that you have the docker and docker-distribution services running, with a few containers available, you can start investigating the Docker environment and looking into what makes up a container. Run docker with the version and info options to get a feel for your Docker environment.

docker version: The version option shows which versions of different Docker components are installed.

# docker version
Client:
 Version:         1.10.3
 API version:     1.22
Package version: docker-common-1.10.3-44.el7.x86_64
 Go version:      go1.4.2
 Git commit:      7ffc8ee-unsupported
 Built:           Fri Jun 17 15:27:21 2016
 OS/Arch:         linux/amd64

Server:
 Version:         1.10.3
 API version:     1.22
 Package version: docker-common-1.10.3-44.el7.x86_64
 Go version:      go1.4.2
 Git commit:      7ffc8ee-unsupported
 Built:           Fri Jun 17 15:27:21 2016
 OS/Arch:         linux/amd64

docker info: The info option lets you see the locations of different components, such as how many local containers and images there are, as well as information on the size and location of Docker storage areas.

# docker info
Containers: 9
Images: 25
Server Version: 1.10.3
Storage Driver: devicemapper
 Pool Name: docker-253:0-21214-pool
 Pool Blocksize: 65.54 kB
 Base Device Size: 107.4 GB
 Backing Filesystem:
 Data file: /dev/loop0
 Metadata file: /dev/loop1
 Data Space Used: 5.367 GB
 Data Space Total: 107.4 GB
 Data Space Available: 5.791 GB
 Metadata Space Used: 4.706 MB
 Metadata Space Total: 2.147 GB
 Metadata Space Available: 2.143 GB
 Udev Sync Supported: true
 Deferred Removal Enabled: false
 Deferred Deletion Enabled: false
 Deferred Deleted Device Count: 0
 Data loop file: /var/lib/docker/devicemapper/devicemapper/data
 Metadata loop file: /var/lib/docker/devicemapper/devicemapper/metadata
 Library Version: 1.02.107-RHEL7 (2015-12-01)
Execution Driver: native-0.2
Logging Driver: json-file
Kernel Version: 3.10.0-327.18.2.el7.x86_64
Operating System: Red Hat Enterprise Linux Atomic Host 7.2
CPUs: 1
Total Memory: 1.907 GiB
Name: atomic-7.2-12
ID: JSDA:MGJV:ALYX:N6RC:YXER:M4OJ:GYR2:GYQK:BPZX:GQOA:F476:WLQY

Docker images that are now on your system (whether they have been run or not) can be managed in several ways. The docker run command lets you say which command to run in a container. Once a container is running, you can stop, start, and restart it. You can remove containers you no longer need (in fact you probably want to). Before you run an image, it is a good idea to investigate its contents.

Investigate a container image After you pull an image to your local system and before you run it, it is a good idea to investigate that image. Reasons for investigating an image before you run it include:

Tools (such as openscap) are being integrated with container tools to allow them to scan a container image before you run it. In the mean time, however, you can use docker inspect to get some basic information about what an image does. You also have the option of mounting the image to your host system and using tools from the host to investigate what’s in the image. Here is an example of investigating what a container image does before you run it:

In the near future, look for software scanning features, such as Openscap or Black Duck, to be available for scanning your container images. When they are, you will be able to use the atomic scan command to scan your images.

Running Docker containers

When you execute a docker run command, you essentially spin up and create a new container from a Docker image. That container consists of the contents of the image, plus features based on any additional options you pass on the docker run command line.

The command you pass on the docker run command line sees the inside the container as its running environment so, by default, very little can be seen of the host system. For example, by default, the running applications sees:

If you want to make a directory from the host available to the container, map network ports from the container to the host, limit the amount of memory the container can use, or expand the CPU shares available to the container, you can do those things from the docker run command line. Here are some examples of docker run command lines that enable different features.

EXAMPLE #1 (Run a quick command): This docker command runs the ip addr show eth0 command to see address information for the eth0 network interface within a container that is generated from the RHEL image. Because this is a bare-bones container, we mount the /usr/sbin directory from the RHEL 7 host system for this demonstration (mounting is done by the -v option), because it contains the ip command we want to run. After the container runs the command, which shows the IP address (172.17.0.2/16) and other information about eth0, the container stops and is deleted (--rm).

# docker run -v /usr/sbin:/usr/sbin \
    --rm rhel /usr/sbin/ip addr show eth0
20: eth0:  mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 4e:90:00:27:a2:5d brd ff:ff:ff:ff:ff:ff
    inet 172.17.0.10/16 scope global eth0
       valid_lft forever preferred_lft forever
    inet6 fe80::4c90:ff:fe27:a25d/64 scope link tentative
       valid_lft forever preferred_lft forever

If you feel that this is a container you wanted to keep around and use again, consider assigning a name to it, so you can start it again later by name. For example, I named this container myipaddr:

# docker run -v /usr/sbin:/usr/sbin \
    --name=myipaddr rhel /usr/sbin/ip addr show eth0
20: eth0:  mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 4e:90:00:27:a2:5d brd ff:ff:ff:ff:ff:ff
    inet 172.17.0.10/16 scope global eth0
       valid_lft forever preferred_lft forever
    inet6 fe80::4c90:ff:fe27:a25d/64 scope link tentative
       valid_lft forever preferred_lft forever

# docker start -i myipaddr
22: eth0:  mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 4e:90:00:27:a2:5d brd ff:ff:ff:ff:ff:ff
    inet 172.17.0.10/16 scope global eth0
       valid_lft forever preferred_lft forever
    inet6 fe80::4c90:ff:fe27:a25d/64 scope link tentative
       valid_lft forever preferred_lft forever

EXAMPLE #2 (View the Dockerfile in the container): This is another example of running a quick command to inspect the content of a container from the host. All layered images that Red Hat provides include the Dockerfile from which they are built in /root/buildinfo. In this case you do not need to mount any volumes from the host.

# docker run --rm registry.access.redhat.com/rhel7/rsyslog  ls /root/buildinfo
Dockerfile-rhel7-rsyslog-7.2-21

Now you know what the Dockerfile is called, you can list its contents:

# docker run --rm registry.access.redhat.com/rhel7/rsyslog \
    cat /root/buildinfo/Dockerfile-rhel7-rsyslog-7.2-21
FROM 6c3a84d798dc449313787502060b6d5b4694d7527d64a7c99ba199e3b2df834e
MAINTAINER Red Hat, Inc.
ENV container docker
RUN yum -y update; yum -y install rsyslog; yum clean all

LABEL BZComponent="rsyslog-docker"
LABEL Name="rhel7/rsyslog"
LABEL Version="7.2"
LABEL Release="21"
LABEL Architecture="x86_64"
...

EXAMPLE #3 (Run a shell inside the container): Using a container to launch a bash shell lets you look inside the container and change the contents. Here, I set the name of the container to mybash. The -i creates an interactive session and -t opens a terminal session. Without -i, the shell would open and then exit. Without -t, the shell would stay open, but you wouldn’t be able to type anything to the shell.

Once you run the command, you are presented with a shell prompt and you can start running commands from inside the container:

# docker run --name=mybash -it rhel /bin/bash
[root@49830c4f9cc4/]#

Although there are very few applications available inside the base RHEL image, you can add more software using the yum command. With the shell open inside the container, run the following commands:

[root@49830c4f9cc4/]# cat /etc/redhat-release
Red Hat Enterprise Linux Server release 7.2 (Maipo)
[root@49830c4f9cc4/]# nmap
bash: nmap: command not found
[root@49830c4f9cc4/]# yum install -y nmap
[root@49830c4f9cc4/]# # nmap 192.168.122.1

Starting Nmap 6.40 ( http://nmap.org ) at 2016-05-10 08:55 EDT
Nmap scan report for 192.168.122.1
Host is up (0.00042s latency).
Not shown: 996 filtered ports
PORT     STATE SERVICE
22/tcp   open  ssh
53/tcp   open  domain
5000/tcp open  upnp
...
[root@49830c4f9cc4/]# exit

Notice that the container is a RHEL 7.2 container. The nmap command is not included in the RHEL base image. However, you can install it with yum as shown above, then run it within that container. To leave the container, type exit.

Although the container is no longer running once you exit, the container still exists with the new software package still installed. Use docker ps -a to list the container:

# docker ps -a
CONTAINER ID  IMAGE COMMAND      CREATED        STATUS                    PORTS  NAMES
49830c4f9cc4  rhel  "/bin/bash"  2 minutes ago  Exited (0) 2 seconds ago        mybash
...

You could start that container again using docker start with the -ai options. For example:

# docker start -ai mybash
[root@a0aee493a605/]#

EXAMPLE #4 (Bind mounting log files): One way to make log messages from inside a container available to the host system is to bind mount the host’s /dev/log device inside the container. This example illustrates how to run an application in a RHEL container that is named log_test that generates log messages (just the logger command in this case) and directs those messages to the /dev/log device that is mounted in the container from the host. The --rm option removes the container after it runs.

# docker run --name="log_test" -v /dev/log:/dev/log --rm rhel logger "Testing logging to the host"
# journalctl -b | grep Testing
May 10 09:00:32 atomic-7.2-12 logger[15377]: Testing logging to the host

Investigating from outside of a Docker container

Let’s say you have one or more Docker containers running on your host. To work with containers from the host system, you can open a shell and try some of the following commands.

docker ps: The ps option shows all containers that are currently running:

# docker ps
CONTAINER ID IMAGE       COMMAND     CREATED         STATUS       PORTS    NAMES
0d8b2ded3af0 rhel:latest "/bin/bash" 10 minutes ago  Up 3 minutes          mybash

If there are containers that are not running, but were not removed (--rm option), the containers are still hanging around and can be restarted. The docker ps -a command shows all containers, running or stopped.

# docker ps -a
CONTAINER ID  IMAGE       COMMAND    CREATED      STATUS                PORTS  NAMES
92b7ed0c039b  rhel:latest /bin/bash  2 days ago   Exited (0) 2 days ago        agitated_hopper
eaa96236afa6  rhel:latest /bin/bash  2 days ago   Exited (0) 2 days ago        prickly_newton

See the section "Working with Docker containers" for information on starting, stopping, and removing containers that exist on your system.

docker inspect: To inspect the metadata of an existing container, use the docker inspect command. You can show all metadata or just selected metadata for the container. For example, to show all metadata for a selected container, type:

# docker inspect mybash
[{
        "Labels": {
            "Architecture": "x86_64",
            "Authoritative_Registry": "registry.access.redhat.com",
            "BZComponent": "rhel-server-docker",
            "Build_Host": "rcm-img03.build.eng.bos.redhat.com",
            "Name": "rhel7/rhel",
            "Release": "56",
            "Vendor": "Red Hat, Inc.",
            "Version": "7.2"
    ...

docker inspect --format: You can also use inspect to pull out particular pieces of information from a container. The information is stored in a hierarchy. So to see the container’s IP address (IPAddress under NetworkSettings), use the --format option and the identity of the container. For example:

# docker inspect --format='{{.NetworkSettings.IPAddress}}' mybash
172.17.0.2

Examples of other pieces of information you might want to inspect include .Path (to see the command run with the container), .Args (arguments to the command), .Config.ExposedPorts (TCP or UDP ports exposed from the container), .State.Pid (to see the process id of the container) and .HostConfig.PortBindings (port mapping from container to host). Here’s an example of .State.Pid and .HostConfig.PortBindings:

# docker inspect --format='{{.State.Pid}}' mybash
5007
# docker inspect --format='{{.HostConfig.PortBindings}}' mybash
map[8000/tcp:[map[HostIp: HostPort:8000]]]

Investigating within a running Docker container

To investigate within a running Docker container, you can use the docker exec command. With docker exec, you can run a command (such as /bin/bash) to enter a running Docker container process to investigate that container.

The reason for using docker exec, instead of just launching the container into a bash shell, is that you can investigate the container as it is running its intended application. By attaching to the container as it is performing its intended task, you get a better view of what the container actually does, without necessarily interrupting the container’s activity.

Here is an example using docker exec to look into a running container named myrhel_httpd, then look around inside that container.

The commands just run from the bash shell (running inside the container) show you several things. The container holds a RHEL Server release 7.1 system. The process table (ps -ef) shows that the httpd command is process ID 1 (followed by five other httpd processes), /bin/bash is PID 12 and ps -ef is PID 35. Processes running in the host’s process table cannot be seen from within the container. The container’s file system consumes 414M of the 9.8G available root file system space.

There is no separate kernel running in the container (uname -r shows the host system’s kernel: 3.10.0-229.1.2.el7.x86_64). The rpm -qa command lets you see the RPM packages that are included inside the container. In other words, there is an RPM database inside of the container. Viewing memory (free -m) shows the available memory on the host (although what the container can actually use can be limited using cgroups).

Starting and stopping containers

If you ran a container, but didn’t remove it (--rm), that container is stored on your local system and ready to run again. To start a previously run container that wasn’t removed, use the start option. To stop a running container, use the stop option.

Starting containers: A docker container that doesn’t need to run interactively can start with only the start option and the container ID or name:

# docker start myrhel_httpd
myrhel_httpd

To start a container so you can work with it from the local shell, use the -a (attach) and -i (interactive) options. Once the bash shell starts, run the commands you want inside the container and type exit to kill the shell and stop the container.

# docker start -a -i agitated_hopper
bash-4.2# exit

Stopping containers: To stop a running container that is not attached to a terminal session, use the stop option and the container ID or number. For example:

# docker stop myrhel_httpd
myrhel_httpd

The stop option sends a SIGTERM signal to terminate a running container. If the container doesn’t stop after a grace period (10 seconds by default), docker sends a SIGKILL signal. You could also use the docker kill command to kill a container (SIGKILL) or send a different signal to a container. Here’s an example of sending a SIGHUP signal to a container (if supported by the application, a SIGHUP causes the application to re-read its configuration files):

# docker kill --signal="SIGHUP" myrhel_httpd

Removing containers

To see a list of containers that are still hanging around your system, run the docker ps -a command. To remove containers you no longer need, use the docker rm command, with the container ID or name as an option. Here is an example:

# docker rm goofy_wozniak

You can remove multiple containers on the same command line:

# docker rm clever_yonath furious_shockley drunk_newton

If you want to clear out all your containers, you could use a command like the following to remove all containers (not images) from your local system (make sure you mean it before you do this!):

# docker rm $(docker ps -a -q)

So far we have grabbed some existing docker container images and worked with them in various ways. To make the process of running the exact container you want less manual, you can create a Docker image from scratch or from a container you ran that combines an existing image with some other content or settings.

You can add names to images to make it more intuitive to understand what they contain. Using the docker tag command, you essentially add an alias to the image, that can consist of several parts. Those parts can include:

registryhost/username/NAME:tag

You can add just NAME if you like. For example:

# docker tag 474ff279782b myrhel7

In the previous example, the rhel7 image had a image ID of 474ff279782b. Using docker tag, the name myrhel7 now also is attached to the image ID. So you could run this container by name (rhel7 or myrhel7) or by image ID. Notice that without adding a :tag to the name, it was assigned :latest as the tag. You could have set the tag to 7.2 as follows:

# docker tag 474ff279782b myrhel7:7.2

To the beginning of the name, you can optionally add a user name and/or a registry name. The user name is actually the repository on Docker.io that relates to the user account that owns the repository. Tagging an image with a registry name was shown in the "Tagging Images" section earlier in this document. Here’s an example of adding a user name:

# docker tag 474ff279782b cnegus/myrhel7
# docker images | grep 474ff279782b
rhel7           latest  474ff279782b  7 months ago  139.6 MB
myrhel7         latest  474ff279782b  7 months ago  139.6 MB
myrhel7         7.1     474ff279782b  7 months ago  139.6 MB
cnegus/myrhel7  latest  474ff279782b  7 months ago  139.6 MB

Above, you can see all the image names assigned to the single image ID.

If you want to save a Docker image you created, you can use docker save to save the image to a tarball. After that, you can store it or send it to someone else, then reload the image later to reuse it. Here is an example of saving an image as a tarball:

# docker save -o myrhel7.tar myrhel7:latest

The myrhel7.tar file should now be stored in your current directory. Later, when you ready to reuse the tarball as a container image, you can import it to another docker environment as follows:

# cat myrhel7.tar | docker import - cnegus/myrhel7

To see a list of images that are on your system, run the docker images command. To remove images you no longer need, use the docker rmi command, with the image ID or name as an option. (You must stop any containers using an image before you can remove the image.) Here is an example:

# docker rmi rhel

You can remove multiple images on the same command line:

# docker rmi rhel fedora

If you want to clear out all your images, you could use a command like the following to remove all images from your local registry (make sure you mean it before you do this!):

# docker rmi $(docker images -a -q)

At this point, you should be able to get Red Hat Docker installed with the docker and docker-distribution services working. You should also have one or more Docker images to work with, as well as know how to run containers and build your own images.