Working with Docker Container - Labeling and filtering containers

Labeling and filtering containers

With Docker 1.6, a feature has been added to label containers and images, through which we can attach arbitrary key-value metadata to them. You can think of them as environment variables, which are not available to running applications inside containers but they are available to programs (Docker CLI) that are managing images and containers. Labels attached to images also get applied to containers started via them. We can also attach labels to containers while starting them.

Docker also provides filters to containers, images, and events, which we can use in conjunction with labels to narrow down our searches.

For this document, let's assume that we have an image with the label, distro=centos.

root@Docker:~# docker images
REPOSITORY                 TAG                 IMAGE ID                CREATED            SIZE
nskselvan/nsk                   latest              b2f0c17eed23        2 months ago        197MB
centos-latest                     latest              2083898799b1       2 months ago        197MB
wordpress                         latest              224b7eef6944        3 months ago        408MB
mysql                                 5.7                 b4e78b89bcf3        3 months ago        412MB
registry                               2                    28525f9a6e46       3 months ago        33.2MB
localhost:5000/reg            latest               28525f9a6e46       3 months ago        33.2MB
centos                               latest              196e0ce0c9fb        3 months ago        197MB
localhost:5000/centos-ka  latest              196e0ce0c9fb        3 months ago        197MB

As you can see from the preceding screenshot, if we use filters with the docker images command, we only get an image where the corresponding label is found in the image's metadata.

To start the container with the --label/-l option, run the following command:
root@Docker:~# docker run --label environment=test centos date
Thu Feb 15 06:21:09 UTC 2018

Let's start a container without a label and start two others with the same label:
root@Docker:~# docker run --name container1 centos date
Thu Feb 15 06:22:13 UTC 2018
root@Docker:~# docker run --name tcontainer1 --label environment=test centos date
Thu Feb 15 06:23:01 UTC 2018
root@Docker:~# docker run --name tcontainer2 --label environment=test centos date
Thu Feb 15 06:23:20 UTC 2018

If we list all the containers without a label, we will see all the containers, but if we use label, then we get only containers, which matches the label.

Docker attaches label metadata to containers while starting them and matches the label while listing them or other related operations
We can list all the labels attached to a container through the inspect command

root@Docker:~# docker inspect -f '{{.Config.Labels}}' tcontainer1
map[build-date:20170911 environment:test license:GPLv2 name:CentOS Base Image vendor:CentOS]
root@Docker:~# docker inspect -f '{{.Config.Labels}}' tcontainer2
map[license:GPLv2 name:CentOS Base Image vendor:CentOS build-date:20170911 environment:test]