Labels

Labels are the mechanism you use to organize Kubernetes objects. A label is a key-value pair with certain restrictions concerning length and allowed values but without any pre-defined meaning. So you’re free to choose labels as you see fit, for example, to express environments such as ‘this pod is running in production’ or ownership, like ‘department X owns that pod’.

Let’s create a pod that initially has one label (env=development):

kubectl apply -f https://raw.githubusercontent.com/openshift-evangelists/kbe/main/specs/labels/pod.yaml

kubectl get pods --show-labels

NAME       READY     STATUS    RESTARTS   AGE    LABELS
labelex    1/1       Running   0          10m    env=development

In above get pods command note the --show-labels option that output the labels of an object in an additional column.

You can add a label to the pod as:

kubectl label pods labelex owner=michael

kubectl get pods --show-labels

NAME        READY     STATUS    RESTARTS   AGE    LABELS
labelex     1/1       Running   0          16m    env=development,owner=michael

To use a label for filtering, for example to list only pods that have an owner that equals michael, use the --selector option:

kubectl get pods --selector owner=michael

NAME      READY     STATUS    RESTARTS   AGE
labelex   1/1       Running   0          27m

The --selector option can be abbreviated to -l, so to select pods that are labelled with env=development, do:

kubectl get pods -l env=development

NAME      READY     STATUS    RESTARTS   AGE
labelex   1/1       Running   0          27m

Oftentimes, Kubernetes objects also support set-based selectors. Let’s launch another pod that has two labels (env=production and owner=michael):

kubectl apply -f https://raw.githubusercontent.com/openshift-evangelists/kbe/main/specs/labels/anotherpod.yaml

Now, let’s list all pods that are either labelled with env=development or with env=production:

kubectl get pods -l 'env in (production, development)'

NAME           READY     STATUS    RESTARTS   AGE
labelex        1/1       Running   0          43m
labelexother   1/1       Running   0          3m

Other verbs also support label selection, for example, you could remove both of these pods with:

kubectl delete pods -l 'env in (production, development)'

Beware that this will destroy any pods with those labels.

You can also delete them directly, via their names, with:

kubectl delete pods labelex

kubectl delete pods labelexother

Note that labels are not restricted to pods. In fact you can apply them to all sorts of objects, such as nodes or services.

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