See these pages for more details about Secrets:
https://kubernetes.io/docs/concepts/configuration/secret/
https://kubernetes.io/docs/tasks/inject-data-application/distribute-credentials-secure/
https://kubernetes.io/docs/tasks/configmap-secret/managing-secret-using-kubectl/

Secrets allows creating data objects that can be consumed by Pods at runtime.

Here's an example of a Secret (YAML):

apiVersion: v1
data:
  key1: dmFsdWUx
  key2: dmFsdWUy
kind: Secret
metadata:
  name: secretexample
type: Opaque

The manifest file of the Secret is pretty simple. It contains the fields apiVersion, kind, and metadata. It also contain the data field that contains the keys-values of the Secret.

The data in Secrets is stored as key-value entries:

• Key: is a string value formed by alphanumeric, dot (.), dash (-), and underscore (_) characters.
  
  
• Value: is a base64-encoded string value.
  
  

The key and the value, in the Secrets, are separated with a colon (key:value).

The name of the Secrets must be a valid DNS subdomain name.

The data stored in a Secrets can be consumed in a container in one the following ways:

• Environment variables.
  
  
• Command-line arguments of the container command.
  
  
• Files in volumes.
  
  
• Custom code (read the Secrets directly from the Kubernetes API).
  
  

To encode a text into a base64 string:

$ echo -n 'value1' | base64

dmFsdWUx

The -n flag ensures that the generated output doesn't have an extra newline character at the end of the text. This is to avoid that the extra newline character gets encoded along with the text.

To decode a base64 data:

$ echo 'dmFsdWUx' | base64 --decode

value1

Note: If you update a Secret that was already posted to the Kubernetes API server, then Pods that were already created will be able to leverage the new data only if it was injected as files in a volume. Secrets data injected as environment variables won't be updated in a running Pod unless you recreate it.

The data of the Secrets can be specified using one of the following methods:

• generic --from-literal:
  This command allows you to specify a key and literal value to insert in the Secret.
  
  You need to escape the special characters $, \, *, =, and ! (otherwise they will be interpreted by the shell).
  
  One way to escape the special characters is to enclose the value within single quotes (').
  
  Create the Secrets:
  

  $ kubectl create secret generic secretfromliteral --from-literal="key1=value1" --from-literal=key2='value2'

  secret/secretfromliteral created

  Check the Secret:
  

  $ kubectl get secret secretfromliteral

  NAME                TYPE     DATA   AGE
  secretfromliteral   Opaque   2      9m5s

  Describe the Secret:
  

  $ kubectl describe secret secretfromliteral

  Name:         secretfromliteral
  Labels:       <none>
  Annotations:  <none>
  
  Type:  Opaque
  
  Data
  ====
  key1:  6 bytes
  key2:  6 bytes

  View the YAML file of the Secret:
  

  $ kubectl get secrets secretfromliteral -o yaml

  apiVersion: v1
  data:
    key1: dmFsdWUx
    key2: dmFsdWUy
  kind: Secret
  metadata:
    name: secretfromliteral
  type: Opaque

  To decode the Secret of a specific key, you need to decode its base64 data:
  

  $ kubectl get secrets secretfromliteral -o jsonpath='{.data.key1}' | base64 --decode -

  value1

  $ kubectl get secrets secretfromliteral -o jsonpath='{.data.key2}' | base64 --decode -

  value2

  
  
• generic --from-file:
  This command allows you to specify a file path (--from-file="./secrets/file1.txt"), in which case the file base name will be used as the Secret key.
  
  Optionally you can use a key with the file path (--from-file="mykey1=./secrets/file1.txt"), in which case the given key will be used as the Secret key.
  
  If you specify a directory (--from-file="./secrets/") then kubectl will iterate each file in the directory and create a new entry for each file whose base name is a valid Secret key.
  
  The path to the file (or directory) can be an absolute path or relative to the location from where the "kubectl create secret" is executed.
  
  The content of the file can be anything. You don't have to escape special characters in the text that you put in the file.
  
  Let's crate a directory to hold the Secrets data (this can be any location you want):
  

  $ mkdir ./secrets/

  Create a sample file "file1.txt" (single line text):
  

  $ echo -n 'value1' > "./secrets/file1.txt"

  Create a sample file "file2.txt" (multiple lines text):
  

  $ echo 'value2-1' > "./secrets/file2.txt"
  $ echo 'value2-2' >> "./secrets/file2.txt"

  • Create the Secret from a file without a key:
    

    $ kubectl create secret generic secret1fromfile --from-file="./secrets/file1.txt" --from-file="./secrets/file2.txt"

    secret/secret1fromfile created

    View the Secrets (notice that the content of the file is indented properly):
    

    $ kubectl get secret secret1fromfile -o yaml

    apiVersion: v1
    data:
      file1.txt: dmFsdWUx # note that the name of the key is file1.txt
      file2.txt: dmFsdWUyLTEKdmFsdWUyLTIK # note that the name of the key is file2.txt
    kind: Secret
    metadata:
      name: secret1fromfile
    type: Opaque

  • Create the Secret from a file with a key:
    

    $ kubectl create secret generic secret2fromfile --from-file="mykey1=./secrets/file1.txt" --from-file="mykey2=./secrets/file2.txt"

    secret/secret2fromfile created

    View the Secrets (notice that the content of the file is indented properly):
    

    $ kubectl get secret secret2fromfile -o yaml

    apiVersion: v1
    data:
      mykey1: dmFsdWUx # note that the name of the key is mykey1
      mykey2: dmFsdWUyLTEKdmFsdWUyLTIK # note that the name of the key is mykey2
    kind: Secret
    metadata:
      name: secret2fromfile
    type: Opaque

  • Create the Secret from a directory:
    

    $ kubectl create secret generic secretfromdirectory --from-file="./secrets/"

    secret/secretfromdirectory created

    View the Secrets (notice that the content of the file is indented properly):
    

    $ kubectl get secret secretfromdirectory -o yaml

    apiVersion: v1
    data:
      file1.txt: dmFsdWUx # note that the name of the key is file1.txt
      file2.txt: dmFsdWUyLTEKdmFsdWUyLTIK # note that the name of the key is file2.txt
    kind: Secret
    metadata:
      name: secretfromdirectory
    type: Opaque

  
  
• --from-env-file:
  This command allows you to specify the path to an environement (properties) file (--from-file="./secrets/file1.properties") to read lines of key=value pairs to insert in the Secret.
  
  Create a sample property file:
  

  $ vi "./secrets/file1.properties"

  key1=value1
  key2=value2

  Create the Secret:
  

  $ kubectl create secret generic secretfromenvfile --from-env-file="./secrets/file1.properties"

  secret/secretfromenvfile created

  View the Secret (notice that each entry in the property file is represented by a key:value pairs in the Secret):
  

  $ kubectl get secret secretfromenvfile -o yaml

  apiVersion: v1
  data:
    key1: dmFsdWUx
    key2: dmFsdWUy
  kind: Secret
  metadata:
    name: secretfromenvfile
  type: Opaque

The manifest YAML file defines Secrets with key:value entries.

The values must be base64 encoded strings.

$ vi secretkeyvalue.yaml

apiVersion: v1
kind: Secret
metadata:
  name: secretkeyvalue
data:
  key1: dmFsdWUx
  key2: dmFsdWUy

To apply the file:

$ kubectl apply -f secretkeyvalue.yaml

secret/secretkeyvalue created

If you have created a Secret using the manifest YAML file, then to change that Secret you can adjust the YAML file and run the command kubectl apply -f SECRET-YAML-FILE-PATH

You can also use the command kubectl edit secret SECRET-NAME to edit a Secret. The command will open the Secret in your default text editor. Make your changes then save and exit the editor. Your changes will be submitted to API Server.

For example to view and edit the Secret "secretkeyvalue":

$ kubectl edit secret secretkeyvalue

# Please edit the object below.
# Lines beginning with a '#' will be ignored, and an empty file will abort the edit.
# If an error occurs while saving this file will be reopened with the relevant failures.
#
apiVersion: v1
data:
  key1: dmFsdWUx
  key2: dmFsdWUy
kind: Secret
metadata:
  annotations:
    kubectl.kubernetes.io/last-applied-configuration: |
      {"apiVersion":"v1","data":{"key1":"dmFsdWUx","key2":"dmFsdWUy"},"kind":"Secret","metadata":{"annotations":{},"name":"secretkeyvalue"}}
  name: secretkeyvalue
type: Opaque

secret/secretkeyvalue edited

In a Pod YAML file, You can reference the entries of the Secrets directly in the "spec.containers.env" field. The environment variables will be set like any other standard Linux environment variables. You can check them using the env command directly from inside the container.

• env/valueFrom/secretKeyRef
  
  Let's use the Secret "secretkeyvalue" (see above) with the Pod "hello-busybox-secret-keyref":
  

  $ vi hello-busybox-secret-keyref.yaml

  apiVersion: v1
  kind: Pod
  metadata:
    name: hello-busybox-secret-keyref
  spec:
    containers:
      - name: hello-busybox-secret-keyref
        image: busybox:latest
        tty: true # (TeleTYpewriter) allocates a TTY / terminal console on the container
        stdin: true # Keeps stdin open on the container
        env:
          - name: mykey1
            valueFrom:
              secretKeyRef:
                name: secretkeyvalue
                key: key1
          - name: mykey2
            valueFrom:
              secretKeyRef:
                name: secretkeyvalue
                key: key2

  Note: You can give the environment variable a different name than the Secrets key name (i.e. name: mykey1).
  
  Apply the Pod:
  

  $ kubectl apply -f hello-busybox-secret-keyref.yaml

  pod/hello-busybox-secret-keyref created

  Check the environment variables:
  

  $ kubectl exec hello-busybox-secret-keyref -- env | grep key

  mykey1=value1
  mykey2=value2

  Note that the secrets are clear text!
  
  
• envFrom/secretRef
  
  Instead of referencing individual entries of the Secret, you can use the "spec.containers.envFrom" field to reference all entries of the Secret:
  

  $ vi hello-busybox-secret-ref.yaml

  apiVersion: v1
  kind: Pod
  metadata:
    name: hello-busybox-secret-ref
  spec:
    containers:
      - name: hello-busybox-secret-ref
        image: busybox:latest
        tty: true
        stdin: true
        envFrom:
          - secretRef:
              name: secretkeyvalue

  Apply the Pod:
  

  $ kubectl apply -f hello-busybox-secret-ref.yaml

  pod/hello-busybox-secret-ref created

  Check the environment variables:
  

  $ kubectl exec hello-busybox-secret-ref -- env | grep key

  key1=value1
  key2=value2

Secrets can also be referenced using volumes.

Each entry of the Secrets will be referenced as a separate file in the volume.

The changes to the Secrets will be reflect directly in the volumes.

The Secrets is defined as a volume (spec.volumes) and mounted to the container (spec.containers.volumeMounts).

• In the following example, the "spec.volumes" field creates a volume (named secretkeyvalue) from the "secretkeyvalue" Secrets.
  The "spec.containers.volumeMounts" field mounts the volume (secretkeyvalue) into the container (under /tmp/secretkeyvalue).
  The volume will be populated with files where the file name is the key of the Secrets (key1, key2) and the content of the file is the value of the key (value1, value2).
  
  

  $ vi hello-busybox-secret-volume.yaml

  apiVersion: v1
  kind: Pod
  metadata:
    name: hello-busybox-secret-volume
  spec:
    containers:
      - name: hello-busybox-secret-volume
        image: busybox:latest
        tty: true
        stdin: true
        volumeMounts:
          - name: secretkeyvalue
            mountPath: /tmp/secretkeyvalue
    volumes:
      - name: secretkeyvalue
        secret:
          secretName: secretkeyvalue

  Apply the Pod:
  

  $ kubectl apply -f hello-busybox-secret-volume.yaml

  pod/hello-busybox-secret-volume created

  Check the files in the volume (notice two files are created: key1, key2):
  

  $ kubectl exec hello-busybox-secret-volume -- ls -l /tmp/secretkeyvalue

  lrwxrwxrwx    1    root    root    key1 -> ..data/key1
  lrwxrwxrwx    1    root    root    key2 -> ..data/key2

  Check the content of the files:
  

  $ kubectl exec hello-busybox-secret-volume -- cat /tmp/secretkeyvalue/key1

  value1

• You can also decide what entries of the Secret can be mounted.
  
  You can also decide the names of the files in the volume "path".
  
  

  $ vi hello-busybox-secret-volume-item.yaml

  apiVersion: v1
  kind: Pod
  metadata:
    name: hello-busybox-secret-volume-item
  spec:
    containers:
      - name: hello-busybox-secret-volume-item
        image: busybox:latest
        tty: true
        stdin: true
        volumeMounts:
          - name: secretkeyvalue
            mountPath: /tmp/secretkeyvalue
    volumes:
      - name: secretkeyvalue
        secret:
          secretName: secretkeyvalue
          items:
            - key: key1
              path: key1path

  Apply the Pod:
  

  $ kubectl apply -f hello-busybox-secret-volume-item.yaml

  pod/ello-busybox-secret-volume-item created

  Check the files in the volume (notice the "key1path" file was created):
  

  $ kubectl exec hello-busybox-secret-volume-item -- ls -l /tmp/secretkeyvalue

  lrwxrwxrwx    1    root    root    key1path -> ..data/key1path

  Check the content of the files:
  

  $ kubectl exec hello-busybox-secret-volume-item -- cat /tmp/secretkeyvalue/key1path

  value1

You can use the data of the Secret to set the arguments of the container command.

You need to use the following syntax to reference the environment variable: $(ENV_VAR_NAME).

As previously mentioned, changes to the Secret won't be reflected on the environment variables already set.

$ vi hello-busybox-secret-keyref-arg.yaml

apiVersion: v1
kind: Pod
metadata:
  name: hello-busybox-secret-keyref-arg
spec:
  containers:
    - name: hello-busybox-secret-keyref-arg
      image: busybox:latest
      command: ['sh', '-c', 'echo "mykey1:$(mykey1) | mykey2:$(mykey2)"; sleep 300;']
      env:
        - name: mykey1
          valueFrom:
            secretKeyRef:
              name: secretkeyvalue
              key: key1
        - name: mykey2
          valueFrom:
            secretKeyRef:
              name: secretkeyvalue
              key: key2

Apply the Pod:

$ kubectl apply -f hello-busybox-secret-keyref-arg.yaml

pod/hello-busybox-secret-keyref-arg created

Check the logs of the Pod:

$ kubectl logs hello-busybox-secret-keyref-arg

mykey1:value1 | mykey2:value2

To delete a Secret: using manifest file (kubectl delete):

$ kubectl delete -f secretkeyvalue.yaml

secret "secretkeyvalue" deleted

To delete a Secret using its name:

$ kubectl delete secret secretkeyvalue

secret "secretkeyvalue" deleted