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