利用环境变量（env.value or env.valueFrom）配置容器应用
在 Kubernetes 中使用此类镜像启动容器时，也可以在 Pod 资源或 Pod 模板资源的定义中，为容器配置段使用 env 参数来定义所使用的环境变量列表。
环境变量配置容器化应用时，需要在容器配置段中嵌套使用 env 字段，它的值是一个由环境变量构建的列表。
pod.spec.containers 中嵌套字段 env 和 envFrom 解释如下：
[root@mh-k8s-master-247-10 ~]# kubectl explain pod.spec.containers
KIND: Pod
VERSION: v1
RESOURCE: containers <[]Object>
DESCRIPTION:
List of containers belonging to the pod. Containers cannot currently be
added or removed. There must be at least one container in a Pod. Cannot be
updated.
A single application container that you want to run within a pod.
FIELDS:
args <[]string>
Arguments to the entrypoint. The docker image's CMD is used if this is not
provided. Variable references $(VAR_NAME) are expanded using the
container's environment. If a variable cannot be resolved, the reference in
the input string will be unchanged. The $(VAR_NAME) syntax can be escaped
with a double $$, ie: $$(VAR_NAME). Escaped references will never be
expanded, regardless of whether the variable exists or not. Cannot be
updated. More info:
https://kubernetes.io/docs/tasks/inject-data-application/define-command-argument-container/#running-a-command-in-a-shell
command <[]string>
Entrypoint array. Not executed within a shell. The docker image's
ENTRYPOINT is used if this is not provided. Variable references $(VAR_NAME)
are expanded using the container's environment. If a variable cannot be
resolved, the reference in the input string will be unchanged. The
$(VAR_NAME) syntax can be escaped with a double $$, ie: $$(VAR_NAME).
Escaped references will never be expanded, regardless of whether the
variable exists or not. Cannot be updated. More info:
https://kubernetes.io/docs/tasks/inject-data-application/define-command-argument-container/#running-a-command-in-a-shell
env <[]Object>
List of environment variables to set in the container. Cannot be updated.
envFrom <[]Object>
List of sources to populate environment variables in the container. The
keys defined within a source must be a C_IDENTIFIER. All invalid keys will
be reported as an event when the container is starting. When a key exists
in multiple sources, the value associated with the last source will take
precedence. Values defined by an Env with a duplicate key will take
precedence. Cannot be updated.
……
pod.spec.containers.env 字段定义如下：
[root@mh-k8s-master-247-10 ~]# kubectl explain pod.spec.containers.env
KIND: Pod
VERSION: v1
RESOURCE: env <[]Object>
DESCRIPTION:
List of environment variables to set in the container. Cannot be updated.
EnvVar represents an environment variable present in a Container.
FIELDS:
name <string> -required-
Name of the environment variable. Must be a C_IDENTIFIER.
value <string>
Variable references $(VAR_NAME) are expanded using the previous defined
environment variables in the container and any service environment
variables. If a variable cannot be resolved, the reference in the input
string will be unchanged. The $(VAR_NAME) syntax can be escaped with a
double $$, ie: $$(VAR_NAME). Escaped references will never be expanded,
regardless of whether the variable exists or not. Defaults to "".
valueFrom <Object>
Source for the environment variable's value. Cannot be used if value is not
empty.
[root@mh-k8s-master-247-10 ~]#
pod.spec.containers.envFrom 字段定义如下：
[root@mh-k8s-master-247-10 ~]# kubectl explain pod.spec.containers.envFrom
KIND: Pod
VERSION: v1
RESOURCE: envFrom <[]Object>
DESCRIPTION:
List of sources to populate environment variables in the container. The
keys defined within a source must be a C_IDENTIFIER. All invalid keys will
be reported as an event when the container is starting. When a key exists
in multiple sources, the value associated with the last source will take
precedence. Values defined by an Env with a duplicate key will take
precedence. Cannot be updated.
EnvFromSource represents the source of a set of ConfigMaps
FIELDS:
configMapRef <Object>
The ConfigMap to select from
prefix <string>
An optional identifier to prepend to each key in the ConfigMap. Must be a
C_IDENTIFIER.
secretRef <Object>
The Secret to select from
[root@mh-k8s-master-247-10 ~]#
环境变量通常由 name 和 value（或 valueFrom）字段构成：
-
- name <string>: 环境变量的名称，必选字段。
- value <string>: 环境变量的值，通过 $(VAR_NAME)引用，逃逸格式为 "$$(VAR_NAME)"，默认值为空。
- valueFrom <Object>: 环境变量值的引用源。例如，当前 Pod 资源的名称、名称空间、标签等，不能与非空值的 value 字段同时使用，即环境变量的值要么源于 value 字段，要么源于 valueFrom 字段，二者不可同时提供服数据。
valueFrom 字段可引用的值有多种来源，包括当前 Pod 资源的属性值，容器相关的系统资源配置、ConfigMap 对象中的 Key 以及 Secret 对象中的 Key，它们应分别使用不同的嵌套字段进行定义：
- configMapRef <Object>：ConfigMap 对象中的特定 Key。
- secretKeyRef <Object>: Secret 对象中的特定 Key。