Add support for environment variable substitution

[kadel]

You can use environment variables in you Kedge files. Expression is [[ variable_name ]].

fixes #224

TODO:

• move variable substitution logic from pkg/cmd/util.go to its own package
• documentation

move variable substitution logic from pkg/cmd/util.go to its own package

I'm keeping this as it is, for now. We need clean up whole pkg/cmd package. Created #301

[concaf]

This also works for YAML fields and not only values, is that the expected behavior?
e.g.

name: httpd
containers:
- [[ KFIELD ]]: centos/httpd
services:
- name: httpd
  type: NodePort
  ports:
  - port: 8080
    targetPort: 80

and this works - KFIELD=image kedge generate -f docs/examples/simplest/httpd.yaml

[concaf]

Works perfectly for me, code LGTM, feel free to merge 🎉

[concaf]

nit: environment

[kadel]

typo fixed, thx

[kadel]

This also works for YAML fields and not only values, is that the expected behavior?

Yep, that is a side effect of the way how it is implemented.

I wouldn't suggest doing it anywhere but it is possible.

[surajssd]

@kadel @containscafeine

Am I doing anything wrong?

$ kedge version
0.2.0 (f9c6b37)

$ cat file.yaml 
name: httpd
containers:
- image: [[ imagename ]]

$ imagename=foo
$ echo $imagename
foo

$ kedge generate -f .
failed to replace variables: undefined variable(s): imagename

[surajssd]

This worked alright:

$ export imagename=foo
$ echo $imagename
foo

$ kedge generate -f .
---
apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  creationTimestamp: null
  labels:
    app: httpd
  name: httpd
spec:
  strategy: {}
  template:
    metadata:
      creationTimestamp: null
      labels:
        app: httpd
      name: httpd
    spec:
      containers:
      - image: foo
        name: httpd
        resources: {}
status: {}

[surajssd]

$ imagename=foo kedge generate -f .
---
apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  creationTimestamp: null
  labels:
    app: httpd
  name: httpd
spec:
...

This also worked fine!

[kadel]

@kadel @containscafeine

Am I doing anything wrong?

$ kedge version
0.2.0 (f9c6b37)

$ cat file.yaml
name: httpd
containers:

• image: [[ imagename ]]

$ imagename=foo
$ echo $imagename
foo

$ kedge generate -f .
failed to replace variables: undefined variable(s): imagename

This is OK it shouldn't work. This is how env variables work in bash.

If you don't export it the variable is not passed to any child processes of given shell.

imagename=foo ./kedge

Above works because In this case variable is directly passed to kedge process.

export imagename=foo
 ./kedge

Above is also working because export is used and that means that any child process of given shell will also see this variable

But following doesn't work as kedge process is a new child process of given shell, and it doesn't receive unexported variables.

imagename=foo 
./kedge

it works with echo, because echo is build in bash function.

[surajssd]

@kadel thanks for that explanation!

This LGTM!