app.yaml environment variables substitution

AAA README.md

This is a code snippet that reads an "app.yaml" file on stdin; then it:

• seek for env_variables entry in the file
• for each key: value pair, it looks for an environment variables named value
• if it exists, it substitute the value with value found in the environment.

Exemple:

cat app.yaml

runtime: nodejs12

service: default

handlers:
  - url: /fonts
    static_dir: build/fonts
  - url: /img
    static_dir: build/img
  - url: /static/js/(.*)
    static_files: build/static/js/\1
    upload: build/static/js/(.*)
  - url: /static/css/(.*)
    static_files: build/static/css/\1
    upload: build/static/css/(.*)
  - url: /static/media/(.*)
    static_files: build/static/media/\1
    upload: build/static/media/(.*)
  - url: /(.*\.(json|ico|svg|png))$
    static_files: build/\1
    upload: build/.*\.(json|ico|svg|png)$
  - url: /manifest.webmanifest
    static_files: build/manifest.webmanifest
    upload: build/manifest.webmanifest
  - url: /
    static_files: build/index.html
    upload: build/index.html
  - url: /.*
    static_files: build/index.html
    upload: build/index.html

env_variables:
  TERM: TERM
  BLABLA: BLABLA
  TOTO: TITI

export BLABLA=coucou
cat app.yaml | go run .
2022/01/03 17:41:47 variable TITI not found
runtime: nodejs12
service: default
handlers:
- url: /fonts
  static_dir: build/fonts
- url: /img
  static_dir: build/img
- url: /static/js/(.*)
  static_files: build/static/js/\1
  upload: build/static/js/(.*)
- url: /static/css/(.*)
  static_files: build/static/css/\1
  upload: build/static/css/(.*)
- url: /static/media/(.*)
  static_files: build/static/media/\1
  upload: build/static/media/(.*)
- url: /(.*\.(json|ico|svg|png))$
  static_files: build/\1
  upload: build/.*\.(json|ico|svg|png)$
- url: /manifest.webmanifest
  static_files: build/manifest.webmanifest
  upload: build/manifest.webmanifest
- url: /
  static_files: build/index.html
  upload: build/index.html
- url: /.*
  static_files: build/index.html
  upload: build/index.html
env_variables:
  TERM: xterm-256color
  BLABLA: coucou
  TOTO: ""

main.go

package main

import (
	"log"
	"os"

	"gopkg.in/yaml.v2"
)

func main() {
	dec := yaml.NewDecoder(os.Stdin)
	content := yaml.MapSlice{}
	err := dec.Decode(&content)
	if err != nil {
		log.Fatal(err)
	}
	for _, item := range content {
		if item.Key.(string) == "env_variables" {
			keyValue := item.Value.(yaml.MapSlice)
			for i, v := range keyValue {
				val, ok := os.LookupEnv(v.Value.(string))
				if !ok {
					log.Printf("variable %v not found", v.Value)
				}
				keyValue[i].Value = val
			}
		}
	}
	enc := yaml.NewEncoder(os.Stdout)
	enc.Encode(content)
}