- Change the order in which the environment variables from the .env f…

.env.demo

@@ -7,3 +7,6 @@ DOT.TED=dotted
 SHARP=sharp after a var # is not a comment
 # This is a most likely override
 SHELL=invalid
+YOUR_NAME=Felix
+GREETING=Hello ${YOUR_NAME}!
+TEST_HOME_PATH=${HOME}

.gitignore

@@ -13,3 +13,4 @@
 
 # Dependency directories (remove the comment below to include it)
 # vendor/
+.vscode/settings.json

Makefile

@@ -1,2 +1,4 @@
 demo:
 	LOCAL=demo go run . -f .env.demo env | sort
+demo-convert:
+	LOCAL=demo2 go run . -f .env.demo env | sort | sed -E 's/(.*)/-e \1/' | tr '\n' ' '

main.go

@@ -18,12 +18,14 @@ const (
 	CommentRx = `^[\s]*#`
 	// NameRx is much tighter than Posix, which accepts anything but NUL and '=',
 	// but laxer than shells, which do not accept dots. Names are assumed to be pre-trimmed.
-	NameRx = `^[[:alpha:]][-._a-zA-Z0-9]*`
+	NameRx          = `^[[:alpha:]][-._a-zA-Z0-9]*`
+	EnvKeyReplaceRx = `\$\{[^}]+\}`
 )
 
 var (
-	commentRx = regexp.MustCompile(CommentRx)
-	nameRx    = regexp.MustCompile(NameRx)
+	commentRx       = regexp.MustCompile(CommentRx)
+	nameRx          = regexp.MustCompile(NameRx)
+	envKeyReplaceRx = regexp.MustCompile(EnvKeyReplaceRx)
 )
 
 type env map[string]string
@@ -76,6 +78,28 @@ func (e env) Merge(f env) env {
 	return res
 }
 
+// replaces ${ENV_KEY} placeholder in env values
+func (e env) ReplaceEnvKeys() env {
+	res := make(env, len(e))
+	for k, v := range e {
+		res[k] = envKeyReplaceRx.ReplaceAllStringFunc(v, func(part string) string {
+			plen := len(part)
+			if plen > 3 {
+				envKey := part[2 : plen-1]
+				if envPart, ok := e[envKey]; ok {
+					return envPart
+				} else {
+					log.Printf(`Replacing the variable failed, env key is unknown: "%s"`, part)
+				}
+				return part
+			}
+			log.Printf(`Replacing the variable failed, env key is empty: "%s"`, part)
+			return part
+		})
+	}
+	return res
+}
+
 func readCloser(args []string) (io.ReadCloser, *flag.FlagSet, error) {
 	if len(args) < 2 {
 		return nil, nil, errors.New("need at least a command to run")
@@ -138,7 +162,9 @@ func main() {
 	}()
 
 	env := envFromReader(rc)
-	env = env.Merge(envFromEnv())
+	osEnv := envFromEnv()
+	env = osEnv.Merge(env)
+	env = env.ReplaceEnvKeys()
 	toRun := fs.Args()
 	// Length checked during readCloser().
 	name := toRun[0]