diff --git a/.env.demo b/.env.demo
index d485f46..a990266 100644
--- a/.env.demo
+++ b/.env.demo
@@ -7,3 +7,8 @@ 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 ${YOUR_NAME}!
+# Test of the -o Flag, the current directory should be at the beginning of the path
+PATH=.:${PATH}
+TEST_DEFAULT=Hello ${UNKNOWN_NAME|Peter}!
\ No newline at end of file
diff --git a/.gitignore b/.gitignore
index 66fd13c..35e523e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -13,3 +13,4 @@
 
 # Dependency directories (remove the comment below to include it)
 # vendor/
+.vscode/*
diff --git a/Makefile b/Makefile
index 2e609d9..c7881c5 100644
--- a/Makefile
+++ b/Makefile
@@ -1,2 +1,6 @@
 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' ' '
+demo-override:
+	LOCAL=demo2 go run . -o -f .env.demo env | grep PATH
\ No newline at end of file
diff --git a/main.go b/main.go
index bf16c74..8aef5a0 100644
--- a/main.go
+++ b/main.go
@@ -18,14 +18,19 @@ 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 = `\$\{[^}]+\}`
+	VarAndDefaultRx = `([^|]+)(?:\|(.+))?`
 )
 
 var (
-	commentRx = regexp.MustCompile(CommentRx)
-	nameRx    = regexp.MustCompile(NameRx)
+	commentRx       = regexp.MustCompile(CommentRx)
+	nameRx          = regexp.MustCompile(NameRx)
+	envKeyReplaceRx = regexp.MustCompile(EnvKeyReplaceRx)
+	varAndDefaultRx = regexp.MustCompile(VarAndDefaultRx)
 )
 
+type options = map[string]any
 type env map[string]string
 
 func envFromEnv() env {
@@ -76,23 +81,64 @@ func (e env) Merge(f env) env {
 	return res
 }
 
-func readCloser(args []string) (io.ReadCloser, *flag.FlagSet, error) {
+// replaces ${ENV_KEY} keys in env with source env values
+func (e env) ReplaceEnvKeys(source env) 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 {
+				envKeyAndDefault := part[2 : plen-1]
+				mr := varAndDefaultRx.FindStringSubmatch(envKeyAndDefault)
+				envKey := mr[1]
+				if envPart, ok := source[envKey]; ok {
+					return envPart
+				}
+				// default value?
+				if len(mr) > 2 && mr[2] != "" {
+					return mr[2]
+				} else {
+					log.Printf(`Replace variable failed, env key is unknown and no default value was defined: "%s"`, part)
+					return part
+				}
+			} else {
+				log.Printf(`Replacing the variable failed, env key is empty: "%s"`, part)
+				return part
+			}
+		})
+	}
+	return res
+}
+
+func parseArgs(args []string) (options, *flag.FlagSet, error) {
+	opts := make(map[string]any)
 	if len(args) < 2 {
 		return nil, nil, errors.New("need at least a command to run")
 	}
+
 	fs := flag.NewFlagSet(args[0], flag.ContinueOnError)
-	inName := fs.String("f", ".env", "The file from which to read the environment variables")
+	inFile := fs.String("f", ".env", "The file from which to read the environment variables")
+	overrideEnv := fs.Bool("o", false, "Do override existing environment variables with own values.\n"+
+		"But you can still use the original environment variable names as variables inside the .env file.")
 	if err := fs.Parse(args[1:]); err != nil {
-		return nil, nil, fmt.Errorf("failed parsing flags: %w", err)
+		return opts, nil, fmt.Errorf("failed parsing flags: %w", err)
 	}
+	opts["<override_envs>"] = *overrideEnv
+	opts["<env_file_name>"] = *inFile
+
 	if len(fs.Args()) == 0 {
-		return nil, nil, errors.New("no command to run")
+		return opts, nil, errors.New("no command to run")
 	}
-	inFile, err := os.Open(*inName)
+
+	return opts, fs, nil
+}
+
+func readCloser(opts options) (io.ReadCloser, error) {
+	inFile, err := os.Open(opts["<env_file_name>"].(string))
 	if err != nil {
-		return nil, fs, fmt.Errorf("failed reading %s: %v", *inName, err)
+		return nil, fmt.Errorf("failed reading %s: %v", opts["<env_file_name>"], err)
 	}
-	return inFile, fs, nil
+	return inFile, nil
 }
 
 func run(env env, name string, args []string) error {
@@ -126,7 +172,12 @@ func main() {
 		err      error
 		exitCode int
 	)
-	rc, fs, err := readCloser(os.Args)
+
+	opts, fs, err := parseArgs(os.Args)
+	if err != nil {
+		log.Fatal(err)
+	}
+	rc, err := readCloser(opts)
 	if err != nil {
 		log.Fatal(err)
 	}
@@ -138,7 +189,14 @@ func main() {
 	}()
 
 	env := envFromReader(rc)
-	env = env.Merge(envFromEnv())
+	osEnv := envFromEnv()
+	if opts["<override_envs>"].(bool) {
+		env = env.ReplaceEnvKeys(osEnv)
+		env = osEnv.Merge(env)
+	} else {
+		env = env.Merge(osEnv)
+		env = env.ReplaceEnvKeys(env)
+	}
 	toRun := fs.Args()
 	// Length checked during readCloser().
 	name := toRun[0]
@@ -147,6 +205,7 @@ func main() {
 	}
 	exit, ok := err.(*exec.ExitError)
 	if !ok {
+
 		log.Printf("non-exit error running %s: %v", name, err)
 		exitCode = 1
 	}