add initIfSet option

README.md

@@ -105,6 +105,7 @@ Here are all the options available for the `env` tag:
 - `,expand`: expands environment variables, e.g. `FOO_${BAR}`
 - `,file`: instructs that the content of the variable is a path to a file that should be read
 - `,init`: initialize nil pointers
+- `,initIfSet`: initialize nil pointers when a value is found in the environment.
 - `,notEmpty`: make the field errors if the environment variable is empty
 - `,required`: make the field errors if the environment variable is not set
 - `,unset`: unset the environment variable after use

env.go

@@ -371,7 +371,7 @@ func doParseField(
 		return err
 	}
 
-	if params.Init && isStructPtr(refField) && refField.IsNil() {
+	if isStructPtr(refField) && refField.IsNil() && shouldInit(params, optionsWithEnvPrefix(refTypeField, opts)) {
 		refField.Set(reflect.New(refField.Type().Elem()))
 		refField = refField.Elem()
 
@@ -391,6 +391,18 @@ func doParseField(
 	return nil
 }
 
+func shouldInit(fieldParams FieldParams, opts Options) bool {
+	if fieldParams.Init {
+		return true
+	}
+
+	if fieldParams.InitIfSet {
+		return hasValue(opts)
+	}
+
+	return false
+}
+
 func isSliceOfStructs(refTypeField reflect.StructField, opts Options) bool {
 	field := refTypeField.Type
 	if reflect.Ptr == field.Kind() {
@@ -533,6 +545,7 @@ type FieldParams struct {
 	NotEmpty        bool
 	Expand          bool
 	Init            bool
+	InitIfSet       bool
 }
 
 func parseFieldParams(field reflect.StructField, opts Options) (FieldParams, error) {
@@ -567,6 +580,8 @@ func parseFieldParams(field reflect.StructField, opts Options) (FieldParams, err
 			result.Expand = true
 		case "init":
 			result.Init = true
+		case "initIfSet":
+			result.InitIfSet = true
 		default:
 			return FieldParams{}, newNoSupportedTagOptionError(tag)
 		}
@@ -619,6 +634,16 @@ func get(fieldParams FieldParams, opts Options) (val string, err error) {
 	return val, err
 }
 
+// hasValue checks if the struct has any values in the environment variables.
+func hasValue(opts Options) bool {
+	for key, _ := range opts.Environment {
+		if strings.HasPrefix(key, opts.Prefix) {
+			return true
+		}
+	}
+	return false
+}
+
 // split the env tag's key into the expected key and desired option, if any.
 func parseKeyForOption(key string) (string, []string) {
 	opts := strings.Split(key, ",")

env_test.go

@@ -2214,3 +2214,22 @@ func TestParseWithOptionsRenamedPrefix(t *testing.T) {
 	isNoErr(t, Parse(cfg))
 	isEqual(t, "101", cfg.Foo.Str)
 }
+
+func TestInitIfSet(t *testing.T) {
+	type Test struct {
+		Str string `env:"TEST"`
+	}
+	type ComplexConfig struct {
+		Foo *Test `envPrefix:"FOO_" env:",init"`
+		Bar *Test `envPrefix:"BAR_" env:",initIfSet"`
+		Baz *Test `envPrefix:"BAZ_" env:",initIfSet"`
+	}
+
+	t.Setenv("BAR_TEST", "lel")
+
+	cfg := ComplexConfig{}
+	isNoErr(t, Parse(&cfg))
+	isEqual(t, &Test{}, cfg.Foo)
+	isEqual(t, &Test{Str: "lel"}, cfg.Bar)
+	isEqual(t, nil, cfg.Baz)
+}