diff --git a/.golangci.yml b/.golangci.yml
index 4aa9f33..9189dc6 100644
--- a/.golangci.yml
+++ b/.golangci.yml
@@ -10,16 +10,6 @@ linters-settings:
     lines: 200
     statements: 120
 
-linters:
-  disable:
-    - gochecknoinits
-    - gocyclo
-    - godox
-    - gocognit
-    - maligned
-    - dupl
-    - gochecknoglobals
-
 issues:
     # List of regexps of issue texts to exclude, empty list by default.
     # But independently from this option we use default exclude patterns,
diff --git a/cmd/envdef/envdef.go b/cmd/envdef/envdef.go
index 4a4ab26..205935d 100644
--- a/cmd/envdef/envdef.go
+++ b/cmd/envdef/envdef.go
@@ -7,23 +7,27 @@ import (
 	"github.com/locona/envdef"
 )
 
-var (
-	defaultSource = ".env.sample"
-	defaultDist   = ".env"
-)
-
 func main() {
+	defaultSource := ".env.sample"
+	defaultDist := ".env"
+
 	var source string
 	flag.StringVar(&source, "s", defaultSource, "source .env file")
 	var dist string
 	flag.StringVar(&dist, "d", defaultDist, "distribution .env file")
+	var overwrite bool
+	flag.BoolVar(&overwrite, "o", false, "Whether to overwrite when the key already exists.")
+
 	flag.Parse()
 
-	result, err := envdef.Diff(source, dist)
+	result, err := envdef.Diff(source, dist, overwrite)
 	if err != nil {
 		log.Fatal(err)
 	}
 
-	result.Write()
+	err = result.Write()
+	if err != nil {
+		log.Fatal(err)
+	}
 	result.Print()
 }
diff --git a/envdef.go b/envdef.go
index 16d72ee..4cef835 100644
--- a/envdef.go
+++ b/envdef.go
@@ -6,10 +6,7 @@ import (
 	"github.com/joho/godotenv"
 )
 
-var iconFormat = "%v %v"
-var envFormat = "%v=%v"
-
-func Diff(source, dist string) (*Result, error) {
+func Diff(source, dist string, overwrite bool) (*Result, error) {
 	var (
 		insertSlice   InsertSlice
 		updateSlice   UpdateSlice
@@ -30,23 +27,27 @@ func Diff(source, dist string) (*Result, error) {
 	for k, v := range sourceEnv {
 		// update
 		if _, ok := distEnv[k]; ok {
-			if v != distEnv[k] {
-				updateSlice = append(updateSlice, fmt.Sprintf(envFormat, k, v))
+			if v == distEnv[k] {
+				// nochange
+				noChangeSlice = append(noChangeSlice, envFormat(k, v))
 				continue
 			}
 
-			noChangeSlice = append(noChangeSlice, fmt.Sprintf(envFormat, k, v))
-
+			if overwrite {
+				updateSlice = append(updateSlice, envFormat(k, v))
+			} else {
+				updateSlice = append(updateSlice, envFormat(k, distEnv[k]))
+			}
 			continue
 		}
 
 		// insert
-		insertSlice = append(insertSlice, fmt.Sprintf(envFormat, k, v))
+		insertSlice = append(insertSlice, envFormat(k, v))
 	}
 
 	for k, v := range distEnv {
 		if _, ok := sourceEnv[k]; !ok {
-			deleteSlice = append(deleteSlice, fmt.Sprintf(envFormat, k, v))
+			deleteSlice = append(deleteSlice, envFormat(k, v))
 		}
 	}
 
@@ -58,6 +59,11 @@ func Diff(source, dist string) (*Result, error) {
 	}, nil
 }
 
+func envFormat(k, v string) string {
+	format := "%v=%v"
+	return fmt.Sprintf(format, k, v)
+}
+
 func Read(path string) (map[string]string, error) {
 	e, err := godotenv.Read(path)
 	if err != nil {
diff --git a/envdef_test.go b/envdef_test.go
index 2928aca..a561878 100644
--- a/envdef_test.go
+++ b/envdef_test.go
@@ -40,13 +40,15 @@ func TestRead(t *testing.T) {
 
 func TestDiff(t *testing.T) {
 	testdata := []struct {
-		source   string
-		dist     string
-		expected Result
+		source    string
+		dist      string
+		overwrite bool
+		expected  Result
 	}{
 		{
-			source: "testdata/.env.sample",
-			dist:   "testdata/.env",
+			source:    "testdata/.env.sample",
+			dist:      "testdata/.env",
+			overwrite: true,
 			expected: Result{
 				InsertSlice:   InsertSlice{"INSERT=insert"},
 				UpdateSlice:   UpdateSlice{"UPDATE=default"},
@@ -54,10 +56,21 @@ func TestDiff(t *testing.T) {
 				NoChangeSlice: NoChangeSlice{"NOCHANGE=nochange"},
 			},
 		},
+		{
+			source:    "testdata/.env.sample",
+			dist:      "testdata/.env",
+			overwrite: false,
+			expected: Result{
+				InsertSlice:   InsertSlice{"INSERT=insert"},
+				UpdateSlice:   UpdateSlice{"UPDATE=update"},
+				DeleteSlice:   DeleteSlice{"DELETE=delete"},
+				NoChangeSlice: NoChangeSlice{"NOCHANGE=nochange"},
+			},
+		},
 	}
 
 	for _, d := range testdata {
-		res, _ := Diff(d.source, d.dist)
+		res, _ := Diff(d.source, d.dist, d.overwrite)
 		// check len
 
 		if len(d.expected.InsertSlice) != len(res.InsertSlice) {
diff --git a/result.go b/result.go
index 2578d3f..b34cb49 100644
--- a/result.go
+++ b/result.go
@@ -39,7 +39,7 @@ func (es InsertSlice) Print() {
 	iconInsert := "+"
 
 	for _, e := range es {
-		color.Yellow(fmt.Sprintf(iconFormat, iconInsert, e))
+		color.Yellow(iconFormat(iconInsert, e))
 	}
 }
 
@@ -49,7 +49,7 @@ func (es UpdateSlice) Print() {
 	iconUpdate := "~"
 
 	for _, e := range es {
-		color.Cyan(fmt.Sprintf(iconFormat, iconUpdate, e))
+		color.Cyan(iconFormat(iconUpdate, e))
 	}
 }
 
@@ -59,7 +59,7 @@ func (es DeleteSlice) Print() {
 	iconDelete := "-"
 
 	for _, e := range es {
-		color.Red(fmt.Sprintf(iconFormat, iconDelete, e))
+		color.Red(iconFormat(iconDelete, e))
 	}
 }
 
@@ -70,3 +70,8 @@ func (es NoChangeSlice) Print() {
 		color.White(e)
 	}
 }
+
+func iconFormat(icon, msg string) string {
+	iconFormat := "%v %v"
+	return fmt.Sprintf(iconFormat, icon, msg)
+}