diff --git a/internal/fn/fn.go b/internal/fn/fn.go
index 7788524..1a13582 100644
--- a/internal/fn/fn.go
+++ b/internal/fn/fn.go
@@ -29,6 +29,13 @@ import (
 	"gopkg.in/yaml.v2"
 )
 
+var (
+	readFile  = os.ReadFile
+	writeFile = os.WriteFile
+	stat      = os.Stat
+	mkdirAll  = os.MkdirAll
+)
+
 func GetUserInputFromStdin() string {
 	var lines []string
 	scanner := bufio.NewScanner(os.Stdin)
@@ -130,7 +137,7 @@ func GetPathContent(src string) ([]byte, error) {
 
 	var content []byte
 	for _, filePath := range filePaths {
-		fileContent, err := os.ReadFile(filePath)
+		fileContent, err := readFile(filePath)
 		if err != nil {
 			return nil, fmt.Errorf("no valid SSH config found in %s: %w", src, err)
 		}
@@ -145,7 +152,7 @@ func Save(dest string, content []byte) error {
 		return err
 	}
 
-	info, err := os.Stat(destDir)
+	info, err := stat(destDir)
 	if err != nil {
 		return fmt.Errorf("can not write to destination file: %v", err)
 	}
@@ -154,7 +161,7 @@ func Save(dest string, content []byte) error {
 		return fmt.Errorf("can not write to destination file: directory %s is not writable", destDir)
 	}
 
-	if err := os.WriteFile(dest, content, 0644); err != nil {
+	if err := writeFile(dest, content, 0644); err != nil {
 		return fmt.Errorf("can not write to destination file: %v", err)
 	}
 	return nil
@@ -173,7 +180,7 @@ func TidyLastEmptyLines(input []byte) []byte {
 }
 
 func ensureDirectory(destDir string) error {
-	info, err := os.Stat(destDir)
+	info, err := stat(destDir)
 	if err == nil {
 		if !info.IsDir() {
 			return fmt.Errorf("can not create destination directory: %s is not a directory", destDir)
@@ -187,7 +194,7 @@ func ensureDirectory(destDir string) error {
 
 	parent := filepath.Dir(destDir)
 	if parent != destDir {
-		if parentInfo, parentErr := os.Stat(parent); parentErr == nil {
+		if parentInfo, parentErr := stat(parent); parentErr == nil {
 			if !parentInfo.IsDir() {
 				return fmt.Errorf("can not create destination directory: parent %s is not a directory", parent)
 			}
@@ -197,7 +204,7 @@ func ensureDirectory(destDir string) error {
 		}
 	}
 
-	if err := os.MkdirAll(destDir, 0755); err != nil {
+	if err := mkdirAll(destDir, 0755); err != nil {
 		return fmt.Errorf("can not create destination directory: %v", err)
 	}
 
diff --git a/internal/fn/fn_errors_test.go b/internal/fn/fn_errors_test.go
new file mode 100644
index 0000000..b804469
--- /dev/null
+++ b/internal/fn/fn_errors_test.go
@@ -0,0 +1,170 @@
+package fn
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestGetPathContentReadFileError(t *testing.T) {
+	tmpDir := t.TempDir()
+	configPath := filepath.Join(tmpDir, "config")
+	if err := os.WriteFile(configPath, []byte("Host example\n  HostName example.com\n"), 0644); err != nil {
+		t.Fatalf("failed to create config file: %v", err)
+	}
+
+	originalReadFile := readFile
+	readFile = func(path string) ([]byte, error) {
+		if path == configPath {
+			return nil, errors.New("read failure")
+		}
+		return originalReadFile(path)
+	}
+	t.Cleanup(func() { readFile = originalReadFile })
+
+	_, err := GetPathContent(tmpDir)
+	if err == nil {
+		t.Fatalf("expected error but got nil")
+	}
+	if !strings.Contains(err.Error(), "no valid SSH config found") {
+		t.Fatalf("unexpected error message: %v", err)
+	}
+}
+
+func TestSaveStatError(t *testing.T) {
+	tmpDir := t.TempDir()
+	dest := filepath.Join(tmpDir, "sub", "file.txt")
+	destDir := filepath.Dir(dest)
+
+	callCount := 0
+	originalStat := stat
+	stat = func(path string) (os.FileInfo, error) {
+		if path == destDir {
+			callCount++
+			if callCount == 2 {
+				return nil, errors.New("stat failure")
+			}
+		}
+		return originalStat(path)
+	}
+	t.Cleanup(func() { stat = originalStat })
+
+	err := Save(dest, []byte("content"))
+	if err == nil {
+		t.Fatalf("expected error but got nil")
+	}
+	if !strings.Contains(err.Error(), "can not write to destination file") {
+		t.Fatalf("unexpected error message: %v", err)
+	}
+}
+
+func TestSaveWriteFileError(t *testing.T) {
+	tmpDir := t.TempDir()
+	dest := filepath.Join(tmpDir, "file.txt")
+
+	originalWriteFile := writeFile
+	writeFile = func(path string, data []byte, perm os.FileMode) error {
+		if path == dest {
+			return errors.New("write failure")
+		}
+		return originalWriteFile(path, data, perm)
+	}
+	t.Cleanup(func() { writeFile = originalWriteFile })
+
+	err := Save(dest, []byte("content"))
+	if err == nil {
+		t.Fatalf("expected error but got nil")
+	}
+	if !strings.Contains(err.Error(), "can not write to destination file") {
+		t.Fatalf("unexpected error message: %v", err)
+	}
+}
+
+func TestEnsureDirectoryExistingFile(t *testing.T) {
+	tmpDir := t.TempDir()
+	destDir := filepath.Join(tmpDir, "existing")
+
+	if err := os.WriteFile(destDir, []byte("content"), 0644); err != nil {
+		t.Fatalf("failed to create file: %v", err)
+	}
+
+	err := ensureDirectory(destDir)
+	if err == nil {
+		t.Fatalf("expected error but got nil")
+	}
+	if !strings.Contains(err.Error(), "is not a directory") {
+		t.Fatalf("unexpected error message: %v", err)
+	}
+}
+
+func TestEnsureDirectoryParentNotDir(t *testing.T) {
+	tmpDir := t.TempDir()
+	parent := filepath.Join(tmpDir, "parent")
+	if err := os.WriteFile(parent, []byte("content"), 0644); err != nil {
+		t.Fatalf("failed to create parent file: %v", err)
+	}
+	destDir := filepath.Join(parent, "child")
+
+	originalStat := stat
+	stat = func(path string) (os.FileInfo, error) {
+		if path == destDir {
+			return nil, os.ErrNotExist
+		}
+		return originalStat(path)
+	}
+	t.Cleanup(func() { stat = originalStat })
+
+	err := ensureDirectory(destDir)
+	if err == nil {
+		t.Fatalf("expected error but got nil")
+	}
+	if !strings.Contains(err.Error(), "parent") {
+		t.Fatalf("unexpected error message: %v", err)
+	}
+}
+
+func TestEnsureDirectoryMkdirError(t *testing.T) {
+	tmpDir := t.TempDir()
+	destDir := filepath.Join(tmpDir, "newdir")
+
+	originalMkdirAll := mkdirAll
+	mkdirAll = func(path string, perm os.FileMode) error {
+		if path == destDir {
+			return errors.New("mkdir failure")
+		}
+		return originalMkdirAll(path, perm)
+	}
+	t.Cleanup(func() { mkdirAll = originalMkdirAll })
+
+	err := ensureDirectory(destDir)
+	if err == nil {
+		t.Fatalf("expected error but got nil")
+	}
+	if !strings.Contains(err.Error(), "can not create destination directory") {
+		t.Fatalf("unexpected error message: %v", err)
+	}
+}
+
+func TestEnsureDirectoryStatUnexpectedError(t *testing.T) {
+	tmpDir := t.TempDir()
+	destDir := filepath.Join(tmpDir, "unexpected")
+
+	originalStat := stat
+	stat = func(path string) (os.FileInfo, error) {
+		if path == destDir {
+			return nil, os.ErrPermission
+		}
+		return originalStat(path)
+	}
+	t.Cleanup(func() { stat = originalStat })
+
+	err := ensureDirectory(destDir)
+	if err == nil {
+		t.Fatalf("expected error but got nil")
+	}
+	if !strings.Contains(err.Error(), "can not create destination directory") {
+		t.Fatalf("unexpected error message: %v", err)
+	}
+}