diff --git a/bun/cmd/check.go b/bun/cmd/check.go
index 4e48959..bb0c072 100644
--- a/bun/cmd/check.go
+++ b/bun/cmd/check.go
@@ -1,6 +1,7 @@
 package cmd
 
 import (
+	"context"
 	"fmt"
 	"os"
 	"sort"
@@ -36,7 +37,8 @@ func preRun(cmd *cobra.Command, args []string) {
 		fmt.Printf("Error while detecting a working directory: %v\n", err.Error())
 		os.Exit(1)
 	}
-	b, err := bun.NewBundle(path)
+	ctx := context.WithValue(context.Background(), "fs", bun.OSFS{})
+	b, err := bun.NewBundle(ctx, path)
 	if err != nil {
 		fmt.Printf("Error while identifying basic bundle parameters: %v\n", err.Error())
 		os.Exit(1)
diff --git a/bundle.go b/bundle.go
index f89717f..0543a09 100644
--- a/bundle.go
+++ b/bundle.go
@@ -1,8 +1,7 @@
 package bun
 
 import (
-	"io"
-	"io/ioutil"
+	"context"
 	"log"
 	"path/filepath"
 	"regexp"
@@ -13,7 +12,7 @@ type Bundle struct {
 	Hosts map[string]Host // IP to Host map
 }
 
-func NewBundle(path string) (Bundle, error) {
+func NewBundle(ctx context.Context, path string) (Bundle, error) {
 	b := Bundle{Hosts: make(map[string]Host)}
 	var err error
 	b.Path, err = filepath.Abs(path)
@@ -21,7 +20,8 @@ func NewBundle(path string) (Bundle, error) {
 		log.Printf("Error occurred while detecting absolute path: %v", err)
 		return b, err
 	}
-	infos, err := ioutil.ReadDir(b.Path)
+	fs := ctx.Value("fs").(FileSystem)
+	infos, err := fs.ReadDir(b.Path)
 	if err != nil {
 		return b, err
 	}
@@ -53,7 +53,8 @@ func NewBundle(path string) (Bundle, error) {
 	return b, nil
 }
 
-// OpenFile opens bundle file.  Caller is responsible for closing the file.
-func (b Bundle) OpenFile(fileType string) (io.ReadCloser, error) {
+// OpenFile opens a file in a root directory of the bundle. The caller is
+// responsible for closing the file.
+func (b Bundle) OpenFile(fileType string) (File, error) {
 	return OpenFile(b.Path, fileType)
 }
diff --git a/file.go b/file.go
index 1b99453..de2c0ec 100644
--- a/file.go
+++ b/file.go
@@ -54,25 +54,26 @@ func GetFileType(typeName string) (FileType, error) {
 }
 
 // OpenFile opens bundle file.  Caller is responsible for closing the file.
-func OpenFile(basePath string, typeName string) (rc io.ReadCloser, err error) {
+func OpenFile(basePath string, typeName string) (file File, err error) {
 	fileType, err := GetFileType(typeName)
 	if err != nil {
 		return
 	}
 	filePath := path.Join(basePath, fileType.Path)
-	rc, err = os.Open(filePath)
+	file, err = os.Open(filePath)
 	if os.IsNotExist(err) {
-		var gzrc io.ReadCloser
-		if gzrc, err = os.Open(filePath + ".gz"); err != nil {
+		var gzfile File
+		if gzfile, err = os.Open(filePath + ".gz"); err != nil {
 			return
 		}
-		if rc, err = gzip.NewReader(gzrc); err != nil {
+		if file, err = gzip.NewReader(gzfile); err != nil {
 			return
 		}
-		rc = struct {
+		file = struct {
 			io.Reader
 			io.Closer
-		}{io.Reader(rc), bulkCloser{rc, gzrc}}
+		}{io.Reader(file),
+			bulkCloser{file, gzfile}}
 	}
 	return
 }
diff --git a/file_system.go b/file_system.go
new file mode 100644
index 0000000..415fb12
--- /dev/null
+++ b/file_system.go
@@ -0,0 +1,45 @@
+package bun
+
+import (
+	"io"
+	"io/ioutil"
+	"os"
+)
+
+type FileSystem interface {
+	// ReadDir reads the directory named by dirname and returns
+	// a list of directory entries sorted by filename.
+	// It's mocking the io/ioutil.ReadDir.
+	ReadDir(string) ([]os.FileInfo, error)
+
+	// Open opens the named file for reading. If successful, methods on
+	// the returned file can be used for reading.
+	// It's partially mocking the os.Open function.
+	Open(string) (File, error)
+
+	// Getwd returns a rooted path name corresponding to the
+	// current directory. If the current directory can be
+	// reached via multiple paths (due to symbolic links),
+	// Getwd may return any one of them.
+	// It's mocking os.Getwd.
+	Getwd() (string, error)
+}
+
+type File interface {
+	io.ReadCloser
+}
+
+// osFS implements FileSystem
+type OSFS struct {
+}
+
+func (osfs OSFS) ReadDir(dirname string) ([]os.FileInfo, error) {
+	return ioutil.ReadDir(dirname)
+}
+func (osfs OSFS) Open(name string) (File, error) {
+	file, err := os.Open(name)
+	return File(file), err
+}
+func (osfs OSFS) Getwd() (string, error) {
+	return os.Getwd()
+}
diff --git a/test/file_system.go b/test/file_system.go
new file mode 100644
index 0000000..15940cd
--- /dev/null
+++ b/test/file_system.go
@@ -0,0 +1,18 @@
+package test
+
+import (
+	"os"
+
+	"github.com/adyatlov/bun/fs"
+)
+
+type FileSystem struct {
+}
+
+func (fs *FileSystem) ReadDir(name string) ([]os.FileInfo, error) {
+	return nil, nil
+}
+
+func (fs *FileSystem) Open(name string) (fs.File, error) {
+	return nil, nil
+}