Golang testing — gotest.tools fs

Published on Sep 14, 2018 by Vincent Demeester.


Let’s continue the gotest.tools serie, this time with the fs package.

Package fs provides tools for creating temporary files, and testing the contents and structure of a directory.

This package is heavily using functional arguments (as we saw in functional arguments for wonderful builders). Functional arguments is, in a nutshell, a combinaison of two Go features : variadic functions (... operation in a function signature) and the fact that func are first class citizen. This looks more or less like that.

type Config struct {}

func MyFn(ops ...func(*Config)) *Config {
        c := &Config{} // with default values
        for _, op := range ops {
        return c

// Calling it
conf := MyFn(withFoo, withBar("baz"))

The fs package has too main purpose :

  1. create folders and files required for testing in a simple manner
  2. compare two folders structure (and content)

Create folder structures

Sometimes, you need to create folder structures (and files) in tests. Doing i/o work takes time so try to limit the number of tests that needs to do that, especially in unit tests. Doing it in tests adds a bit of boilerplate that could be avoid. As stated before :

One of the most important characteristic of a unit test (and any type of test really) is readability. This means it should be easy to read but most importantly it should clearly show the intent of the test. The setup (and cleanup) of the tests should be as small as possible to avoid the noise.

In a test you usually end up using ioutil function to create what you need. This looks somewhat like the following.

path, err := ioutil.TempDir("", "bar")
if err != nil { // or using `assert.Assert`
if err := os.Mkdir(filepath.Join(path, "foo"), os.FileMode(0755)); err != nil {
if err := ioutil.WriteFile(filepath.Join(path, "foo", "bar"), []byte("content"), os.FileMode(0777)); err != nil {
defer os.RemoveAll(path) // to clean up at the end of the test

The fs package intends to help reduce the noise and comes with a bunch function to create folder structure :

  • two main function NewFile and NewDir
  • a bunch of operators : WithFile, WithDir, …
func NewDir(t assert.TestingT, prefix string, ops ...PathOp) *Dir {

func NewFile(t assert.TestingT, prefix string, ops ...PathOp) *File {

The With* function are all satisfying the PathOp interface, making NewFile and NewDir extremely composable. Let’s first see how our above example would look like using the fs package, and then, we’ll look a bit more at the main PathOp function…

dir := fs.NewDir(t, "bar", fs.WithDir("foo",
        fs.WithFile("bar", fs.WithContent("content"), fs.WithMode(os.FileMode(0777))),
defer dir.Remove()

It’s clean and simple to read. The intent is well described and there is not that much of noise. fs functions tends to have sane and safe defaults value (for os.FileMode for example). Let’s list the main, useful, PathOp provided by gotest.tools/fs.

  • WithDir creates a sub-directory in the directory at path.
  • WithFile creates a file in the directory at path with content.
  • WithSymlink creates a symlink in the directory which links to target. Target must be a path relative to the directory.
  • WithHardlink creates a link in the directory which links to target. Target must be a path relative to the directory.
  • WithContent and WWithBytes write content to a file at Path (from a string or a []byte slice).
  • WithMode sets the file mode on the directory or file at path.
  • WithTimestamps sets the access and modification times of the file system object at path.
  • FromDir copies the directory tree from the source path into the new Dir. This is pretty useful when you have a huge folder structure already present in you testdata folder or elsewhere.
  • AsUser changes ownership of the file system object at Path.

Also, note that PathOp being an function type, you can provide your own implementation for specific use-cases. Your function just has to satisfy PathOp signature.

type PathOp func(path Path) error

Compare folder structures

Sometimes, the code you’re testing is creating a folder structure, and you would like to be able to tests that, with the given arguments, it creates the specified structure. fs allows you to do that too.

The package provides a Equal function, which returns a Comparison, that the assert package understand. It works by comparing a Manifest type provided by the test and a Manifest representation of the specified folder.

Equal compares a directory to the expected structured described by a manifest and returns success if they match. If they do not match the failure message will contain all the differences between the directory structure and the expected structure defined by the Manifest.

A Manifest stores the expected structure and properties of files and directories in a file-system. You can create a Manifest using either the functions Expected or ManifestFromDir.

We’re going to focus on the Expected function, as ManifestFromDir does pretty much what you would expected : it takes the specified path, and returns a Manifest that represent this folder.

func Expected(t assert.TestingT, ops ...PathOp) Manifest

Expected is close to NewDir function : it takes the same PathOp functional arguments. This makes creating a Manifest straightforward, as it’s working the same. Any function that satisfy PathOp can be used for Manifest the exact same way you’re using them on fs.NewDir.

There is a few additional functions that are only useful with Manifest :

  • MatchAnyFileContent updates a Manifest so that the file at path may contain any content.
  • MatchAnyFileMode updates a Manifest so that the resource at path will match any file mode.
  • MatchContentIgnoreCarriageReturn ignores cariage return discrepancies.
  • MatchExtraFiles updates a Manifest to allow a directory to contain unspecified files.
path := operationWhichCreatesFiles()
expected := fs.Expected(t,
    fs.WithFile("one", "",
        fs.WithBytes(golden.Get(t, "one.golden")),
            fs.WithFile("config", "", fs.MatchAnyFileContent)),

assert.Assert(t, fs.Equal(path, expected))

The following example compares the result of operationWhichCreatesFiles to the expected Manifest. As you can see it also integrates well with other part of the gotest.tools library, with the golden package in this example.


… that’s a wrap. In my opinion, this is one the most useful package provided by gotest.tools after assert. It allows to create simple or complex folder structure without the noise that usually comes with it.