FakeFS build status

Mocking calls to FileUtils or File means tightly coupling tests with the implementation.

“uby it “creates a directory” do FileUtils.expects(:mkdir).with(“directory”).once Library.add “directory” end

The above test will break if mkdir_p is used instead. Refactoring code should not necessitate refactoring tests.

A better approach is to use a temp directory if you are working with relative directories.

“by require ‘tmpdir’

it “creates a directory” do Dir.mktmpdir do |dir| Dir.chdir dir do Library.add “directory” assert File.directory?(“directory”) end end end

But if you are working with absolute directories or do not want to use temporary directories, use FakeFS instead:

“uby it “creates a directory” do FakeFS do Library.add “directory” assert File.directory?(“directory”) end end


“sh gem install fakefs


To fake out the FS:

“uby require ‘fakefs’

Temporarily faking the FS

“uby require ‘fakefs/safe’


your code



FakeFS do # your code end


In rails projects, add this to your Gemfile:

“uby gem “fakefs”, require: “fakefs/safe”


Include FakeFS::SpecHelpers to turn FakeFS on and off in an example group:

“uby require ‘fakefs/spec_helpers’

describe “my spec” do include FakeFS::SpecHelpers end

See lib/fakefs/spec_helpers.rb for more info.

To use FakeFS within a single test and be guaranteed a fresh fake filesystem: “uby require ‘fakefs/safe’

describe “my spec” do context “my context” do it “does something to the filesystem” FakeFS.with_fresh do # whatever it does end end end end

FakeFs — TypeError: superclass mismatch for class File

pp and fakefs may collide, even if you’re not actually explicitly using pp. Adding require 'pp' before require 'fakefs' should fix the problem locally. For a module-level fix, try adding it to the Gemfile:

“by source “https://rubygems.org”

require ‘pp’

list of gems

The problem may not be limited to pp; any gems that add to File may be affected.

Working with existing files

Clone existing directories or files to reuse them during tests, they are safe to modify.

“by FakeFS do config = File.expand_path(‘../../config’, FILE)

FakeFS::FileSystem.clone(config) expect(File.read(“#config/foo.yml”)).to include(“original-content-of-foo”)

File.write(“#config/foo.yml”), “NEW”) expect(File.read(“#config/foo.yml”)).to eq “NEW” end

Integrating with other filesystem libraries

Third-party libraries may add methods to filesystem-related classes. FakeFS doesn’t support these methods out of the box, but you can define fake versions yourself on the equivalent FakeFS classes. For example, FileMagic adds File#content_type. A fake version can be provided as follows:

“uby FakeFS::File.class_eval do def content_type ‘fake/file’ end end

MockFS comparison

FakeFS provides a test suite and works with symlinks. It’s also strictly a test-time dependency: your actual library does not need to use or know about FakeFS.


FakeFS internally uses the Pathname and FileUtils constants. If you use these in your app, be certain you’re properly requiring them and not counting on FakeFS’ own require.

As of v0.5.0, FakeFS’s current working directory (i.e. Dir.pwd) is independent of the real working directory. Previously if the real working directory were, for example, /Users/donovan/Desktop, then FakeFS would use that as the fake working directory too, even though it most likely didn’t exist. This caused all kinds of subtle bugs. Now the default working directory is the only thing that is guaranteed to exist, namely the root (i.e. /). This may be important when upgrading from v0.4.x to v0.5.x, especially if you depend on the real working directory while using FakeFS.

FakeFS replaces File and FileUtils, but is not a filesystem replacement, so gems that use strange commands or C might circumvent it. For example, the sqlite3 gem will completely ignore any faked filesystem.




Once you’ve made your great commits:

  1. Fork FakeFS
  2. Create a topic branch - git checkout -b my_branch
  3. Push to your branch - git push origin my_branch
  4. Open a Pull Request
  5. That’s it!



  1. bundle exec rake bump:patch or minor/major
  2. bundle exec rake release