MemFs provides a fake file system that can be used for tests. Strongly inspired by FakeFS.
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spec Fixing FileUtils specs for Ruby 2.5 (fixes #27) Feb 6, 2018
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README.md

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MemFs is an in-memory filesystem that can be used for your tests.

When you're writing code that manipulates files, directories, symlinks, you need to be able to test it without touching your hard drive. MemFs is made for it.

MemFs is intended for tests but you can use it for any other scenario needing in memory file system.

MemFs is greatly inspired by the awesome FakeFs.

The main goal of MemFs is to be 100% compatible with the Ruby libraries like FileUtils.

For French people, the answer is yes, the joke in the name is intended ;)

Take a look

Here is a simple example of MemFs usage:

MemFs.activate!
File.open('/test-file', 'w') { |f| f.puts "hello world" }
File.read('/test-file') #=> "hello world\n"
MemFs.deactivate!

File.exists?('/test-file') #=> false

# Or with the block syntax

MemFs.activate do
  FileUtils.touch('/test-file', verbose: true, noop: true)
  File.exists?('/test-file') #=> true
end

File.exists?('/test-file') #=> false

Why you may prefer MemFs over FakeFS?

While FakeFS is pretty cool it overrides classes like FileUtils. This kind of override is problematic when you rely on real behavior from this kind of tool.

For instance, trying to test the following with FakeFS will not work, the noop option will be ignored:

FileUtils.touch('somefile.txt', noop: true)

MemFs tries to be compliant with the Ruby API by overriding only the low level classes (C classes) like File, Dir or File::Stat leaving the stdlib classes untouched and still working, being less intrusive that way.

Some stdlib classes may be overriden at some point if they don't use File or Dir, like Pathname, etc.

Another key point is that MemFs aims to implement every single method provided by Ruby classes (when possible) and to behave and return exactly the same way as the original classes.

Installation

Add this line to your application's Gemfile:

gem 'memfs'

And then execute:

$ bundle

Or install it yourself as:

$ gem install memfs

Usage in tests

Global activation

Add the following to your spec_helper.rb:

RSpec.configure do |config|
  config.before do
    MemFs.activate!
  end

  config.after do
    MemFs.deactivate!
  end
end

All the spec will be sandboxed in MemFs.

If you want to set it globally with flag activation, you can do the following in you spec_helper.rb file:

Rspec.configure do |c|
  c.around(:each, memfs: true) do |example|
    MemFs.activate { example.run }
  end
end

And then write your specs like this:

it "creates a file", memfs: true do
  subject.create_file('test.rb')
  expect(File.exists?('test.rb')).to be true
end

Local activation

You can choose to activate MemFs only for a specific test:

describe FileCreator do
  describe '.create_file' do
    it "creates a file" do
      MemFs.activate do
        subject.create_file('test.rb')
        expect(File.exists?('test.rb')).to be true
      end
    end
  end
end

No real file will be created during the test.

You can also use it for a specific describe block:

describe FileCreator do
  before { MemFs.activate! }
  after { MemFs.deactivate! }

  describe '.create_file' do
    it "creates a file" do
      subject.create_file('test.rb')
      expect(File.exists?('test.rb')).to be true
    end
  end
end

Utilities

You can use MemFs.touch to quickly create a file and its parent directories:

MemFs.activate do
  MemFs.touch('/path/to/some/file.rb')
  File.exist?('/path/to/some/file.rb') # => true
end

Requirements

  • Ruby 2.0 or newer

Known issues

  • MemFs doesn't implement IO so methods like FileUtils.copy_stream and IO.write are still the originals.
  • Similarly, MemFs doesn't implement Kernel, so don't use a naked open() call. This uses the Kernel class via method_missing, which MemFs will not intercept.
  • Pipes and Sockets are not handled for now.
  • require "pp" will raise a superclass mismatch exception since MemFs::File does not inherit from IO. The best thing to do is to require pp before MemFs.

TODO

  • Implement missing methods from File, Dir and Stat

Contributing

  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request