A FileSystem Abstraction System for Go
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A FileSystem Abstraction System for Go

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Afero is an filesystem framework providing a simple, uniform and universal API interacting with any filesystem, as an abstraction layer providing interfaces, types and methods. Afero has an exceptionally clean interface and simple design without needless constructors or initialization methods.

Afero is also a library providing a base set of interoperable backend filesystems that make it easy to work with afero while retaining all the power and benefit of the os and ioutil packages.

Afero provides significant improvements over using the os package alone, most notably the ability to create mock and testing filesystems without relying on the disk.

It is suitable for use in a any situation where you would consider using the OS package as it provides an additional abstraction that makes it easy to use a memory backed file system during testing. It also adds support for the http filesystem for full interoperability.

Afero Features

  • A single consistent API for accessing a variety of filesystems
  • Interoperation between a variety of file system types
  • A set of interfaces to encourage and enforce interoperability between backends
  • An atomic cross platform memory backed file system
  • Support for compositional (union) file systems by combining multiple file systems acting as one
  • Specialized backends which modify existing filesystems (Read Only, Regexp filtered)
  • A set of utility functions ported from io, ioutil & hugo to be afero aware

Using Afero

Afero is easy to use and easier to adopt.

A few different ways you could use Afero:

  • Use the interfaces alone to define you own file system.
  • Wrap for the OS packages.
  • Define different filesystems for different parts of your application.
  • Use Afero for mock filesystems while testing

Step 1: Install Afero

First use go get to install the latest version of the library.

$ go get github.com/spf13/afero

Next include Afero in your application.

import "github.com/spf13/afero"

Step 2: Declare a backend

First define a package variable and set it to a pointer to a filesystem.

var AppFs afero.Fs = afero.NewMemMapFs()


var AppFs afero.Fs = afero.NewOsFs()

It is important to note that if you repeat the composite literal you will be using a completely new and isolated filesystem. In the case of OsFs it will still use the same underlying filesystem but will reduce the ability to drop in other filesystems as desired.

Step 3: Use it like you would the OS package

Throughout your application use any function and method like you normally would.

So if my application before had:


We would replace it with a call to AppFs.Open('/tmp/foo').

AppFs being the variable we defined above.

List of all available functions

File System Methods Available:

Chmod(name string, mode os.FileMode) : error
Chtimes(name string, atime time.Time, mtime time.Time) : error
Create(name string) : File, error
Mkdir(name string, perm os.FileMode) : error
MkdirAll(path string, perm os.FileMode) : error
Name() : string
Open(name string) : File, error
OpenFile(name string, flag int, perm os.FileMode) : File, error
Remove(name string) : error
RemoveAll(path string) : error
Rename(oldname, newname string) : error
Stat(name string) : os.FileInfo, error

File Interfaces and Methods Available:


Name() : string
Readdir(count int) : []os.FileInfo, error
Readdirnames(n int) : []string, error
Stat() : os.FileInfo, error
Sync() : error
Truncate(size int64) : error
WriteString(s string) : ret int, err error

In some applications it may make sense to define a new package that simply exports the file system variable for easy access from anywhere.

Using Afero's utility functions

Afero provides a set of functions to make it easier to use the underlying file systems. These functions have been primarily ported from io & ioutil with some developed for Hugo.

The afero utilities support all afero compatible backends.

The list of utilities includes:

DirExists(path string) (bool, error)
Exists(path string) (bool, error)
FileContainsBytes(filename string, subslice []byte) (bool, error)
GetTempDir(subPath string) string
IsDir(path string) (bool, error)
IsEmpty(path string) (bool, error)
ReadDir(dirname string) ([]os.FileInfo, error)
ReadFile(filename string) ([]byte, error)
SafeWriteReader(path string, r io.Reader) (err error)
TempDir(dir, prefix string) (name string, err error)
TempFile(dir, prefix string) (f File, err error)
Walk(root string, walkFn filepath.WalkFunc) error
WriteFile(filename string, data []byte, perm os.FileMode) error
WriteReader(path string, r io.Reader) (err error)

For a complete list see Afero's GoDoc

They are available under two different approaches to use. You can either call them directly where the first parameter of each function will be the file system, or you can declare a new Afero, a custom type used to bind these functions as methods to a given filesystem.

Calling utilities directly

fs := new(afero.MemMapFs)
f, err := afero.TempFile(fs,"", "ioutil-test")

Calling via Afero

fs := afero.NewMemMapFs
afs := &Afero{Fs: fs}
f, err := afs.TempFile("", "ioutil-test")

Using Afero for Testing

There is a large benefit to using a mock filesystem for testing. It has a completely blank state every time it is initialized and can be easily reproducible regardless of OS. You could create files to your heart’s content and the file access would be fast while also saving you from all the annoying issues with deleting temporary files, Windows file locking, etc. The MemMapFs backend is perfect for testing.

  • Much faster than performing I/O operations on disk
  • Avoid security issues and permissions
  • Far more control. 'rm -rf /' with confidence
  • Test setup is far more easier to do
  • No test cleanup needed

One way to accomplish this is to define a variable as mentioned above. In your application this will be set to afero.NewOsFs() during testing you can set it to afero.NewMemMapFs().

It wouldn't be uncommon to have each test initialize a blank slate memory backend. To do this I would define my appFS = afero.NewOsFs() somewhere appropriate in my application code. This approach ensures that Tests are order independent, with no test relying on the state left by an earlier test.

Then in my tests I would initialize a new MemMapFs for each test:

func TestExist(t *testing.T) {
    appFS = afero.NewMemMapFs()
    // create test files and directories
    appFS.MkdirAll("src/a", 0755))
    afero.WriteFile(appFS, "src/a/b", []byte("file b"), 0644)
    afero.WriteFile(appFS, "src/c", []byte("file c"), 0644)
    _, err := appFS.Stat("src/c")
    if os.IsNotExist(err) {
        t.Errorf("file \"%s\" does not exist.\n", name)

Available Backends

Operating System Native


The first is simply a wrapper around the native OS calls. This makes it very easy to use as all of the calls are the same as the existing OS calls. It also makes it trivial to have your code use the OS during operation and a mock filesystem during testing or as needed.

appfs := afero.NewOsFs()
appfs.MkdirAll("src/a", 0755))

Memory Backed Storage


Afero also provides a fully atomic memory backed filesystem perfect for use in mocking and to speed up unnecessary disk io when persistence isn’t necessary. It is fully concurrent and will work within go routines safely.

mm := afero.NewMemMapFs()
mm.MkdirAll("src/a", 0755))


As part of MemMapFs, Afero also provides an atomic, fully concurrent memory backed file implementation. This can be used in other memory backed file systems with ease. Plans are to add a radix tree memory stored file system using InMemoryFile.

Network Interfaces


Afero has experimental support for secure file transfer protocol (sftp). Which can be used to perform file operations over a encrypted channel.

Filtering Backends


The BasePathFs restricts all operations to a given path within an Fs. The given file name to the operations on this Fs will be prepended with the base path before calling the source Fs.

bp := afero.NewBasePathFs(afero.NewOsFs(), "/base/path")


A thin wrapper around the source Fs providing a read only view.

fs := afero.NewReadOnlyFs(afero.NewOsFs())
_, err := fs.Create("/file.txt")
// err = syscall.EPERM


A filtered view on file names, any file NOT matching the passed regexp will be treated as non-existing. Files not matching the regexp provided will not be created. Directories are not filtered.

fs := afero.NewRegexpFs(afero.NewMemMapFs(), regexp.MustCompile(`\.txt$`))
_, err := fs.Create("/file.html")
// err = syscall.ENOENT


Afero provides an http compatible backend which can wrap any of the existing backends.

The Http package requires a slightly specific version of Open which returns an http.File type.

Afero provides an httpFs file system which satisfies this requirement. Any Afero FileSystem can be used as an httpFs.

httpFs := afero.NewHttpFs(<ExistingFS>)
fileserver := http.FileServer(httpFs.Dir(<PATH>)))
http.Handle("/", fileserver)

Composite Backends

Afero provides the ability have two filesystems (or more) act as a single file system.


The CacheOnReadFs will lazily make copies of any accessed files from the base layer into the overlay. Subsequent reads will be pulled from the overlay directly permitting the request is within the cache duration of when it was created in the overlay.

If the base filesystem is writeable, any changes to files will be done first to the base, then to the overlay layer. Write calls to open file handles like Write() or Truncate() to the overlay first.

To writing files to the overlay only, you can use the overlay Fs directly (not via the union Fs).

Cache files in the layer for the given time.Duration, a cache duration of 0 means "forever" meaning the file will not be re-requested from the base ever.

A read-only base will make the overlay also read-only but still copy files from the base to the overlay when they're not present (or outdated) in the caching layer.

base := afero.NewOsFs()
layer := afero.NewMemMapFs()
ufs := afero.NewCacheOnReadFs(base, layer, 100 * time.Second)


The CopyOnWriteFs is a read only base file system with a potentially writeable layer on top.

Read operations will first look in the overlay and if not found there, will serve the file from the base.

Changes to the file system will only be made in the overlay.

Any attempt to modify a file found only in the base will copy the file to the overlay layer before modification (including opening a file with a writable handle).

Removing and Renaming files present only in the base layer is not currently permitted. If a file is present in the base layer and the overlay, only the overlay will be removed/renamed.

    base := afero.NewOsFs()
    roBase := afero.NewReadOnlyFs(base)
    ufs := afero.NewCopyOnWriteFs(roBase, afero.NewMemMapFs())

    fh, _ = ufs.Create("/home/test/file2.txt")
    fh.WriteString("This is a test")

In this example all write operations will only occur in memory (MemMapFs) leaving the base filesystem (OsFs) untouched.

Desired/possible backends

The following is a short list of possible backends we hope someone will implement:

  • SSH
  • ZIP
  • TAR
  • S3

About the project

What's in the name

Afero comes from the latin roots Ad-Facere.

"Ad" is a prefix meaning "to".

"Facere" is a form of the root "faciō" making "make or do".

The literal meaning of afero is "to make" or "to do" which seems very fitting for a library that allows one to make files and directories and do things with them.

The English word that shares the same roots as Afero is "affair". Affair shares the same concept but as a noun it means "something that is made or done" or "an object of a particular type".

It's also nice that unlike some of my other libraries (hugo, cobra, viper) it Googles very well.

Release Notes

  • 0.10.0 2015.12.10
    • Full compatibility with Windows
    • Introduction of afero utilities
    • Test suite rewritten to work cross platform
    • Normalize paths for MemMapFs
    • Adding Sync to the file interface
    • Breaking Change Walk and ReadDir have changed parameter order
    • Moving types used by MemMapFs to a subpackage
    • General bugfixes and improvements
  • 0.9.0 2015.11.05
    • New Walk function similar to filepath.Walk
    • MemMapFs.OpenFile handles O_CREATE, O_APPEND, O_TRUNC
    • MemMapFs.Remove now really deletes the file
    • InMemoryFile.Readdir and Readdirnames work correctly
    • InMemoryFile functions lock it for concurrent access
    • Test suite improvements
  • 0.8.0 2014.10.28
    • First public version
    • Interfaces feel ready for people to build using
    • Interfaces satisfy all known uses
    • MemMapFs passes the majority of the OS test suite
    • OsFs passes the majority of the OS test suite


  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


Names in no particular order:


Afero is released under the Apache 2.0 license. See LICENSE.txt