How to mock/abstract filesystem in go?

For those looking to solve the problem of mocking out your filesystem during testing, checkout @spf13's Afero library, https://github.com/spf13/afero. It does everything that the accepted answer does, but with better documentation and examples.

This is straight from Andrew Gerrand's 10 things you (probably) don't know about Go:

var fs fileSystem = osFS{}

type fileSystem interface {
    Open(name string) (file, error)
    Stat(name string) (os.FileInfo, error)
}

type file interface {
    io.Closer
    io.Reader
    io.ReaderAt
    io.Seeker
    Stat() (os.FileInfo, error)
}

// osFS implements fileSystem using the local disk.
type osFS struct{}

func (osFS) Open(name string) (file, error)        { return os.Open(name) }
func (osFS) Stat(name string) (os.FileInfo, error) { return os.Stat(name) }

For this to work, you will need to write your code to take a fileSystem argument (maybe embed it in some other type, or let nil denote the default filesystem).