Source: path.go in package path/filepath

Package filepath implements utility routines for manipulating filename paths in a way compatible with the target operating system-defined file paths. The filepath package uses either forward slashes or backslashes, depending on the operating system. To process paths such as URLs that always use forward slashes regardless of the operating system, see the path package.

package filepath

import (
	"errors"
	"io/fs"
	"os"
	"sort"
	"strings"
)

A lazybuf is a lazily constructed path buffer. It supports append, reading previously appended bytes, and retrieving the final string. It does not allocate a buffer to hold the output until that output diverges from s.

type lazybuf struct {
	path       string
	buf        []byte
	w          int
	volAndPath string
	volLen     int
}

func (b *lazybuf) index(i int) byte {
	if b.buf != nil {
		return b.buf[i]
	}
	return b.path[i]
}

func (b *lazybuf) append(c byte) {
	if b.buf == nil {
		if b.w < len(b.path) && b.path[b.w] == c {
			b.w++
			return
		}
		b.buf = make([]byte, len(b.path))
		copy(b.buf, b.path[:b.w])
	}
	b.buf[b.w] = c
	b.w++
}

func (b *lazybuf) string() string {
	if b.buf == nil {
		return b.volAndPath[:b.volLen+b.w]
	}
	return b.volAndPath[:b.volLen] + string(b.buf[:b.w])
}

const (
	Separator     = os.PathSeparator
	ListSeparator = os.PathListSeparator
)

Clean returns the shortest path name equivalent to path by purely lexical processing. It applies the following rules iteratively until no further processing can be done: 1. Replace multiple Separator elements with a single one. 2. Eliminate each . path name element (the current directory). 3. Eliminate each inner .. path name element (the parent directory) along with the non-.. element that precedes it. 4. Eliminate .. elements that begin a rooted path: that is, replace "/.." by "/" at the beginning of a path, assuming Separator is '/'. The returned path ends in a slash only if it represents a root directory, such as "/" on Unix or `C:\` on Windows. Finally, any occurrences of slash are replaced by Separator. If the result of this process is an empty string, Clean returns the string ".". See also Rob Pike, ``Lexical File Names in Plan 9 or Getting Dot-Dot Right,'' https://9p.io/sys/doc/lexnames.html

func Clean(path string) string {
	originalPath := path
	volLen := volumeNameLen(path)
	path = path[volLen:]
	if path == "" {

should be UNC

			return FromSlash(originalPath)
		}
		return originalPath + "."
	}
	rooted := os.IsPathSeparator(path[0])

Invariants: reading from path; r is index of next byte to process. writing to buf; w is index of next byte to write. dotdot is index in buf where .. must stop, either because it is the leading slash or it is a leading ../../.. prefix.

	n := len(path)
	out := lazybuf{path: path, volAndPath: originalPath, volLen: volLen}
	r, dotdot := 0, 0
	if rooted {
		out.append(Separator)
		r, dotdot = 1, 1
	}

	for r < n {
		switch {

empty path element

r++

. element

r++

.. element: remove to last separator

			r += 2
			switch {

can backtrack

				out.w--
				for out.w > dotdot && !os.IsPathSeparator(out.index(out.w)) {
					out.w--
				}

cannot backtrack, but not rooted, so append .. element.

				if out.w > 0 {
					out.append(Separator)
				}
				out.append('.')
				out.append('.')
				dotdot = out.w
			}

real path element. add slash if needed

			if rooted && out.w != 1 || !rooted && out.w != 0 {
				out.append(Separator)

copy element

			for ; r < n && !os.IsPathSeparator(path[r]); r++ {
				out.append(path[r])
			}
		}
	}

Turn empty string into "."

	if out.w == 0 {
		out.append('.')
	}

	return FromSlash(out.string())
}

ToSlash returns the result of replacing each separator character in path with a slash ('/') character. Multiple separators are replaced by multiple slashes.

func ToSlash(path string) string {
	if Separator == '/' {
		return path
	}
	return strings.ReplaceAll(path, string(Separator), "/")
}

FromSlash returns the result of replacing each slash ('/') character in path with a separator character. Multiple slashes are replaced by multiple separators.

func FromSlash(path string) string {
	if Separator == '/' {
		return path
	}
	return strings.ReplaceAll(path, "/", string(Separator))
}

SplitList splits a list of paths joined by the OS-specific ListSeparator, usually found in PATH or GOPATH environment variables. Unlike strings.Split, SplitList returns an empty slice when passed an empty string.

func SplitList(path string) []string {
	return splitList(path)
}

Split splits path immediately following the final Separator, separating it into a directory and file name component. If there is no Separator in path, Split returns an empty dir and file set to path. The returned values have the property that path = dir+file.

func Split(path string) (dir, file string) {
	vol := VolumeName(path)
	i := len(path) - 1
	for i >= len(vol) && !os.IsPathSeparator(path[i]) {
		i--
	}
	return path[:i+1], path[i+1:]
}

Join joins any number of path elements into a single path, separating them with an OS specific Separator. Empty elements are ignored. The result is Cleaned. However, if the argument list is empty or all its elements are empty, Join returns an empty string. On Windows, the result will only be a UNC path if the first non-empty element is a UNC path.

func Join(elem ...string) string {
	return join(elem)
}

Ext returns the file name extension used by path. The extension is the suffix beginning at the final dot in the final element of path; it is empty if there is no dot.

func Ext(path string) string {
	for i := len(path) - 1; i >= 0 && !os.IsPathSeparator(path[i]); i-- {
		if path[i] == '.' {
			return path[i:]
		}
	}
	return ""
}

EvalSymlinks returns the path name after the evaluation of any symbolic links. If path is relative the result will be relative to the current directory, unless one of the components is an absolute symbolic link. EvalSymlinks calls Clean on the result.

func EvalSymlinks(path string) (string, error) {
	return evalSymlinks(path)
}

Abs returns an absolute representation of path. If the path is not absolute it will be joined with the current working directory to turn it into an absolute path. The absolute path name for a given file is not guaranteed to be unique. Abs calls Clean on the result.

func Abs(path string) (string, error) {
	return abs(path)
}

func unixAbs(path string) (string, error) {
	if IsAbs(path) {
		return Clean(path), nil
	}
	wd, err := os.Getwd()
	if err != nil {
		return "", err
	}
	return Join(wd, path), nil
}

Rel returns a relative path that is lexically equivalent to targpath when joined to basepath with an intervening separator. That is, Join(basepath, Rel(basepath, targpath)) is equivalent to targpath itself. On success, the returned path will always be relative to basepath, even if basepath and targpath share no elements. An error is returned if targpath can't be made relative to basepath or if knowing the current working directory would be necessary to compute it. Rel calls Clean on the result.

func Rel(basepath, targpath string) (string, error) {
	baseVol := VolumeName(basepath)
	targVol := VolumeName(targpath)
	base := Clean(basepath)
	targ := Clean(targpath)
	if sameWord(targ, base) {
		return ".", nil
	}
	base = base[len(baseVol):]
	targ = targ[len(targVol):]
	if base == "." {
		base = ""

Can't use IsAbs - `\a` and `a` are both relative in Windows.

	baseSlashed := len(base) > 0 && base[0] == Separator
	targSlashed := len(targ) > 0 && targ[0] == Separator
	if baseSlashed != targSlashed || !sameWord(baseVol, targVol) {
		return "", errors.New("Rel: can't make " + targpath + " relative to " + basepath)

Position base[b0:bi] and targ[t0:ti] at the first differing elements.

	bl := len(base)
	tl := len(targ)
	var b0, bi, t0, ti int
	for {
		for bi < bl && base[bi] != Separator {
			bi++
		}
		for ti < tl && targ[ti] != Separator {
			ti++
		}
		if !sameWord(targ[t0:ti], base[b0:bi]) {
			break
		}
		if bi < bl {
			bi++
		}
		if ti < tl {
			ti++
		}
		b0 = bi
		t0 = ti
	}
	if base[b0:bi] == ".." {
		return "", errors.New("Rel: can't make " + targpath + " relative to " + basepath)
	}

Base elements left. Must go up before going down.

		seps := strings.Count(base[b0:bl], string(Separator))
		size := 2 + seps*3
		if tl != t0 {
			size += 1 + tl - t0
		}
		buf := make([]byte, size)
		n := copy(buf, "..")
		for i := 0; i < seps; i++ {
			buf[n] = Separator
			copy(buf[n+1:], "..")
			n += 3
		}
		if t0 != tl {
			buf[n] = Separator
			copy(buf[n+1:], targ[t0:])
		}
		return string(buf), nil
	}
	return targ[t0:], nil
}

SkipDir is used as a return value from WalkFuncs to indicate that the directory named in the call is to be skipped. It is not returned as an error by any function.

var SkipDir error = fs.SkipDir

WalkFunc is the type of the function called by Walk to visit each each file or directory. The path argument contains the argument to Walk as a prefix. That is, if Walk is called with root argument "dir" and finds a file named "a" in that directory, the walk function will be called with argument "dir/a". The directory and file are joined with Join, which may clean the directory name: if Walk is called with the root argument "x/../dir" and finds a file named "a" in that directory, the walk function will be called with argument "dir/a", not "x/../dir/a". The info argument is the fs.FileInfo for the named path. The error result returned by the function controls how Walk continues. If the function returns the special value SkipDir, Walk skips the current directory (path if info.IsDir() is true, otherwise path's parent directory). Otherwise, if the function returns a non-nil error, Walk stops entirely and returns that error. The err argument reports an error related to path, signaling that Walk will not walk into that directory. The function can decide how to handle that error; as described earlier, returning the error will cause Walk to stop walking the entire tree. Walk calls the function with a non-nil err argument in two cases. First, if an os.Lstat on the root directory or any directory or file in the tree fails, Walk calls the function with path set to that directory or file's path, info set to nil, and err set to the error from os.Lstat. Second, if a directory's Readdirnames method fails, Walk calls the function with path set to the directory's path, info, set to an fs.FileInfo describing the directory, and err set to the error from Readdirnames.

type WalkFunc func(path string, info fs.FileInfo, err error) error

var lstat = os.Lstat // for testing

walkDir recursively descends path, calling walkDirFn.

func walkDir(path string, d fs.DirEntry, walkDirFn fs.WalkDirFunc) error {
	if err := walkDirFn(path, d, nil); err != nil || !d.IsDir() {

Successfully skipped directory.

			err = nil
		}
		return err
	}

	dirs, err := readDir(path)

Second call, to report ReadDir error.

		err = walkDirFn(path, d, err)
		if err != nil {
			return err
		}
	}

	for _, d1 := range dirs {
		path1 := Join(path, d1.Name())
		if err := walkDir(path1, d1, walkDirFn); err != nil {
			if err == SkipDir {
				break
			}
			return err
		}
	}
	return nil
}

walk recursively descends path, calling walkFn.

func walk(path string, info fs.FileInfo, walkFn WalkFunc) error {
	if !info.IsDir() {
		return walkFn(path, info, nil)
	}

	names, err := readDirNames(path)

If err != nil, walk can't walk into this directory. err1 != nil means walkFn want walk to skip this directory or stop walking. Therefore, if one of err and err1 isn't nil, walk will return.

The caller's behavior is controlled by the return value, which is decided by walkFn. walkFn may ignore err and return nil. If walkFn returns SkipDir, it will be handled by the caller. So walk should return whatever walkFn returns.

		return err1
	}

	for _, name := range names {
		filename := Join(path, name)
		fileInfo, err := lstat(filename)
		if err != nil {
			if err := walkFn(filename, fileInfo, err); err != nil && err != SkipDir {
				return err
			}
		} else {
			err = walk(filename, fileInfo, walkFn)
			if err != nil {
				if !fileInfo.IsDir() || err != SkipDir {
					return err
				}
			}
		}
	}
	return nil
}

WalkDir walks the file tree rooted at root, calling fn for each file or directory in the tree, including root. All errors that arise visiting files and directories are filtered by fn: see the fs.WalkDirFunc documentation for details. The files are walked in lexical order, which makes the output deterministic but requires WalkDir to read an entire directory into memory before proceeding to walk that directory. WalkDir does not follow symbolic links.

func WalkDir(root string, fn fs.WalkDirFunc) error {
	info, err := os.Lstat(root)
	if err != nil {
		err = fn(root, nil, err)
	} else {
		err = walkDir(root, &statDirEntry{info}, fn)
	}
	if err == SkipDir {
		return nil
	}
	return err
}

type statDirEntry struct {
	info fs.FileInfo
}

func (d *statDirEntry) Name() string               { return d.info.Name() }
func (d *statDirEntry) IsDir() bool                { return d.info.IsDir() }
func (d *statDirEntry) Type() fs.FileMode          { return d.info.Mode().Type() }
func (d *statDirEntry) Info() (fs.FileInfo, error) { return d.info, nil }

Walk walks the file tree rooted at root, calling fn for each file or directory in the tree, including root. All errors that arise visiting files and directories are filtered by fn: see the WalkFunc documentation for details. The files are walked in lexical order, which makes the output deterministic but requires Walk to read an entire directory into memory before proceeding to walk that directory. Walk does not follow symbolic links. Walk is less efficient than WalkDir, introduced in Go 1.16, which avoids calling os.Lstat on every visited file or directory.

func Walk(root string, fn WalkFunc) error {
	info, err := os.Lstat(root)
	if err != nil {
		err = fn(root, nil, err)
	} else {
		err = walk(root, info, fn)
	}
	if err == SkipDir {
		return nil
	}
	return err
}

readDir reads the directory named by dirname and returns a sorted list of directory entries.

func readDir(dirname string) ([]fs.DirEntry, error) {
	f, err := os.Open(dirname)
	if err != nil {
		return nil, err
	}
	dirs, err := f.ReadDir(-1)
	f.Close()
	if err != nil {
		return nil, err
	}
	sort.Slice(dirs, func(i, j int) bool { return dirs[i].Name() < dirs[j].Name() })
	return dirs, nil
}

readDirNames reads the directory named by dirname and returns a sorted list of directory entry names.

func readDirNames(dirname string) ([]string, error) {
	f, err := os.Open(dirname)
	if err != nil {
		return nil, err
	}
	names, err := f.Readdirnames(-1)
	f.Close()
	if err != nil {
		return nil, err
	}
	sort.Strings(names)
	return names, nil
}

Base returns the last element of path. Trailing path separators are removed before extracting the last element. If the path is empty, Base returns ".". If the path consists entirely of separators, Base returns a single separator.

func Base(path string) string {
	if path == "" {
		return "."

Strip trailing slashes.

	for len(path) > 0 && os.IsPathSeparator(path[len(path)-1]) {
		path = path[0 : len(path)-1]

Throw away volume name

Find the last element

	i := len(path) - 1
	for i >= 0 && !os.IsPathSeparator(path[i]) {
		i--
	}
	if i >= 0 {
		path = path[i+1:]

If empty now, it had only slashes.

	if path == "" {
		return string(Separator)
	}
	return path
}

Dir returns all but the last element of path, typically the path's directory. After dropping the final element, Dir calls Clean on the path and trailing slashes are removed. If the path is empty, Dir returns ".". If the path consists entirely of separators, Dir returns a single separator. The returned path does not end in a separator unless it is the root directory.

func Dir(path string) string {
	vol := VolumeName(path)
	i := len(path) - 1
	for i >= len(vol) && !os.IsPathSeparator(path[i]) {
		i--
	}
	dir := Clean(path[len(vol) : i+1])

must be UNC

		return vol
	}
	return vol + dir
}

VolumeName returns leading volume name. Given "C:\foo\bar" it returns "C:" on Windows. Given "\\host\share\foo" it returns "\\host\share". On other platforms it returns "".

func VolumeName(path string) string {
	return path[:volumeNameLen(path)]