Golang Files and I/O

Table of Contents

一：读文件

1.1 读一个 whole 文件

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d, err := ioutil.ReadFile("foo.txt")
if err != nil {
    log.Fatalf("ioutil.ReadFile failed with '%s'\n", err)
}
fmt.Printf("Size of 'foo.txt': %d bytes\n", len(d))

1.2 读一个文件，close

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f, err := os.Open("foo.txt")
if err != nil {
    log.Fatalf("os.Open failed with '%s'\n", err)
}
defer f.Close()

1.3 read file by line

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// ReadLines reads all lines from a file
func ReadLines(filePath string) ([]string, error) {
	file, err := os.OpenFile(filePath, os.O_RDONLY, 0644)
	if err != nil {
		return nil, err
	}
	defer file.Close()
	scanner := bufio.NewScanner(file)
	res := make([]string, 0)
	for scanner.Scan() {
		line := scanner.Text()
		res = append(res, line)
	}
	if err = scanner.Err(); err != nil {
		return nil, err
	}
	return res, nil
}

func main() {
	path := "main.go"
	lines, err := ReadLines(path)
	if err != nil {
		log.Fatalf("ReadLines failed with '%s'\n", err)
	}
	fmt.Printf("File %s has %d lines\n", path, len(lines))
}

二：写文件

2.1 Write the whole file

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d := []byte("content of the file")
err := ioutil.WriteFile("foo.txt", d, 0644)
if err != nil {
    log.Fatalf("ioutil.WriteFile failed with '%s'\n", err)
}

2.2 Open file for writing

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f, err := os.Create("foo.txt")
if err != nil {
    log.Fatalf("os.Open failed with '%s'\n", err)
}

2.3 Open file for appending

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f, err := os.OpenFile(filePath, os.O_WRONLY | os.O_APPEND | os.O_CREATE, 0666)
if err != nil {
    log.Fatalf("os.Open failed with '%s'\n", err)
}

2.4 Write to a file

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d := []byte("data to write")
nWritten, err := f.Write(d)

2.5 File permissions when creating files

使用 os.Create 或创建新文件时 os.OpenFile，需要为新文件提供文件权限。

对于大多数情况 0644 是一个不错的选择。

这些是八进制格式的 Unix 样式权限。

三：File Operations

3.1 获取文件 size

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func GetFileSize(path string) (int64, error) {
	st, err := os.Lstat(path)
	if err != nil {
		return -1, err
	}
	return st.Size(), nil
}

3.2 获取文件 information

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	st, err := os.Stat("main.go")
	if err != nil {
		log.Fatalf("GetFileSize failed with '%s'\n", err)
	}
	fmt.Printf(`Name: %s
Size: %d
IsDir: %v
Mode: %x
ModTime: %s
OS info: %#v
`, st.Name(), st.Size(), st.IsDir(), st.Mode, st.ModTime(), st.Sys())

3.3 检查文件是否 Exist

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func IsPathExists(path string) (bool, error) {
	_, err := os.Lstat(path)
	if err == nil {
		return true, nil
	}
	if os.IsNotExist(err) {
		return false, nil
	}
	// error other than not existing e.g. permission denied
	return false, err
}

3.4 Delete File

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path := "foo.txt"
err := os.Remove(path)
if err != nil {
    if os.IsNotExist(err) {
        fmt.Printf("os.Remove failed because file doesn't exist\n")
    } else {
        fmt.Printf("os.Remove failed with '%s'\n", err)
    }
}

3.5 Rename File

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oldPath := "old_name.txt"
newPath := "new_name.txt"
err := os.Rename(oldPath, newPath)
if err != nil {
    fmt.Printf("os.Rename failed with '%s'\n", err)
}

3.6 Copy a File

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// CopyFile copies a src file to dst
func CopyFile(dst, src string) error {
	srcFile, err := os.Open(src)
	if err != nil {
		return err
	}
	defer srcFile.Close()

	dstFile, err := os.Create(dst)
	if err != nil {
		return err
	}
	_, err = io.Copy(dstFile, srcFile)
	err2 := dstFile.Close()
	if err == nil && err2 != nil {
		err = err2
	}
	if err != nil {
		// delete the destination if copy failed
		os.Remove(dst)
	}
	return err
}

四：目录相关操作 Directory Operations

4.1 Create directory

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dir := "my_dir"
err := os.Mkdir(dir, 0755)
if err != nil {
    fmt.Printf("os.Mkdir('%s') failed with '%s'\n", dir)
}
dir := filepath.Join("topdir", "subdir")
err := os.MkdirAll(dir, 0755)
if err != nil {
    fmt.Printf("os.MkdirAll('%s') failed with '%s'\n", dir)
}

  • os.Mkdir only succeeds if parent directory of dir already exists.

  • os.MkdirAll will create all intermediary directories.

  • 0755 describes permissions for the directory.

4.2 Remove Directory

  • os.Remove only works for empty directories i.e. directories without any files of sub-directories. 
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    dir := "my_dir"
err := os.Remove(dir)
if err != nil {
  fmt.Printf("os.Remove('%s') failed with '%s'\n", path, err)
}
  • os.RemoveAll removes the directory and all its children (files and sub-directories). 
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    dir := "my_dir"
err := os.RemoveAll(dir)
if err != nil {
  fmt.Printf("os.RemoveAll('%s') failed with '%s'\n", path, err)
}

4.3 列出目录下面的文件 List files in a directory

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func main() {
	dir := "."
	fileInfos, err := ioutil.ReadDir(dir)
	if err != nil {
		log.Fatalf("ioutil.ReadDir('%s') failed with '%s'\n", dir, err)
	}
	for _, fi := range fileInfos {
		fmt.Printf("Path: %s, is dir: %v, size: %d bytes\n", fi.Name(), fi.IsDir(), fi.Size())
	}
}

4.4 Recursive 递归的 List 文件

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// 递归获取指定目录下的所有文件名
func GetAllFile(pathname string) ([]string, error) {
	result := []string{}

	fis, err := ioutil.ReadDir(pathname)
	if err != nil {
		fmt.Printf("读取文件目录失败，pathname=%v, err=%v \n",pathname, err)
		return result, err
	}

	// 所有文件/文件夹
	for _, fi := range fis {
		fullname := pathname + "/" + fi.Name()
		// 是文件夹则递归进入获取;是文件，则压入数组
		if fi.IsDir() {
			temp, err := GetAllFile(fullname)
			if err != nil {
				fmt.Printf("读取文件目录失败,fullname=%v, err=%v",fullname, err)
				return result, err
			}
			result = append(result, temp...)
		} else {
			result = append(result, fullname)
		}
	}

	return result, nil

五：File Path Operations

5.1 Join a path

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path := filepath.Join("dir", "sub", "file.txt")
fmt.Printf("path: %s\n", path)

// path: dir/sub/file.txt

5.2 Split a path into a directory and file

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path := filepath.Join("dir", "file.txt")
file := filepath.Base(path)
fmt.Printf("path: %s, file: %s\n", path, file)

// path: dir/file.txt, file: file.txt

5.3 Split list of paths

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parts := filepath.SplitList("/usr/bin:/tmp")
fmt.Printf("parts: %#v\n", parts)

// parts: []string{"/usr/bin", "/tmp"}

5.4 Get file name from path

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path := filepath.Join("dir", "file.txt")
dir, file := filepath.Split(path)
fmt.Printf("dir: %s, file: %s\n", dir, file)

// dir: dir/, file: file.txt

5.5 Get directory name from path

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path := filepath.Join("dir", "file.txt")
dir := filepath.Dir(path)
fmt.Printf("path: %s, dif: %s\n", path, dir)

// path: dir/file.txt, dif: dir

5.6 获取文件拓展名 Get file extension

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ext := filepath.Ext("file.txt")
fmt.Printf("ext: %s\n", ext)

// ext: .txt

六：IO related interface

6.1 io.Reader

  • io.Reader 是读取顺序字节流的关键的抽象；
  • Read函数将最多 len(p) byte 读入一个 buffer 缓冲区 p，返回一个读取的字节和错误的状态；
  • 当它到达文件结尾的时候，返回一个io.EOF错误
  • io.Reader不允许返回流中。为此，类型必须实现io.Seeker或io.ReaderAt接口;
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type  Reader  interface  {
    Read (p [] byte)  (n  int ,  err  error)
}

6.2 io.Writer

  • io.Writer用于写入顺序的字节流 bytes stream;
  • Write writes bytes in p and returns the number of bytes written and error status.
  • Write 保证它会写入所有数据或返回错误，即如果返回的 n 是 < len(p) 则 err 必须非零; 
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    type Writer interface{
    Write (p []byte) (n int,err error)
}

6.3 io.Closer

  • Closer describes streams that must be explicitly closed.(Closer 描述必须显式的 close 的流)；
  • Close returns an error because it’s required in some real-world cases. For example, 在对文件进行缓冲写入时，Close 可能需要将剩余的缓冲数据刷新到文件中，这可能会失败.
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type  Closer  interface  {
        Close ()  error
}

6.4 io.ReaderAt

  • io.ReaderAt is like io.Reader but allows to read at any position in the stream.
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type ReaderAt interface {
        ReadAt(p []byte, off int64) (n int, err error)
}

6.5 io.WriterAt

  • io.WriterAt is like io.Write but allows to write at an arbitrary position in the stream. 
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    type WriterAt interface {
      WriteAt(p []byte, off int64) (n int, err error)
}

6.6 io.Seeker

  • io.Seeker allows seeking within the stream. If you can seek, you can also implement io.ReaderAt and io.WriterAt.
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type Seeker interface {
        Seek(offset int64, whence int) (int64, error)
}