golang-fileExists

检查文件是否存在

demo 代码

package main

import (
	"fmt"
	"os"
)

func fileExists(path string) bool {
	fileInfo, err := os.Stat(path)
	fmt.Println(fileInfo.IsDir(), fileInfo.Mode(), fileInfo.ModTime(), fileInfo.Name(), fileInfo.Size(), fileInfo.Sys())
	if err == nil {
		return true
	}
	fmt.Println(err.Error())
	if os.IsNotExist(err) {
		return false
	}
	return true
}
func main() {
	fileExists("a.txt")
}

思路

1. os.Stat(path) 尝试拿到某一具体路径的文件描述

Stat returns a FileInfo describing the named file. If there is an error, it will be of type *PathError.

2. 如果返回了err 用os.IsNotExist(err) 判断是否属于文件或者目录不存在的那种err.
   如果是,那就是不存在.

sNotExist returns a boolean indicating whether the error is known to report that a file or directory does not exist. It is satisfied by ErrNotExist as well as some syscall errors

粗略源码跟踪

• fileInfo, err := os.Stat(path)

这看定义先,根据一个路径返回文件描述结构和error,实际上调用了内部的statNolog
这里先解释下FileInfo

// Stat returns a FileInfo describing the named file.
// If there is an error, it will be of type *PathError.
func Stat(name string) (FileInfo, error) {
	testlog.Stat(name)
	return statNolog(name)
}

// A FileInfo describes a file and is returned by Stat and Lstat.
type FileInfo interface {
	Name() string       // base name of the file
	Size() int64        // length in bytes for regular files; system-dependent for others 这是里bytes,一个中文不是1
	Mode() FileMode     // file mode bits 例如:-rw-r--r--
	ModTime() time.Time // modification time
	IsDir() bool        // abbreviation for Mode().IsDir()
	Sys() interface{}   // underlying data source (can return nil)
}

内部stat

// statNolog stats a file with no test logging.
func statNolog(name string) (FileInfo, error) {
	var fs fileStat
	err := syscall.Stat(name, &fs.sys) //用系统的stat获取fs信息赋值给&fs.sys
	if err != nil {
		return nil, &PathError{"stat", name, err}
	}
	fillFileStatFromSys(&fs, name) // 从&fs.sys里解析出FileInfo的内容
	return &fs, nil
}

在debug过程中发现这个name参数其实走了3次

1. “.”当前目录
2. 当前目录所在的具体目录例如”/Users/vincent/go/src/vincent.com/checkFileDicExist”
3. 文件”./a.txt”

也就说不管是这个文件不存在还是这个目录不存在都会err

---

• os.IsNotExist(err)
  假如爆出err这个函数用来判断err类型

调用内部判断

// IsNotExist returns a boolean indicating whether the error is known to
// report that a file or directory does not exist. It is satisfied by
// ErrNotExist as well as some syscall errors.
func IsNotExist(err error) bool {
	return isNotExist(err)
}

大意就是检查err的内容是否包含特定的string, 是不是感觉有点简陋?

func isNotExist(err error) bool {
	return checkErrMessageContent(err, "does not exist", "not found",
		"has been removed", "no parent")
}

帮助函数

// checkErrMessageContent checks if err message contains one of msgs.
func checkErrMessageContent(err error, msgs ...string) bool {
	if err == nil {
		return false
	}
	err = underlyingError(err) //这个根据err类型把内部的err替换出来为了得到err.Error()
	for _, msg := range msgs {
        // 查看err.Error里是否包含msg
		if contains(err.Error(), msg) {
			return true
		}
	}
	return false
}

// contains is a local version of strings.Contains. It knows len(sep) > 1.
func contains(s, sep string) bool {
	n := len(sep)
    c := sep[0]
    // 逐个字符段判断是否相等
	for i := 0; i+n <= len(s); i++ {
		if s[i] == c && s[i:i+n] == sep {
			return true
		}
	}
	return false
}