文件IO编程最基本、最常用的就属读写文件操作了。ioutil包实现了一些IO实用功能，其中就包括非常简捷、好用的文件读取功能。
ioutil包有7个函数1个变量：
var Discard io.Writer = devNull(0)
func NopCloser(r io.Reader) io.ReadCloser
func ReadAll(r io.Reader) ([]byte, error)
func ReadDir(dirname string) ([]os.FileInfo, error)
func ReadFile(filename string) ([]byte, error)
func TempDir(dir, prefix string) (name string, err error)
func TempFile(dir, prefix string) (f *os.File, err error)
func WriteFile(filename string, data []byte, perm os.FileMode) error
ReadFile和WriteFile这两个函数分别实现了读文件和写文件操作，一行代码即可搞定，非常便捷。
func ReadFile(filename string) ([]byte, error)
ReadFile读取filename指定的文件，返回文件的内容。如果调用成功返回err==nil，而不是err==EOF。因为ReadFile读取整个文件，不会将Read操作遇到的EOF报告为一个错误。
func WriteFile(filename string, data []byte, perm os.FileMode) error
WriteFile将data写入到filename指定的文件。如果文件不存在，WriteFile用权限perm创建文件；如果文件存在，WriteFile函数在写之前先将文件截断。
可以搭配json包实现程序配置的读写功能。下面是一个读写数据库配置的例子。
package main
import (
"fmt"
"io/ioutil"
"log"
"encoding/json"
)
type AppConf struct{
DriverName string `json:"driver_name"`
SqlUser string `json:"sql_user"`
SqlPass string `json:"sql_password"`
SqlUrls string `json:"sql_urls"`
SqlDB string `json:"sql_db"`
}
func main() {
appConf := AppConf{DriverName:"mysql",
SqlUser:"root",
SqlPass:"password",
SqlUrls:"tcp(127.0.0.1:3306)",
SqlDB:"mydb"}
bConf,err := json.MarshalIndent(appConf,"","\t")
if err != nil {
log.Fatal(err)
}
err = ioutil.WriteFile("app.cfg",bConf,0666)
if err != nil {
log.Fatal(err)
}
var appConf2 AppConf
bConf,err =ioutil.ReadFile("app.cfg")
if err != nil {
log.Fatal(err)
}
err = json.Unmarshal(bConf,&appConf2)
if err != nil {
log.Fatal(err)
}
fmt.Println("appConf === appConf2",appConf==appConf2)
}
运行结果如下：
生成的app.cfg内容如下：
{
"driver_name": "mysql",
"sql_user": "root",
"sql_password": "password",
"sql_urls": "tcp(127.0.0.1:3306)",
"sql_db": "mydb"
}
通常WriteFile的权限是0666。关于权限的定义，可以查看golang源码(C:\Go\src\os\types.go)
// A FileMode represents a file‘s mode and permission bits.
// The bits have the same definition on all systems, so that
// information about files can be moved from one system
// to another portably. Not all bits apply to all systems.
// The only required bit is ModeDir for directories.
type FileMode uint32
// The defined file mode bits are the most significant bits of the FileMode.
// The nine least-significant bits are the standard Unix rwxrwxrwx permissions.
// The values of these bits should be considered part of the public API and
// may be used in wire protocols or disk representations: they must not be
// changed, although new bits might be added.
const (
// The single letters are the abbreviations
// used by the String method‘s formatting.
ModeDir FileMode = 1 << (32 - 1 - iota) // d: is a directory
ModeAppend // a: append-only
ModeExclusive // l: exclusive use
ModeTemporary // T: temporary file; Plan 9 only
ModeSymlink // L: symbolic link
ModeDevice // D: device file
ModeNamedPipe // p: named pipe (FIFO)
ModeSocket // S: Unix domain socket
ModeSetuid // u: setuid
ModeSetgid // g: setgid
ModeCharDevice // c: Unix character device, when ModeDevice is set
ModeSticky // t: sticky
// Mask for the type bits. For regular files, none will be set.
ModeType = ModeDir | ModeSymlink | ModeNamedPipe | ModeSocket | ModeDevice
ModePerm FileMode = 0777 // Unix permission bits
)
FileMode在所有系统种的定义相同，与linux系统的权限定义类似，一些可取的值包括如下：
const (
OS_READ = 04
OS_WRITE = 02
OS_EX = 01
OS_USER_SHIFT = 6
OS_GROUP_SHIFT = 3
OS_OTH_SHIFT = 0
OS_USER_R = OS_READ<<OS_USER_SHIFT
OS_USER_W = OS_WRITE<<OS_USER_SHIFT
OS_USER_X = OS_EX<<OS_USER_SHIFT
OS_USER_RW = OS_USER_R | OS_USER_W
OS_USER_RWX = OS_USER_RW | OS_USER_X
OS_GROUP_R = OS_READ<<OS_GROUP_SHIFT
OS_GROUP_W = OS_WRITE<<OS_GROUP_SHIFT
OS_GROUP_X = OS_EX<<OS_GROUP_SHIFT
OS_GROUP_RW = OS_GROUP_R | OS_GROUP_W
OS_GROUP_RWX = OS_GROUP_RW | OS_GROUP_X
OS_OTH_R = OS_READ<<OS_OTH_SHIFT
OS_OTH_W = OS_WRITE<<OS_OTH_SHIFT
OS_OTH_X = OS_EX<<OS_OTH_SHIFT
OS_OTH_RW = OS_OTH_R | OS_OTH_W
OS_OTH_RWX = OS_OTH_RW | OS_OTH_X
OS_ALL_R = OS_USER_R | OS_GROUP_R | OS_OTH_R
OS_ALL_W = OS_USER_W | OS_GROUP_W | OS_OTH_W
OS_ALL_X = OS_USER_X | OS_GROUP_X | OS_OTH_X
OS_ALL_RW = OS_ALL_R | OS_ALL_W
OS_ALL_RWX = OS_ALL_RW | OS_GROUP_X)
WriteFile权限指定为0777，Ubuntu上执行上面的例子，查看app.cfg的详细信息
dell@dell-VirtualBox:~/mygo/src/example/ioutil$ ll app.cfg -rwxrwxr-x 1 dell dell 132 11月 21 22:15 app.cfg*
WriteFile 权限指定为0666，Ubuntu上执行上面的例子，查看app.cfg的详细信息
dell@dell-VirtualBox:~/mygo/src/example/ioutil$ ll app.cfg -rw-rw-r-- 1 dell dell 132 11月 21 22:18 app.cfg
原文：http://www.cnblogs.com/majianguo/p/7875686.html