生命不止，继续 go go go !!!

今天跟大家分享的是golang中的flag package，如果你在golang中用到了命令行参数，那么你就必须要了解一些flag的知识。

Package flag
作用：
Package flag implements command-line flag parsing.
Go语言通过使用标准库里的flag包来处理命令行参数。

定义flags
定义String flag:

func (f *FlagSet) String(name string,value string,usage string) *string

String defines a string flag with specified name,default value,and usage string. The return value is the address of a string variable that stores the value of the flag.

定义int flag:

func Int64(name string,value int64,usage string) *int64

Int64 defines an int64 flag with specified name,and usage string. The return value is the address of an int64 variable that stores the value of the flag.

定义布尔flag:

func Bool(name string,value bool,usage string) *bool

Bool defines a bool flag with specified name,and usage string. The return value is the address of a bool variable that stores the value of the flag.

这里唯一指的注意的就是返回值：是指针

应用示例：

import "flag"
var ip = flag.Int("flagname", 1234,"help message for flagname")

将flag绑定到一个变量

func StringVar(p *string,name string,usage string)

StringVar defines a string flag with specified name,and usage string. The argument p points to a string variable in which to store the value of the flag.

func IntVar(p *int,value int,usage string)

IntVar defines an int flag with specified name,and usage string. The argument p points to an int variable in which to store the value of the flag.

func BoolVar(p *bool,usage string)

BoolVar defines a bool flag with specified name,and usage string. The argument p points to a bool variable in which to store the value of the flag.

应用示例：

var flagvar int
func init() {
    flag.IntVar(&flagvar,"flagname","help message for flagname")
}

绑定自定义的类型
这里就简单提一嘴，我们自定义的类型需要实现value接口，那么就可以通过flag.Var方法了：

flag.Var(&flagVal,"name","help message for flagname")

For such flags,the default value is just the initial value of the variable.

将命令行参数解析到flag中

func Parse()

Parse parses the command-line flags from os.Args[1:]. Must be called after all flags are defined and before flags are accessed by the program.
解析函数将会在碰到第一个非flag命令行参数时停止。
非flag命令行参数是指不满足命令行语法的参数，如命令行参数为cmd --flag=true abc 则第一个非flag命令行参数为“abc”
调用Parse解析后，就可以直接使用flag本身(指针类型)或者绑定的变量了(值类型)

NArg和NFlag
NArg is the number of arguments remaining after flags have been processed.
获得non-flag个数

NFlag returns the number of command-line flags that have been set.
获得flag个数

获取非flag参数

func Arg(i int) string

Arg returns the i’th command-line argument. Arg(0) is the first remaining argument after flags have been processed. Arg returns an empty string if the requested element does not exist.

func Args() []string

Args returns the non-flag command-line arguments.

如何区分non-flag和flag
命令行参数的格式可以是：
-flag xxx （使用空格，一个 - 符号）
–flag xxx （使用空格，两个 - 符号）
-flag=xxx （使用等号，一个 - 符号）
–flag=xxx （使用等号，两个 - 符号）
其中，布尔类型的参数防止解析时的二义性，应该使用等号的方式指定

应用
新建一个test_flag.go，键入代码：

package main

import (
    "flag"
    "fmt"
)

var Input_Name = flag.String("name","dabaojian","input your name")
var Input_Age = flag.Int("age", 20,"input your age")
var Input_Gender = flag.String("gender","male","input your age")
var Input_flagvar int

func Init() {
    flag.IntVar(&Input_flagvar,"help message for flagname")
}

func main() {
    Init()
    flag.Parse()

    fmt.Printf("args=%s,num=%d\n",flag.Args(),flag.NArg())
    fmt.Printf("flag num=%d\n",flag.NFlag())

    for i := 0; i != flag.NArg(); i++ {
        fmt.Printf("arg[%d]=%s\n",i,flag.Arg(i))
    }

    fmt.Println("name=",*Input_Name)
    fmt.Println("age=",*Input_Age)
    fmt.Println("gender=",*Input_Gender)
    fmt.Println("flagname=",Input_flagvar)
}

build:

go build test_flag.go

执行：
D:\go_workspace\src\go_flag\test_flag.exe –name “11” -age=22 -flagname=0x22 fuck hit ds

输出：
args=[fuck hit ds],num=3
flag num=3
arg[0]=fuck
arg[1]=hit
arg[2]=ds
name= 11
age= 22
gender= male
flagname= 34