原文链接： http://www.cloudnoter.com/?p=131
flag包是用来处理命令参数的。总得来说，其通过将命令行标志与某一具体变量进行绑定，开发人员通过使用该变量进行业务逻辑处理。
一、FlagSet是该包的核心类型：type FlagSet struct {
// Usage is the function called when an error occurs while parsing flags.
// The field is a function (not a method) that may be changed to point to
// a custom error handler.
Usage func()
name string
parsed bool
actual map[string]*Flag // 存放命令行中实际输入的标志映射
formal map[string]*Flag // 存放该实例可以处理的命令行标志映射
args []string // arguments after flags 存放非标志的参数列表（即标志后面的参数）
exitOnError bool // does the program exit if there's an error?
errorHandling ErrorHandling
output io.Writer // nil means stderr; use out() accessor
}该类型同时提供了一系列的方法集合【MethodSet】，通过该方法集用于可以实现灵活的命令行标志处理。
二、flag包export的变量:CommandLine
// CommandLine is the default set of command-line flags,parsed from os.Args. // The top-level functions such as BoolVar,Arg,and on are wrappers for the // methods of CommandLine. var CommandLine = NewFlagSet(os.Args[0],ExitOnError)该包提供了一个默认变量：CommandLine，其为FlatSet的一个变量（用面向对象的术语叫做：FlagSet的一个实例）
该flag包export的所有函数本质上都是调用FlagSet类型变量（实例）：CommandLine的方法实现。如下：
// Int defines an int flag with specified name,default value,and usage string.
// The return value is the address of an int variable that stores the value of the flag.
func Int(name string,value int,usage string) *int {
return CommandLine.Int(name,value,usage)
}
三、flag包支持的标志格式
Command line flag Syntax: -flag // 代表bool值，相当于-flag=true -flag=x -flag x // non-boolean flags only 不支持bool值标志 One or two minus signs may be used; they are equivalent. // -flag=value与--flag=value是等效的 The last form is not permitted for boolean flags because the meaning of the command cmd -x * will change if there is a file called 0,false,etc. You must use the -flag=false form to turn off a boolean flag. Flag parsing stops just before the first non-flag argument ("-" is a non-flag argument) or after the terminator "--". // 碰到连续两个"-"号且参数长度为2时则终止标志解析
四、标志绑定相关方法：以绑定int类型为例。
// 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. // FlagSet提供的绑定int类型标志的方法，无返回值。通过传入int类型指针变量进行绑定，当调用该方法后，会将绑定信息存入FlagSet.formal映射中 func (f *FlagSet) IntVar(p *int,name string,usage string) { f.Var(newIntValue(value,p),name,usage) } //与上述方法相对应的flag包export的函数： // 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 IntVar(p *int,usage string) { CommandLine.Var(newIntValue(value,usage) } //使用示例 var flagvar int flag.IntVar(&flagvar,"flagname2",1234,"help message for flagname2") // Int defines an int flag with specified name,and usage string. // The return value is the address of an int variable that stores the value of the flag. // FlagSet提供的绑定int类型标志的方法，有返回值，返回int类型指针，当调用该方法后，会将绑定信息存入FlagSet.formal映射中 func (f *FlagSet) Int(name string,usage string) *int { p := new(int) f.IntVar(p,usage) return p } //与上述方法相对应的flag包export的函数： // Int defines an int flag with specified name,usage) } //使用示例 var flagvar = flag.Int("flagname","help message for flagname")
五、解析标志的相关关键源码
// 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. // flag包export的函数，调用时机为：在设置好标志与变量的绑定关系后，调用flag.Parse()。 func Parse() { // Ignore errors; CommandLine is set for ExitOnError. CommandLine.Parse(os.Args[1:]) } // Parse parses flag definitions from the argument list,which should not // include the command name. Must be called after all flags in the FlagSet // are defined and before flags are accessed by the program. // The return value will be ErrHelp if -help was set but not defined. // FlagSet类型提供的实现方法 func (f *FlagSet) Parse(arguments []string) error { f.parsed = true f.args = arguments for { seen,err := f.parSEOne() if seen { continue } if err == nil { break } switch f.errorHandling { case ContinueOnError: return err case ExitOnError: os.Exit(2) case PanicOnError: panic(err) } } return nil } // parSEOne parses one flag. It reports whether a flag was seen. // 解析每个标志并返回相关结果，若碰到 '-' 或 '--' 时也会直接终止整个标志解析过程，每解析成功一个标志就会将该标志信息放入FlagSet.actual映射中 func (f *FlagSet) parSEOne() (bool,error) { if len(f.args) == 0 { return false,nil } s := f.args[0] if len(s) == 0 || s[0] != '-' || len(s) == 1 { return false,nil } num_minuses := 1 if s[1] == '-' { num_minuses++ if len(s) == 2 { // "--" terminates the flags f.args = f.args[1:] return false,nil } } name := s[num_minuses:] if len(name) == 0 || name[0] == '-' || name[0] == '=' { return false,f.failf("bad flag Syntax: %s",s) } // it's a flag. does it have an argument? f.args = f.args[1:] has_value := false value := "" for i := 1; i < len(name); i++ { // equals cannot be first if name[i] == '=' { value = name[i+1:] has_value = true name = name[0:i] break } } m := f.formal flag,alreadythere := m[name] // BUG if !alreadythere { if name == "help" || name == "h" { // special case for nice help message. f.usage() return false,ErrHelp } return false,f.failf("flag provided but not defined: -%s",name) } if fv,ok := flag.Value.(boolFlag); ok && fv.IsBoolFlag() { // special case: doesn't need an arg if has_value { if err := fv.Set(value); err != nil { return false,f.failf("invalid boolean value %q for -%s: %v",err) } } else { fv.Set("true") } } else { // It must have a value,which might be the next argument. if !has_value && len(f.args) > 0 { // value is the next arg has_value = true value,f.args = f.args[0],f.args[1:] } if !has_value { return false,f.failf("flag needs an argument: -%s",name) } if err := flag.Value.Set(value); err != nil { return false,f.failf("invalid value %q for flag -%s: %v",err) } } if f.actual == nil { f.actual = make(map[string]*Flag) } f.actual[name] = flag return true,nil }原文链接：https://www.f2er.com/go/190564.html
