原文链接： ?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()namestringparsedboolactualmap[string]*Flag // 存放命令行中实际输入的标志映射formalmap[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 ErrorHandlingoutputio.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 themeaning of the commandcmd -x *will change if there is a file called 0, false, etc. You mustuse 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, default value, 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, value int, 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, default value, and usage string.// The argument p points to an int variable in which to store the value of the flag.func IntVar(p *int, name string, value int, usage string) {CommandLine.Var(newIntValue(value, p), name, usage)}//使用示例var flagvar intflag.IntVar(&flagvar, "flagname2", 1234, "help message for flagname2")// 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.// 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, value int, usage string) *int {p := new(int)f.IntVar(p, name, value, 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, 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)}//使用示例var flagvar = flag.Int("flagname", 1234, "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 = truef.args = argumentsfor {seen, err := f.parseOne()if seen {continue}if err == nil {break}switch f.errorHandling {case ContinueOnError:return errcase 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 := 1if s[1] == '-' {num_minuses++if len(s) == 2 { // "–" terminates the flagsf.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 := falsevalue := ""for i := 1; i < len(name); i++ { // equals cannot be firstif name[i] == '=' {value = name[i+1:]has_value = truename = name[0:i]break}}m := f.formalflag, alreadythere := m[name] // BUGif !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 argif has_value {if err := fv.Set(value); err != nil {return false, f.failf("invalid boolean value %q for -%s: %v", value, name, 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 arghas_value = truevalue, 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", value, name, err)}}if f.actual == nil {f.actual = make(map[string]*Flag)}f.actual[name] = flagreturn true, nil}
你的选择是做或不做，但不做就永远不会有机会
