Golang学习笔记--flag包

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, 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 = 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", 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 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", value, name, err)        }    }    if f.actual == nil {        f.actual = make(map[string]*Flag)    }    f.actual[name] = flag    return true, nil}