Golang学习笔记--flag包

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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 int
flag.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
}

---

本文来自:CSDN博客

感谢作者:cza55007

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