我是怎么想到要先看docker中的flag呢，就是因为docker采用了c/s结构，而且daemon和client都是用同一个程序的，因此，为了做出区分，肯定是要用参数来区分的。先来看位于./docker/docker/docker.go下面的main函数代码：
func main() {
//第一次肯定是返回false的，因为没有任何initializer
if reexec.Init() {
return
}
// Set terminal emulation based on platform as required.
stdin, stdout, stderr := term.StdStreams()
logrus.SetOutput(stderr)
flag.Merge(flag.CommandLine, clientFlags.FlagSet, commonFlags.FlagSet)
flag.Usage = func() {
fmt.Fprint(os.Stdout, "Usage: docker [OPTIONS] COMMAND [arg...]\n"+daemonUsage+" docker [ --help | -v | --version ]\n\n")
fmt.Fprint(os.Stdout, "A self-sufficient runtime for containers.\n\nOptions:\n")
flag.CommandLine.SetOutput(os.Stdout)
flag.PrintDefaults()
help := "\nCommands:\n"
for _, cmd := range dockerCommands {
help += fmt.Sprintf(" %-10.10s%s\n", cmd.name, cmd.description)
}
help += "\nRun 'docker COMMAND --help' for more information on a command."
fmt.Fprintf(os.Stdout, "%s\n", help)
}
flag.Parse()
if *flVersion {
showVersion()
return
}
//创建一个docker client
clientCli := client.NewDockerCli(stdin, stdout, stderr, clientFlags)
// TODO: remove once `-d` is retired
handleGlobalDaemonFlag()
if *flHelp {
// if global flag --help is present, regardless of what other options and commands there are,
// just print the usage.
flag.Usage()
return
}
c := cli.New(clientCli, daemonCli)
if err := c.Run(flag.Args()...); err != nil {
if sterr, ok := err.(cli.StatusError); ok {
if sterr.Status != "" {
fmt.Fprintln(os.Stderr, sterr.Status)
os.Exit(1)
}
os.Exit(sterr.StatusCode)
}
fmt.Fprintln(os.Stderr, err)
os.Exit(1)
}
}
从上面我们看到，这个源码中用到了flag，而这个flag来自哪儿呢？我们就看看他的import：
import (
"fmt"
"os"
"github.com/Sirupsen/logrus"
"github.com/docker/docker/api/client"
"github.com/docker/docker/autogen/dockerversion"
"github.com/docker/docker/cli"
flag "github.com/docker/docker/pkg/mflag"
"github.com/docker/docker/pkg/reexec"
"github.com/docker/docker/pkg/term"
"github.com/docker/docker/utils"
)
说明这里的包来自mflag。那么，我们来看看位于./docker/pkg包，这个包中只有一个源码文件就是flag.go：
在flag.go中，他首先申明了几个比较重要的结构：
// Value is the interface to the dynamic value stored in a flag.
// (The default value is represented as a string.)
//
// If a Value has an IsBoolFlag() bool method returning true,
// the command-line parser makes -name equivalent to -name=true
// rather than using the next command-line argument.
type Value interface {
String() string
Set(string) error
}
// A Flag represents the state of a flag.
type Flag struct {
Names []string // name as it appears on command line
Usage string // help message
Value Value // value as set
DefValue string // default value (as text); for usage message
}
上面的Flag为什么要有Names字段呢？原因很简单，那就是像我们在Linux中会使用-h或者--help的格式，因此，这个Names就是用来存储着几种格式的。
// A FlagSet represents a set of defined flags. The zero value of a FlagSet
// has no name and has ContinueOnError error handling.
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()
ShortUsage func()
name string
parsed bool
actual map[string]*Flag
formal map[string]*Flag
args []string // arguments after flags
errorHandling ErrorHandling //type ErrorHandling int output io.Writer // nil means stderr; use Out() accessor
nArgRequirements []nArgRequirement
}
其中的nArgRequirement的定义如下：
type nArgRequirement struct {
Type nArgRequirementType //type nArgRequirementType int N int
}func Parse() {
// Ignore errors; CommandLine is set for ExitOnError.
CommandLine.Parse(os.Args[1:])//这里的os.Args[1:]就是出了程序名字之外的其他所有命令行参数
}
好了，看了，这几个定义，我们还是按照golang的一贯规则来看看，在这个包中定义了哪些const、哪些var以及有哪些init。
var ErrHelp = errors.New("flag: help requested")
var ErrRetry = errors.New("flag: retry")
const (
ContinueOnError ErrorHandling = iota
ExitOnError //1
PanicOnError //2
)
const (
Exact nArgRequirementType = iota
Max //1
Min //2
)从上面的结构定义中，我们看到，有两个比较重要的结构定义，分别是：FlagSet和Flag
在看这两个重要结构的方法之前，我们还要先看看这个包中，还做了一些针对常规类型的封装，针对的类型分别是：
int，int64，unit，string，float64，time.Duration，分别都做了类似如下的封装：
type float64Value float64
func newFloat64Value(val float64, p *float64) *float64Value {
*p = val
return (*float64Value)(p)
}
func (f *float64Value) Set(s string) error {
v, err := strconv.ParseFloat(s, 64)
*f = float64Value(v)
return err
}
func (f *float64Value) Get() interface{} { return float64(*f) }
func (f *float64Value) String() string { return fmt.Sprintf("%v", *f) }
针对上面两个重要的结构，分别绑定了很多重要的方法，具体如下：
// Name returns the name of the FlagSet.
func (fs *FlagSet) Name() string {
return fs.name
}
// Out returns the destination for usage and error messages.
func (fs *FlagSet) Out() io.Writer {
//如果fs中的output为空，则设置为默认的os.Stderr
if fs.output == nil {
return os.Stderr
}
return fs.output
}
// SetOutput sets the destination for usage and error messages.
// If output is nil, os.Stderr is used.
func (fs *FlagSet) SetOutput(output io.Writer) {
fs.output = output
}
// VisitAll visits the flags in lexicographical order, calling fn for each.
// It visits all flags, even those not set.
//这个方法会以字典顺序来访问其中的每一个flag，哪怕这个flag没有被设置，在每一个flag上调用指定的函数
func (fs *FlagSet) VisitAll(fn func(*Flag)) {
for _, flag := range sortFlags(fs.formal) {
fn(flag)
}
}
// Visit visits the flags in lexicographical order, calling fn for each.
// It visits only those flags that have been set.
//这个方法只能以字典顺序访问已经被设置的flag，并在访问到的flag上执行指定的函数
func (fs *FlagSet) Visit(fn func(*Flag)) {
for _, flag := range sortFlags(fs.actual) {
fn(flag)
}
}
// Lookup returns the Flag structure of the named flag, returning nil if none exists.
func (fs *FlagSet) Lookup(name string) *Flag {
return fs.formal[name]
}
// IsSet indicates whether the specified flag is set in the given FlagSet
//判断某个flag是否已经被设置了
func (fs *FlagSet) IsSet(name string) bool {
return fs.actual[name] != nil
}
// Require adds a requirement about the number of arguments for the FlagSet.
// The first parameter can be Exact, Max, or Min to respectively specify the exact,
// the maximum, or the minimal number of arguments required.
// The actual check is done in FlagSet.CheckArgs().
func (fs *FlagSet) Require(nArgRequirementType nArgRequirementType, nArg int) {
fs.nArgRequirements = append(fs.nArgRequirements, nArgRequirement{nArgRequirementType, nArg})
}
// CheckArgs uses the requirements set by FlagSet.Require() to validate
// the number of arguments. If the requirements are not met,
// an error message string is returned.
func (fs *FlagSet) CheckArgs() (message string) {
for _, req := range fs.nArgRequirements {
var arguments string
if req.N == 1 {
arguments = "1 argument"
} else {
arguments = fmt.Sprintf("%d arguments", req.N)
}
str := func(kind string) string {
return fmt.Sprintf("%q requires %s%s", fs.name, kind, arguments)
}
switch req.Type {
case Exact:
if fs.NArg() != req.N {
return str("")
}
case Max:
if fs.NArg() > req.N {
return str("a maximum of ")
}
case Min:
if fs.NArg() < req.N {
return str("a minimum of ")
}
}
}
return ""
}
//设置flag
// Set sets the value of the named flag.
func (fs *FlagSet) Set(name, value string) error {
flag, ok := fs.formal[name]
if !ok {
return fmt.Errorf("no such flag -%v", name)
}
if err := flag.Value.Set(value); err != nil {
return err
}
if fs.actual == nil {
fs.actual = make(map[string]*Flag)
}
fs.actual[name] = flag
return nil
}
// FlagCount returns the number of flags that have been defined.
func (fs *FlagSet) FlagCount() int { return len(sortFlags(fs.formal)) }
// FlagCountUndeprecated returns the number of undeprecated flags that have been defined.
//返回未过时的flag数量
func (fs *FlagSet) FlagCountUndeprecated() int {
count := 0
for _, flag := range sortFlags(fs.formal) {
for _, name := range flag.Names {
if name[0] != '#' {
count++
break
}
}
}
return count
}
// NFlag returns the number of flags that have been set.
//返回已经被设置的flag的数量
func (fs *FlagSet) NFlag() int { return len(fs.actual) }
// Arg returns the i'th argument. Arg(0) is the first remaining argument
// after flags have been processed.
//获得某参数
func (fs *FlagSet) Arg(i int) string {
if i < 0 || i >= len(fs.args) {
return ""
}
return fs.args[i]
}
// NArg is the number of arguments remaining after flags have been processed.
//返回剩下的参数数量
func (fs *FlagSet) NArg() int { return len(fs.args) }
// Args returns the non-flag arguments.
//返回非flag参数
func (fs *FlagSet) Args() []string { return fs.args }
除了上述绑定的函数外，在这里面还定义基本类型的从Var到类似StringVar，再到String的的函数，我们看其中的一个：
func (fs *FlagSet) Var(value Value, names []string, usage string) {
// Remember the default value as a string; it won't change.
flag := &Flag{names, usage, value, value.String()}
for _, name := range names {
name = strings.TrimPrefix(name, "#")
_, alreadythere := fs.formal[name]
if alreadythere {
var msg string
if fs.name == "" {
msg = fmt.Sprintf("flag redefined: %s", name)
} else {
msg = fmt.Sprintf("%s flag redefined: %s", fs.name, name)
}
fmt.Fprintln(fs.Out(), msg)
panic(msg) // Happens only if flags are declared with identical names
}
if fs.formal == nil {
fs.formal = make(map[string]*Flag)
}
fs.formal[name] = flag
}
}
这个方法的基本含义就是就是将names对应的value是指到fs中的formal中，如果已经存在就报错。
//设置fs中那么的值类型为string的name
func (fs *FlagSet) StringVar(p *string, names []string, value string, usage string) {
fs.Var(newStringValue(value, p), names, usage)
}
//比StringVar更进一步的封装
func (fs *FlagSet) String(names []string, value string, usage string) *string {
p := new(string)
fs.StringVar(p, names, value, usage)
return p
}
好了，重头戏来了。
// parseOne parses one flag. It reports whether a flag was seen.
func (fs *FlagSet) parseOne() (bool, string, error) {
if len(fs.args) == 0 {
return false, "", nil
}
s := fs.args[0]
//第一字符必须是'-'
if len(s) == 0 || s[0] != '-' || len(s) == 1 {
return false, "", nil
}
//形如'--不接任何字符串' 这种格式也是错误的,其实这里不算是错误，只是把把单纯的'--'格式给过滤掉了
if s[1] == '-' && len(s) == 2 { // "--" terminates the flags
fs.args = fs.args[1:]
return false, "", nil
}
name := s[1:] //得到flag的下一个字符
//形如‘--=’这种格式是错误的
if len(name) == 0 || name[0] == '=' {
return false, "", fs.failf("bad flag syntax: %s", s)
}
//到这里，说明这个flag是合法的了
// it's a flag. does it have an argument?
fs.args = fs.args[1:] //将指针移动到下一个flag
hasValue := false
value := ""
//如果是形如‘name=’的格式,那么，value=[name[i+1:]
//此时，真正的name=name[:i]
if i := strings.Index(name, "="); i != -1 {
value = trimQuotes(name[i+1:])
hasValue = true
name = name[:i]
}
m := fs.formal
//查看已经规整的formal中是否存在这个name
flag, alreadythere := m[name] // BUG
//如果规整过的参数中还没有这个名字
if !alreadythere {
if name == "-help" || name == "help" || name == "h" { // special case for nice help message.
fs.usage()
return false, "", ErrHelp
}
//连续三个'-'也不行
if len(name) > 0 && name[0] == '-' {
return false, "", fs.failf("flag provided but not defined: -%s", name)
}
return false, name, ErrRetry
}
if fv, ok := flag.Value.(boolFlag); ok && fv.IsBoolFlag() { // special case: doesn't need an arg
if hasValue {
//这个flag有值，却不能设置，那么就要报错
if err := fv.Set(value); err != nil {
return false, "", fs.failf("invalid boolean value %q for -%s: %v", value, name, err)
}
} else {
//如果判断出来没有值，说明这个flag是一个bool类型的
fv.Set("true") //咩有值的情况就设置为true
}
} else {
// It must have a value, which might be the next argument.
if !hasValue && len(fs.args) > 0 {
// value is the next arg
//需要有值，而且这个值是参数表里面的下一个值
hasValue = true
value, fs.args = fs.args[0], fs.args[1:]
}
if !hasValue {
return false, "", fs.failf("flag needs an argument: -%s", name)
}
if err := flag.Value.Set(value); err != nil {
return false, "", fs.failf("invalid value %q for flag -%s: %v", value, name, err)
}
}
if fs.actual == nil {
fs.actual = make(map[string]*Flag)
}
//规整后的值
fs.actual[name] = flag
for i, n := range flag.Names {
if n == fmt.Sprintf("#%s", name) {
replacement := ""
for j := i; j < len(flag.Names); j++ {
if flag.Names[j][0] != '#' {
replacement = flag.Names[j]
break
}
}
if replacement != "" {
fmt.Fprintf(fs.Out(), "Warning: '-%s' is deprecated, it will be replaced by '-%s' soon. See usage.\n", name, replacement)
} else {
fmt.Fprintf(fs.Out(), "Warning: '-%s' is deprecated, it will be removed soon. See usage.\n", name)
}
}
}
return true, "", nil
}
func (fs *FlagSet) Parse(arguments []string) error {
fs.parsed = true
fs.args = arguments
for {
seen, name, err := fs.parseOne()
if seen {
//seen代表已经规整过了，不用再重新规整
continue
}
if err == nil {
//这也是严重错误
break
}
//连续三个'-'的情况
if err == ErrRetry {
if len(name) > 1 {
err = nil
for _, letter := range strings.Split(name, "") {
//将这个-去掉，然后刚在参数的最前面，重新来解析
fs.args = append([]string{"-" + letter}, fs.args...)
seen2, _, err2 := fs.parseOne()
if seen2 {
continue
}
if err2 != nil {
err = fs.failf("flag provided but not defined: -%s", name)
break
}
}
if err == nil {
continue
}
} else {
err = fs.failf("flag provided but not defined: -%s", name)
}
}
switch fs.errorHandling {
case ContinueOnError:
return err
case ExitOnError:
os.Exit(2)
case PanicOnError:
panic(err)
}
}
return nil
}
//将多个flagsets合并到一个flagset上面
func Merge(dest *FlagSet, flagsets ...*FlagSet) error {
for _, fset := range flagsets {
for k, f := range fset.formal {
if _, ok := dest.formal[k]; ok {
var err error
if fset.name == "" {
err = fmt.Errorf("flag redefined: %s", k)
} else {
err = fmt.Errorf("%s flag redefined: %s", fset.name, k)
}
fmt.Fprintln(fset.Out(), err.Error())
// Happens only if flags are declared with identical names
switch dest.errorHandling {
case ContinueOnError:
return err
case ExitOnError:
os.Exit(2)
case PanicOnError:
panic(err)
}
}
newF := *f //flag
newF.Value = mergeVal{f.Value, k, fset}
dest.formal[k] = &newF
}
}
return nil
}
最后再来看，这个包中定义的最后一个变量：
var CommandLine = NewFlagSet(os.Args[0], ExitOnError)CommandLine也是一个FlagSet哦，其中os.Args[0]就是程序的名字。
上面的结构比较完整了，留给我们的就是最后一个函数，他的访问级别死公开的：
func Parse() {
// Ignore errors; CommandLine is set for ExitOnError.
CommandLine.Parse(os.Args[1:])
}
来源：oschina
链接：https://my.oschina.net/u/197860/blog/507069