Source: flag.go in package flag

Package flag implements command-line flag parsing.

Usage

Define flags using flag.String(), Bool(), Int(), etc.

This declares an integer flag, -n, stored in the pointer nFlag, with type *int: import "flag" var nFlag = flag.Int("n", 1234, "help message for flag n") If you like, you can bind the flag to a variable using the Var() functions. var flagvar int func init() { flag.IntVar(&flagvar, "flagname", 1234, "help message for flagname") } Or you can create custom flags that satisfy the Value interface (with pointer receivers) and couple them to flag parsing by flag.Var(&flagVal, "name", "help message for flagname") For such flags, the default value is just the initial value of the variable.

After all flags are defined, call flag.Parse() to parse the command line into the defined flags.

Flags may then be used directly. If you're using the flags themselves, they are all pointers; if you bind to variables, they're values. fmt.Println("ip has value ", *ip) fmt.Println("flagvar has value ", flagvar)

After parsing, the arguments following the flags are available as the slice flag.Args() or individually as flag.Arg(i). The arguments are indexed from 0 through flag.NArg()-1.

Command line flag syntax

The following forms are permitted:

-flag -flag=x -flag x non-boolean flags only One or two minus signs may be used; they are equivalent. The last form is not permitted for boolean flags because the meaning of the command cmd -x * where * is a Unix shell wildcard, 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 "--".

Integer flags accept 1234, 0664, 0x1234 and may be negative. Boolean flags may be: 1, 0, t, f, T, F, true, false, TRUE, FALSE, True, False Duration flags accept any input valid for time.ParseDuration.

The default set of command-line flags is controlled by top-level functions. The FlagSet type allows one to define independent sets of flags, such as to implement subcommands in a command-line interface. The methods of FlagSet are analogous to the top-level functions for the command-line flag set.

package flag

import (
	"errors"
	"fmt"
	"io"
	"os"
	"reflect"
	"sort"
	"strconv"
	"strings"
	"time"
)

ErrHelp is the error returned if the -help or -h flag is invoked but no such flag is defined.

var ErrHelp = errors.New("flag: help requested")

errParse is returned by Set if a flag's value fails to parse, such as with an invalid integer for Int. It then gets wrapped through failf to provide more information.

var errParse = errors.New("parse error")

errRange is returned by Set if a flag's value is out of range. It then gets wrapped through failf to provide more information.

var errRange = errors.New("value out of range")

func numError(err error) error {
	ne, ok := err.(*strconv.NumError)
	if !ok {
		return err
	}
	if ne.Err == strconv.ErrSyntax {
		return errParse
	}
	if ne.Err == strconv.ErrRange {
		return errRange
	}
	return err
}

-- bool Value

type boolValue bool

func newBoolValue(val bool, p *bool) *boolValue {
	*p = val
	return (*boolValue)(p)
}

func (b *boolValue) Set(s string) error {
	v, err := strconv.ParseBool(s)
	if err != nil {
		err = errParse
	}
	*b = boolValue(v)
	return err
}

func (b *boolValue) Get() interface{} { return bool(*b) }

func (b *boolValue) String() string { return strconv.FormatBool(bool(*b)) }

func (b *boolValue) IsBoolFlag() bool { return true }

optional interface to indicate boolean flags that can be supplied without "=value" text

type boolFlag interface {
	Value
	IsBoolFlag() bool
}

-- int Value

type intValue int

func newIntValue(val int, p *int) *intValue {
	*p = val
	return (*intValue)(p)
}

func (i *intValue) Set(s string) error {
	v, err := strconv.ParseInt(s, 0, strconv.IntSize)
	if err != nil {
		err = numError(err)
	}
	*i = intValue(v)
	return err
}

func (i *intValue) Get() interface{} { return int(*i) }

func (i *intValue) String() string { return strconv.Itoa(int(*i)) }

-- int64 Value

type int64Value int64

func newInt64Value(val int64, p *int64) *int64Value {
	*p = val
	return (*int64Value)(p)
}

func (i *int64Value) Set(s string) error {
	v, err := strconv.ParseInt(s, 0, 64)
	if err != nil {
		err = numError(err)
	}
	*i = int64Value(v)
	return err
}

func (i *int64Value) Get() interface{} { return int64(*i) }

func (i *int64Value) String() string { return strconv.FormatInt(int64(*i), 10) }

-- uint Value

type uintValue uint

func newUintValue(val uint, p *uint) *uintValue {
	*p = val
	return (*uintValue)(p)
}

func (i *uintValue) Set(s string) error {
	v, err := strconv.ParseUint(s, 0, strconv.IntSize)
	if err != nil {
		err = numError(err)
	}
	*i = uintValue(v)
	return err
}

func (i *uintValue) Get() interface{} { return uint(*i) }

func (i *uintValue) String() string { return strconv.FormatUint(uint64(*i), 10) }

-- uint64 Value

type uint64Value uint64

func newUint64Value(val uint64, p *uint64) *uint64Value {
	*p = val
	return (*uint64Value)(p)
}

func (i *uint64Value) Set(s string) error {
	v, err := strconv.ParseUint(s, 0, 64)
	if err != nil {
		err = numError(err)
	}
	*i = uint64Value(v)
	return err
}

func (i *uint64Value) Get() interface{} { return uint64(*i) }

func (i *uint64Value) String() string { return strconv.FormatUint(uint64(*i), 10) }

-- string Value

type stringValue string

func newStringValue(val string, p *string) *stringValue {
	*p = val
	return (*stringValue)(p)
}

func (s *stringValue) Set(val string) error {
	*s = stringValue(val)
	return nil
}

func (s *stringValue) Get() interface{} { return string(*s) }

func (s *stringValue) String() string { return string(*s) }

-- float64 Value

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)
	if err != nil {
		err = numError(err)
	}
	*f = float64Value(v)
	return err
}

func (f *float64Value) Get() interface{} { return float64(*f) }

func (f *float64Value) String() string { return strconv.FormatFloat(float64(*f), 'g', -1, 64) }

-- time.Duration Value

type durationValue time.Duration

func newDurationValue(val time.Duration, p *time.Duration) *durationValue {
	*p = val
	return (*durationValue)(p)
}

func (d *durationValue) Set(s string) error {
	v, err := time.ParseDuration(s)
	if err != nil {
		err = errParse
	}
	*d = durationValue(v)
	return err
}

func (d *durationValue) Get() interface{} { return time.Duration(*d) }

func (d *durationValue) String() string { return (*time.Duration)(d).String() }

type funcValue func(string) error

func (f funcValue) Set(s string) error { return f(s) }

func (f funcValue) String() string { return "" }

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. Set is called once, in command line order, for each flag present. The flag package may call the String method with a zero-valued receiver, such as a nil pointer.

type Value interface {
	String() string
	Set(string) error
}

Getter is an interface that allows the contents of a Value to be retrieved. It wraps the Value interface, rather than being part of it, because it appeared after Go 1 and its compatibility rules. All Value types provided by this package satisfy the Getter interface, except the type used by Func.

type Getter interface {
	Value
	Get() interface{}
}

ErrorHandling defines how FlagSet.Parse behaves if the parse fails.

type ErrorHandling int

These constants cause FlagSet.Parse to behave as described if the parse fails.

const (
	ContinueOnError ErrorHandling = iota // Return a descriptive error.
	ExitOnError                          // Call os.Exit(2) or for -h/-help Exit(0).
	PanicOnError                         // Call panic with a descriptive error.
)

A FlagSet represents a set of defined flags. The zero value of a FlagSet has no name and has ContinueOnError error handling. Flag names must be unique within a FlagSet. An attempt to define a flag whose name is already in use will cause a panic.

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. What happens after Usage is called depends on the ErrorHandling setting; for the command line, this defaults to ExitOnError, which exits the program after calling Usage.

	Usage func()

	name          string
	parsed        bool
	actual        map[string]*Flag
	formal        map[string]*Flag
	args          []string // arguments after flags
	errorHandling ErrorHandling
	output        io.Writer // nil means stderr; use Output() accessor
}

A Flag represents the state of a flag.

type Flag struct {
	Name     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
}

sortFlags returns the flags as a slice in lexicographical sorted order.

func sortFlags(flags map[string]*Flag) []*Flag {
	result := make([]*Flag, len(flags))
	i := 0
	for _, f := range flags {
		result[i] = f
		i++
	}
	sort.Slice(result, func(i, j int) bool {
		return result[i].Name < result[j].Name
	})
	return result
}

Output returns the destination for usage and error messages. os.Stderr is returned if output was not set or was set to nil.

func (f *FlagSet) Output() io.Writer {
	if f.output == nil {
		return os.Stderr
	}
	return f.output
}

Name returns the name of the flag set.

func (f *FlagSet) Name() string {
	return f.name
}

ErrorHandling returns the error handling behavior of the flag set.

func (f *FlagSet) ErrorHandling() ErrorHandling {
	return f.errorHandling
}

SetOutput sets the destination for usage and error messages. If output is nil, os.Stderr is used.

func (f *FlagSet) SetOutput(output io.Writer) {
	f.output = output
}

VisitAll visits the flags in lexicographical order, calling fn for each. It visits all flags, even those not set.

func (f *FlagSet) VisitAll(fn func(*Flag)) {
	for _, flag := range sortFlags(f.formal) {
		fn(flag)
	}
}

VisitAll visits the command-line flags in lexicographical order, calling fn for each. It visits all flags, even those not set.

func VisitAll(fn func(*Flag)) {
	CommandLine.VisitAll(fn)
}

Visit visits the flags in lexicographical order, calling fn for each. It visits only those flags that have been set.

func (f *FlagSet) Visit(fn func(*Flag)) {
	for _, flag := range sortFlags(f.actual) {
		fn(flag)
	}
}

Visit visits the command-line flags in lexicographical order, calling fn for each. It visits only those flags that have been set.

func Visit(fn func(*Flag)) {
	CommandLine.Visit(fn)
}

Lookup returns the Flag structure of the named flag, returning nil if none exists.

func (f *FlagSet) Lookup(name string) *Flag {
	return f.formal[name]
}

Lookup returns the Flag structure of the named command-line flag, returning nil if none exists.

func Lookup(name string) *Flag {
	return CommandLine.formal[name]
}

Set sets the value of the named flag.

func (f *FlagSet) Set(name, value string) error {
	flag, ok := f.formal[name]
	if !ok {
		return fmt.Errorf("no such flag -%v", name)
	}
	err := flag.Value.Set(value)
	if err != nil {
		return err
	}
	if f.actual == nil {
		f.actual = make(map[string]*Flag)
	}
	f.actual[name] = flag
	return nil
}

Set sets the value of the named command-line flag.

func Set(name, value string) error {
	return CommandLine.Set(name, value)
}

isZeroValue determines whether the string represents the zero value for a flag.

Build a zero value of the flag's Value type, and see if the result of calling its String method equals the value passed in. This works unless the Value type is itself an interface type.

	typ := reflect.TypeOf(flag.Value)
	var z reflect.Value
	if typ.Kind() == reflect.Ptr {
		z = reflect.New(typ.Elem())
	} else {
		z = reflect.Zero(typ)
	}
	return value == z.Interface().(Value).String()
}

UnquoteUsage extracts a back-quoted name from the usage string for a flag and returns it and the un-quoted usage. Given "a `name` to show" it returns ("name", "a name to show"). If there are no back quotes, the name is an educated guess of the type of the flag's value, or the empty string if the flag is boolean.

Look for a back-quoted name, but avoid the strings package.

	usage = flag.Usage
	for i := 0; i < len(usage); i++ {
		if usage[i] == '`' {
			for j := i + 1; j < len(usage); j++ {
				if usage[j] == '`' {
					name = usage[i+1 : j]
					usage = usage[:i] + name + usage[j+1:]
					return name, usage
				}
			}
			break // Only one back quote; use type name.
		}

No explicit name, so use type if we can find one.

	name = "value"
	switch flag.Value.(type) {
	case boolFlag:
		name = ""
	case *durationValue:
		name = "duration"
	case *float64Value:
		name = "float"
	case *intValue, *int64Value:
		name = "int"
	case *stringValue:
		name = "string"
	case *uintValue, *uint64Value:
		name = "uint"
	}
	return
}

PrintDefaults prints, to standard error unless configured otherwise, the default values of all defined command-line flags in the set. See the documentation for the global function PrintDefaults for more information.

func (f *FlagSet) PrintDefaults() {
	f.VisitAll(func(flag *Flag) {
		s := fmt.Sprintf("  -%s", flag.Name) // Two spaces before -; see next two comments.
		name, usage := UnquoteUsage(flag)
		if len(name) > 0 {
			s += " " + name

Boolean flags of one ASCII letter are so common we treat them specially, putting their usage on the same line.

		if len(s) <= 4 { // space, space, '-', 'x'.
			s += "\t"

Four spaces before the tab triggers good alignment for both 4- and 8-space tab stops.

			s += "\n    \t"
		}
		s += strings.ReplaceAll(usage, "\n", "\n    \t")

		if !isZeroValue(flag, flag.DefValue) {

put quotes on the value

				s += fmt.Sprintf(" (default %q)", flag.DefValue)
			} else {
				s += fmt.Sprintf(" (default %v)", flag.DefValue)
			}
		}
		fmt.Fprint(f.Output(), s, "\n")
	})
}

PrintDefaults prints, to standard error unless configured otherwise, a usage message showing the default settings of all defined command-line flags. For an integer valued flag x, the default output has the form -x int usage-message-for-x (default 7) The usage message will appear on a separate line for anything but a bool flag with a one-byte name. For bool flags, the type is omitted and if the flag name is one byte the usage message appears on the same line. The parenthetical default is omitted if the default is the zero value for the type. The listed type, here int, can be changed by placing a back-quoted name in the flag's usage string; the first such item in the message is taken to be a parameter name to show in the message and the back quotes are stripped from the message when displayed. For instance, given flag.String("I", "", "search `directory` for include files") the output will be -I directory search directory for include files. To change the destination for flag messages, call CommandLine.SetOutput.

func PrintDefaults() {
	CommandLine.PrintDefaults()
}

defaultUsage is the default function to print a usage message.

func (f *FlagSet) defaultUsage() {
	if f.name == "" {
		fmt.Fprintf(f.Output(), "Usage:\n")
	} else {
		fmt.Fprintf(f.Output(), "Usage of %s:\n", f.name)
	}
	f.PrintDefaults()
}

NOTE: Usage is not just defaultUsage(CommandLine) because it serves (via godoc flag Usage) as the example for how to write your own usage function.

Usage prints a usage message documenting all defined command-line flags to CommandLine's output, which by default is os.Stderr. It is called when an error occurs while parsing flags. The function is a variable that may be changed to point to a custom function. By default it prints a simple header and calls PrintDefaults; for details about the format of the output and how to control it, see the documentation for PrintDefaults. Custom usage functions may choose to exit the program; by default exiting happens anyway as the command line's error handling strategy is set to ExitOnError.

var Usage = func() {
	fmt.Fprintf(CommandLine.Output(), "Usage of %s:\n", os.Args[0])
	PrintDefaults()
}

NFlag returns the number of flags that have been set.

func (f *FlagSet) NFlag() int { return len(f.actual) }

NFlag returns the number of command-line flags that have been set.

func NFlag() int { return len(CommandLine.actual) }

Arg returns the i'th argument. Arg(0) is the first remaining argument after flags have been processed. Arg returns an empty string if the requested element does not exist.

func (f *FlagSet) Arg(i int) string {
	if i < 0 || i >= len(f.args) {
		return ""
	}
	return f.args[i]
}

Arg returns the i'th command-line argument. Arg(0) is the first remaining argument after flags have been processed. Arg returns an empty string if the requested element does not exist.

func Arg(i int) string {
	return CommandLine.Arg(i)
}

NArg is the number of arguments remaining after flags have been processed.

func (f *FlagSet) NArg() int { return len(f.args) }

NArg is the number of arguments remaining after flags have been processed.

func NArg() int { return len(CommandLine.args) }

Args returns the non-flag arguments.

func (f *FlagSet) Args() []string { return f.args }

Args returns the non-flag command-line arguments.

func Args() []string { return CommandLine.args }

BoolVar defines a bool flag with specified name, default value, and usage string. The argument p points to a bool variable in which to store the value of the flag.

func (f *FlagSet) BoolVar(p *bool, name string, value bool, usage string) {
	f.Var(newBoolValue(value, p), name, usage)
}

BoolVar defines a bool flag with specified name, default value, and usage string. The argument p points to a bool variable in which to store the value of the flag.

func BoolVar(p *bool, name string, value bool, usage string) {
	CommandLine.Var(newBoolValue(value, p), name, usage)
}

Bool defines a bool flag with specified name, default value, and usage string. The return value is the address of a bool variable that stores the value of the flag.

func (f *FlagSet) Bool(name string, value bool, usage string) *bool {
	p := new(bool)
	f.BoolVar(p, name, value, usage)
	return p
}

Bool defines a bool flag with specified name, default value, and usage string. The return value is the address of a bool variable that stores the value of the flag.

func Bool(name string, value bool, usage string) *bool {
	return CommandLine.Bool(name, value, usage)
}

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 (f *FlagSet) IntVar(p *int, name string, value int, usage string) {
	f.Var(newIntValue(value, p), name, usage)
}

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)
}

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 (f *FlagSet) Int(name string, value int, usage string) *int {
	p := new(int)
	f.IntVar(p, name, value, usage)
	return p
}

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)
}

Int64Var defines an int64 flag with specified name, default value, and usage string. The argument p points to an int64 variable in which to store the value of the flag.

func (f *FlagSet) Int64Var(p *int64, name string, value int64, usage string) {
	f.Var(newInt64Value(value, p), name, usage)
}

Int64Var defines an int64 flag with specified name, default value, and usage string. The argument p points to an int64 variable in which to store the value of the flag.

func Int64Var(p *int64, name string, value int64, usage string) {
	CommandLine.Var(newInt64Value(value, p), name, usage)
}

Int64 defines an int64 flag with specified name, default value, and usage string. The return value is the address of an int64 variable that stores the value of the flag.

func (f *FlagSet) Int64(name string, value int64, usage string) *int64 {
	p := new(int64)
	f.Int64Var(p, name, value, usage)
	return p
}

Int64 defines an int64 flag with specified name, default value, and usage string. The return value is the address of an int64 variable that stores the value of the flag.

func Int64(name string, value int64, usage string) *int64 {
	return CommandLine.Int64(name, value, usage)
}

UintVar defines a uint flag with specified name, default value, and usage string. The argument p points to a uint variable in which to store the value of the flag.

func (f *FlagSet) UintVar(p *uint, name string, value uint, usage string) {
	f.Var(newUintValue(value, p), name, usage)
}

UintVar defines a uint flag with specified name, default value, and usage string. The argument p points to a uint variable in which to store the value of the flag.

func UintVar(p *uint, name string, value uint, usage string) {
	CommandLine.Var(newUintValue(value, p), name, usage)
}

Uint defines a uint flag with specified name, default value, and usage string. The return value is the address of a uint variable that stores the value of the flag.

func (f *FlagSet) Uint(name string, value uint, usage string) *uint {
	p := new(uint)
	f.UintVar(p, name, value, usage)
	return p
}

Uint defines a uint flag with specified name, default value, and usage string. The return value is the address of a uint variable that stores the value of the flag.

func Uint(name string, value uint, usage string) *uint {
	return CommandLine.Uint(name, value, usage)
}

Uint64Var defines a uint64 flag with specified name, default value, and usage string. The argument p points to a uint64 variable in which to store the value of the flag.

func (f *FlagSet) Uint64Var(p *uint64, name string, value uint64, usage string) {
	f.Var(newUint64Value(value, p), name, usage)
}

Uint64Var defines a uint64 flag with specified name, default value, and usage string. The argument p points to a uint64 variable in which to store the value of the flag.

func Uint64Var(p *uint64, name string, value uint64, usage string) {
	CommandLine.Var(newUint64Value(value, p), name, usage)
}

Uint64 defines a uint64 flag with specified name, default value, and usage string. The return value is the address of a uint64 variable that stores the value of the flag.

func (f *FlagSet) Uint64(name string, value uint64, usage string) *uint64 {
	p := new(uint64)
	f.Uint64Var(p, name, value, usage)
	return p
}

Uint64 defines a uint64 flag with specified name, default value, and usage string. The return value is the address of a uint64 variable that stores the value of the flag.

func Uint64(name string, value uint64, usage string) *uint64 {
	return CommandLine.Uint64(name, value, usage)
}

StringVar defines a string flag with specified name, default value, and usage string. The argument p points to a string variable in which to store the value of the flag.

func (f *FlagSet) StringVar(p *string, name string, value string, usage string) {
	f.Var(newStringValue(value, p), name, usage)
}

StringVar defines a string flag with specified name, default value, and usage string. The argument p points to a string variable in which to store the value of the flag.

func StringVar(p *string, name string, value string, usage string) {
	CommandLine.Var(newStringValue(value, p), name, usage)
}

String defines a string flag with specified name, default value, and usage string. The return value is the address of a string variable that stores the value of the flag.

func (f *FlagSet) String(name string, value string, usage string) *string {
	p := new(string)
	f.StringVar(p, name, value, usage)
	return p
}

String defines a string flag with specified name, default value, and usage string. The return value is the address of a string variable that stores the value of the flag.

func String(name string, value string, usage string) *string {
	return CommandLine.String(name, value, usage)
}

Float64Var defines a float64 flag with specified name, default value, and usage string. The argument p points to a float64 variable in which to store the value of the flag.

func (f *FlagSet) Float64Var(p *float64, name string, value float64, usage string) {
	f.Var(newFloat64Value(value, p), name, usage)
}

Float64Var defines a float64 flag with specified name, default value, and usage string. The argument p points to a float64 variable in which to store the value of the flag.

func Float64Var(p *float64, name string, value float64, usage string) {
	CommandLine.Var(newFloat64Value(value, p), name, usage)
}

Float64 defines a float64 flag with specified name, default value, and usage string. The return value is the address of a float64 variable that stores the value of the flag.

func (f *FlagSet) Float64(name string, value float64, usage string) *float64 {
	p := new(float64)
	f.Float64Var(p, name, value, usage)
	return p
}

Float64 defines a float64 flag with specified name, default value, and usage string. The return value is the address of a float64 variable that stores the value of the flag.

func Float64(name string, value float64, usage string) *float64 {
	return CommandLine.Float64(name, value, usage)
}

DurationVar defines a time.Duration flag with specified name, default value, and usage string. The argument p points to a time.Duration variable in which to store the value of the flag. The flag accepts a value acceptable to time.ParseDuration.

func (f *FlagSet) DurationVar(p *time.Duration, name string, value time.Duration, usage string) {
	f.Var(newDurationValue(value, p), name, usage)
}

func DurationVar(p *time.Duration, name string, value time.Duration, usage string) {
	CommandLine.Var(newDurationValue(value, p), name, usage)
}

Duration defines a time.Duration flag with specified name, default value, and usage string. The return value is the address of a time.Duration variable that stores the value of the flag. The flag accepts a value acceptable to time.ParseDuration.

func (f *FlagSet) Duration(name string, value time.Duration, usage string) *time.Duration {
	p := new(time.Duration)
	f.DurationVar(p, name, value, usage)
	return p
}

func Duration(name string, value time.Duration, usage string) *time.Duration {
	return CommandLine.Duration(name, value, usage)
}

Func defines a flag with the specified name and usage string. Each time the flag is seen, fn is called with the value of the flag. If fn returns a non-nil error, it will be treated as a flag value parsing error.

func (f *FlagSet) Func(name, usage string, fn func(string) error) {
	f.Var(funcValue(fn), name, usage)
}

func Func(name, usage string, fn func(string) error) {
	CommandLine.Func(name, usage, fn)
}

Var defines a flag with the specified name and usage string. The type and value of the flag are represented by the first argument, of type Value, which typically holds a user-defined implementation of Value. For instance, the caller could create a flag that turns a comma-separated string into a slice of strings by giving the slice the methods of Value; in particular, Set would decompose the comma-separated string into the slice.

Remember the default value as a string; it won't change.

	flag := &Flag{name, usage, value, value.String()}
	_, alreadythere := f.formal[name]
	if alreadythere {
		var msg string
		if f.name == "" {
			msg = fmt.Sprintf("flag redefined: %s", name)
		} else {
			msg = fmt.Sprintf("%s flag redefined: %s", f.name, name)
		}
		fmt.Fprintln(f.Output(), msg)
		panic(msg) // Happens only if flags are declared with identical names
	}
	if f.formal == nil {
		f.formal = make(map[string]*Flag)
	}
	f.formal[name] = flag
}

func Var(value Value, name string, usage string) {
	CommandLine.Var(value, name, usage)
}

failf prints to standard error a formatted error and usage message and returns the error.

func (f *FlagSet) failf(format string, a ...interface{}) error {
	err := fmt.Errorf(format, a...)
	fmt.Fprintln(f.Output(), err)
	f.usage()
	return err
}

usage calls the Usage method for the flag set if one is specified, or the appropriate default usage function otherwise.

func (f *FlagSet) usage() {
	if f.Usage == nil {
		f.defaultUsage()
	} else {
		f.Usage()
	}
}

parseOne parses one flag. It reports whether a flag was seen.

func (f *FlagSet) parseOne() (bool, error) {
	if len(f.args) == 0 {
		return false, nil
	}
	s := f.args[0]
	if len(s) < 2 || s[0] != '-' {
		return false, nil
	}
	numMinuses := 1
	if s[1] == '-' {
		numMinuses++
		if len(s) == 2 { // "--" terminates the flags
			f.args = f.args[1:]
			return false, nil
		}
	}
	name := s[numMinuses:]
	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:]
	hasValue := false
	value := ""
	for i := 1; i < len(name); i++ { // equals cannot be first
		if name[i] == '=' {
			value = name[i+1:]
			hasValue = 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 hasValue {
			if err := fv.Set(value); err != nil {
				return false, f.failf("invalid boolean value %q for -%s: %v", value, name, err)
			}
		} else {
			if err := fv.Set("true"); err != nil {
				return false, f.failf("invalid boolean flag %s: %v", name, err)
			}
		}

It must have a value, which might be the next argument.

value is the next arg

			hasValue = true
			value, f.args = f.args[0], f.args[1:]
		}
		if !hasValue {
			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
}

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 or -h were set but not defined.

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:
			if err == ErrHelp {
				os.Exit(0)
			}
			os.Exit(2)
		case PanicOnError:
			panic(err)
		}
	}
	return nil
}

Parsed reports whether f.Parse has been called.

func (f *FlagSet) Parsed() bool {
	return f.parsed
}

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.

Ignore errors; CommandLine is set for ExitOnError.

	CommandLine.Parse(os.Args[1:])
}

Parsed reports whether the command-line flags have been parsed.

func Parsed() bool {
	return CommandLine.Parsed()
}

CommandLine is the default set of command-line flags, parsed from os.Args. The top-level functions such as BoolVar, Arg, and so on are wrappers for the methods of CommandLine.

var CommandLine = NewFlagSet(os.Args[0], ExitOnError)

Override generic FlagSet default Usage with call to global Usage. Note: This is not CommandLine.Usage = Usage, because we want any eventual call to use any updated value of Usage, not the value it has when this line is run.

	CommandLine.Usage = commandLineUsage
}

func commandLineUsage() {
	Usage()
}

NewFlagSet returns a new, empty flag set with the specified name and error handling property. If the name is not empty, it will be printed in the default usage message and in error messages.

func NewFlagSet(name string, errorHandling ErrorHandling) *FlagSet {
	f := &FlagSet{
		name:          name,
		errorHandling: errorHandling,
	}
	f.Usage = f.defaultUsage
	return f
}

Init sets the name and error handling property for a flag set. By default, the zero FlagSet uses an empty name and the ContinueOnError error handling policy.

func (f *FlagSet) Init(name string, errorHandling ErrorHandling) {
	f.name = name
	f.errorHandling = errorHandling