Package flag

import "flag"

Overview

Index

Examples

Overview ▸

Overview ▾

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   // double dashes are also permitted
-flag=x
-flag x  // non-boolean flags only

One or two dashes 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.

▸ Example

▾ Example

// These examples demonstrate more intricate uses of the flag package.
package main

import (
	"errors"
	"flag"
	"fmt"
	"strings"
	"time"
)

// Example 1: A single string flag called "species" with default value "gopher".
var species = flag.String("species", "gopher", "the species we are studying")

// Example 2: Two flags sharing a variable, so we can have a shorthand.
// The order of initialization is undefined, so make sure both use the
// same default value. They must be set up with an init function.
var gopherType string

func init() {
	const (
		defaultGopher = "pocket"
		usage         = "the variety of gopher"
	)
	flag.StringVar(&gopherType, "gopher_type", defaultGopher, usage)
	flag.StringVar(&gopherType, "g", defaultGopher, usage+" (shorthand)")
}

// Example 3: A user-defined flag type, a slice of durations.
type interval []time.Duration

// String is the method to format the flag's value, part of the flag.Value interface.
// The String method's output will be used in diagnostics.
func (i *interval) String() string {
	return fmt.Sprint(*i)
}

// Set is the method to set the flag value, part of the flag.Value interface.
// Set's argument is a string to be parsed to set the flag.
// It's a comma-separated list, so we split it.
func (i *interval) Set(value string) error {
	// If we wanted to allow the flag to be set multiple times,
	// accumulating values, we would delete this if statement.
	// That would permit usages such as
	//	-deltaT 10s -deltaT 15s
	// and other combinations.
	if len(*i) > 0 {
		return errors.New("interval flag already set")
	}
	for _, dt := range strings.Split(value, ",") {
		duration, err := time.ParseDuration(dt)
		if err != nil {
			return err
		}
		*i = append(*i, duration)
	}
	return nil
}

// Define a flag to accumulate durations. Because it has a special type,
// we need to use the Var function and therefore create the flag during
// init.

var intervalFlag interval

func init() {
	// Tie the command-line flag to the intervalFlag variable and
	// set a usage message.
	flag.Var(&intervalFlag, "deltaT", "comma-separated list of intervals to use between events")
}

func main() {
	// All the interesting pieces are with the variables declared above, but
	// to enable the flag package to see the flags defined there, one must
	// execute, typically at the start of main (not init!):
	//	flag.Parse()
	// We don't call it here because this code is a function called "Example"
	// that is part of the testing suite for the package, which has already
	// parsed the flags. When viewed at pkg.go.dev, however, the function is
	// renamed to "main" and it could be run as a standalone example.
}

Index ▸

Index ▾

Variables

func Arg(i int) string

func Args() []string

func Bool(name string, value bool, usage string) *bool

func BoolFunc(name, usage string, fn func(string) error)

func BoolVar(p *bool, name string, value bool, usage string)

func Duration(name string, value time.Duration, usage string) *time.Duration

func DurationVar(p *time.Duration, name string, value time.Duration, usage string)

func Float64(name string, value float64, usage string) *float64

func Float64Var(p *float64, name string, value float64, usage string)

func Func(name, usage string, fn func(string) error)

func Int(name string, value int, usage string) *int

func Int64(name string, value int64, usage string) *int64

func Int64Var(p *int64, name string, value int64, usage string)

func IntVar(p *int, name string, value int, usage string)

func Set(name, value string) error

func String(name string, value string, usage string) *string

func StringVar(p *string, name string, value string, usage string)

func TextVar(p encoding.TextUnmarshaler, name string, value encoding.TextMarshaler, usage string)

func Uint(name string, value uint, usage string) *uint

func Uint64(name string, value uint64, usage string) *uint64

func Uint64Var(p *uint64, name string, value uint64, usage string)

func UintVar(p *uint, name string, value uint, usage string)

func UnquoteUsage(flag *Flag) (name string, usage string)

func Var(value Value, name string, usage string)

func Visit(fn func(*Flag))

func VisitAll(fn func(*Flag))

type ErrorHandling

type Flag

func Lookup(name string) *Flag

type FlagSet

func NewFlagSet(name string, errorHandling ErrorHandling) *FlagSet

func (f *FlagSet) Arg(i int) string

func (f *FlagSet) Args() []string

func (f *FlagSet) Bool(name string, value bool, usage string) *bool

func (f *FlagSet) BoolFunc(name, usage string, fn func(string) error)

func (f *FlagSet) BoolVar(p *bool, name string, value bool, usage string)

func (f *FlagSet) Duration(name string, value time.Duration, usage string) *time.Duration

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

func (f *FlagSet) ErrorHandling() ErrorHandling

func (f *FlagSet) Float64(name string, value float64, usage string) *float64

func (f *FlagSet) Float64Var(p *float64, name string, value float64, usage string)

func (f *FlagSet) Func(name, usage string, fn func(string) error)

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

func (f *FlagSet) Int(name string, value int, usage string) *int

func (f *FlagSet) Int64(name string, value int64, usage string) *int64

func (f *FlagSet) Int64Var(p *int64, name string, value int64, usage string)

func (f *FlagSet) IntVar(p *int, name string, value int, usage string)

func (f *FlagSet) Lookup(name string) *Flag

func (f *FlagSet) NArg() int

func (f *FlagSet) NFlag() int

func (f *FlagSet) Name() string

func (f *FlagSet) Output() io.Writer

func (f *FlagSet) Parse(arguments []string) error

func (f *FlagSet) Parsed() bool

func (f *FlagSet) PrintDefaults()

func (f *FlagSet) Set(name, value string) error

func (f *FlagSet) SetOutput(output io.Writer)

func (f *FlagSet) String(name string, value string, usage string) *string

func (f *FlagSet) StringVar(p *string, name string, value string, usage string)

func (f *FlagSet) TextVar(p encoding.TextUnmarshaler, name string, value encoding.TextMarshaler, usage string)

func (f *FlagSet) Uint(name string, value uint, usage string) *uint

func (f *FlagSet) Uint64(name string, value uint64, usage string) *uint64

func (f *FlagSet) Uint64Var(p *uint64, name string, value uint64, usage string)

func (f *FlagSet) UintVar(p *uint, name string, value uint, usage string)

func (f *FlagSet) Var(value Value, name string, usage string)

func (f *FlagSet) Visit(fn func(*Flag))

func (f *FlagSet) VisitAll(fn func(*Flag))

type Getter

type Value

Examples (Expand All)

Package files

flag.go

Variables

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)

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")

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()
}

func Arg ¶

func Arg(i int) string

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 Args ¶

func Args() []string

Args returns the non-flag command-line arguments.

func Bool ¶

func Bool(name string, value bool, usage string) *bool

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 BoolFunc ¶ 1.21

func BoolFunc(name, usage string, fn func(string) error)

BoolFunc defines a flag with the specified name and usage string without requiring values. 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.

▸ Example

▾ Example

package main

import (
	"flag"
	"fmt"
	"os"
)

func main() {
	fs := flag.NewFlagSet("ExampleBoolFunc", flag.ContinueOnError)
	fs.SetOutput(os.Stdout)

	fs.BoolFunc("log", "logs a dummy message", func(s string) error {
		fmt.Println("dummy message:", s)
		return nil
	})
	fs.Parse([]string{"-log"})
	fs.Parse([]string{"-log=0"})

}

dummy message: true
dummy message: 0

func BoolVar ¶

func BoolVar(p *bool, name string, value bool, usage string)

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 Duration ¶

func Duration(name string, value time.Duration, usage string) *time.Duration

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 DurationVar ¶

func DurationVar(p *time.Duration, name string, value time.Duration, usage string)

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 Float64 ¶

func Float64(name string, value float64, usage string) *float64

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 Float64Var ¶

func Float64Var(p *float64, name string, value float64, usage string)

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 Func ¶ 1.16

func Func(name, usage string, fn func(string) error)

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.

▸ Example

▾ Example

package main

import (
	"errors"
	"flag"
	"fmt"
	"net"
	"os"
)

func main() {
	fs := flag.NewFlagSet("ExampleFunc", flag.ContinueOnError)
	fs.SetOutput(os.Stdout)
	var ip net.IP
	fs.Func("ip", "`IP address` to parse", func(s string) error {
		ip = net.ParseIP(s)
		if ip == nil {
			return errors.New("could not parse IP")
		}
		return nil
	})
	fs.Parse([]string{"-ip", "127.0.0.1"})
	fmt.Printf("{ip: %v, loopback: %t}\n\n", ip, ip.IsLoopback())

	// 256 is not a valid IPv4 component
	fs.Parse([]string{"-ip", "256.0.0.1"})
	fmt.Printf("{ip: %v, loopback: %t}\n\n", ip, ip.IsLoopback())

}

{ip: 127.0.0.1, loopback: true}

invalid value "256.0.0.1" for flag -ip: could not parse IP
Usage of ExampleFunc:
  -ip IP address
    	IP address to parse
{ip: <nil>, loopback: false}

func Int ¶

func Int(name string, value int, usage string) *int

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 Int64 ¶

func Int64(name string, value int64, usage string) *int64

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 Int64Var ¶

func Int64Var(p *int64, name string, value int64, usage string)

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 IntVar ¶

func IntVar(p *int, name string, value int, usage string)

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 NArg ¶

func NArg() int

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

func NFlag ¶

func NFlag() int

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

func Parse ¶

func Parse()

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.

func Parsed ¶

func Parsed() bool

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

func PrintDefaults ¶

func PrintDefaults()

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 Set ¶

func Set(name, value string) error

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

func String ¶

func String(name string, value string, usage string) *string

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 StringVar ¶

func StringVar(p *string, name string, value string, usage string)

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 TextVar ¶ 1.19

func TextVar(p encoding.TextUnmarshaler, name string, value encoding.TextMarshaler, usage string)

TextVar defines a flag with a specified name, default value, and usage string. The argument p must be a pointer to a variable that will hold the value of the flag, and p must implement encoding.TextUnmarshaler. If the flag is used, the flag value will be passed to p's UnmarshalText method. The type of the default value must be the same as the type of p.

▸ Example

▾ Example

package main

import (
	"flag"
	"fmt"
	"net"
	"os"
)

func main() {
	fs := flag.NewFlagSet("ExampleTextVar", flag.ContinueOnError)
	fs.SetOutput(os.Stdout)
	var ip net.IP
	fs.TextVar(&ip, "ip", net.IPv4(192, 168, 0, 100), "`IP address` to parse")
	fs.Parse([]string{"-ip", "127.0.0.1"})
	fmt.Printf("{ip: %v}\n\n", ip)

	// 256 is not a valid IPv4 component
	ip = nil
	fs.Parse([]string{"-ip", "256.0.0.1"})
	fmt.Printf("{ip: %v}\n\n", ip)

}

{ip: 127.0.0.1}

invalid value "256.0.0.1" for flag -ip: invalid IP address: 256.0.0.1
Usage of ExampleTextVar:
  -ip IP address
    	IP address to parse (default 192.168.0.100)
{ip: <nil>}

func Uint ¶

func Uint(name string, value uint, usage string) *uint

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 Uint64 ¶

func Uint64(name string, value uint64, usage string) *uint64

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 Uint64Var ¶

func Uint64Var(p *uint64, name string, value uint64, usage string)

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 UintVar ¶

func UintVar(p *uint, name string, value uint, usage string)

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 UnquoteUsage ¶ 1.5

func UnquoteUsage(flag *Flag) (name string, usage 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.

func Var ¶

func Var(value Value, name string, usage string)

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.

func Visit ¶

func Visit(fn func(*Flag))

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

func VisitAll ¶

func VisitAll(fn func(*Flag))

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

type ErrorHandling ¶

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.
)

type Flag ¶

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
}

func Lookup ¶

func Lookup(name string) *Flag

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

type FlagSet ¶

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.

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. 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()
    // contains filtered or unexported fields
}

▸ Example

▾ Example

package main

import (
	"flag"
	"fmt"
	"time"
)

func main() {

	start := func(args []string) {
		// A real program (not an example) would use flag.ExitOnError.
		fs := flag.NewFlagSet("start", flag.ContinueOnError)
		addr := fs.String("addr", ":8080", "`address` to listen on")
		if err := fs.Parse(args); err != nil {
			fmt.Printf("error: %s", err)
			return
		}
		fmt.Printf("starting server on %s\n", *addr)
	}

	stop := func(args []string) {
		fs := flag.NewFlagSet("stop", flag.ContinueOnError)
		timeout := fs.Duration("timeout", time.Second, "stop timeout duration")
		if err := fs.Parse(args); err != nil {
			fmt.Printf("error: %s", err)
			return
		}
		fmt.Printf("stopping server (timeout=%v)\n", *timeout)
	}

	main := func(args []string) {
		subArgs := args[2:] // Drop program name and command.
		switch args[1] {
		case "start":
			start(subArgs)
		case "stop":
			stop(subArgs)
		default:
			fmt.Printf("error: unknown command - %q\n", args[1])
			// In a real program (not an example) print to os.Stderr and exit the program with non-zero value.
		}
	}

	main([]string{"httpd", "start", "-addr", ":9999"})
	main([]string{"httpd", "stop"})
	main([]string{"http", "start", "-log-level", "verbose"})

}

starting server on :9999
stopping server (timeout=1s)
error: flag provided but not defined: -log-level

func NewFlagSet ¶

func NewFlagSet(name string, errorHandling ErrorHandling) *FlagSet

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 (*FlagSet) Arg ¶

func (f *FlagSet) Arg(i int) string

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 (*FlagSet) Args ¶

func (f *FlagSet) Args() []string

Args returns the non-flag arguments.

func (*FlagSet) Bool ¶

func (f *FlagSet) Bool(name string, value bool, usage string) *bool

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 (*FlagSet) BoolFunc ¶ 1.21

func (f *FlagSet) BoolFunc(name, usage string, fn func(string) error)

func (*FlagSet) BoolVar ¶

func (f *FlagSet) BoolVar(p *bool, name string, value bool, usage string)

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 (*FlagSet) Duration ¶

func (f *FlagSet) Duration(name string, value time.Duration, usage string) *time.Duration

func (*FlagSet) DurationVar ¶

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

func (*FlagSet) ErrorHandling ¶ 1.10

func (f *FlagSet) ErrorHandling() ErrorHandling

ErrorHandling returns the error handling behavior of the flag set.

func (*FlagSet) Float64 ¶

func (f *FlagSet) Float64(name string, value float64, usage string) *float64

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 (*FlagSet) Float64Var ¶

func (f *FlagSet) Float64Var(p *float64, name string, value float64, usage string)

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 (*FlagSet) Func ¶ 1.16

func (f *FlagSet) Func(name, usage string, fn func(string) error)

func (*FlagSet) Init ¶

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

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 (*FlagSet) Int ¶

func (f *FlagSet) Int(name string, value int, usage string) *int

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 (*FlagSet) Int64 ¶

func (f *FlagSet) Int64(name string, value int64, usage string) *int64

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 (*FlagSet) Int64Var ¶

func (f *FlagSet) Int64Var(p *int64, name string, value int64, usage string)

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 (*FlagSet) IntVar ¶

func (f *FlagSet) IntVar(p *int, name string, value int, usage string)

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 (*FlagSet) Lookup ¶

func (f *FlagSet) Lookup(name string) *Flag

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

func (*FlagSet) NArg ¶

func (f *FlagSet) NArg() int

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

func (*FlagSet) NFlag ¶

func (f *FlagSet) NFlag() int

NFlag returns the number of flags that have been set.

func (*FlagSet) Name ¶ 1.10

func (f *FlagSet) Name() string

Name returns the name of the flag set.

func (*FlagSet) Output ¶ 1.10

func (f *FlagSet) Output() io.Writer

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

func (*FlagSet) Parse ¶

func (f *FlagSet) Parse(arguments []string) error

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 (*FlagSet) Parsed ¶

func (f *FlagSet) Parsed() bool

Parsed reports whether f.Parse has been called.

func (*FlagSet) PrintDefaults ¶

func (f *FlagSet) PrintDefaults()

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 (*FlagSet) Set ¶

func (f *FlagSet) Set(name, value string) error

Set sets the value of the named flag.

func (*FlagSet) SetOutput ¶

func (f *FlagSet) SetOutput(output io.Writer)

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

func (*FlagSet) String ¶

func (f *FlagSet) String(name string, value string, usage string) *string

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 (*FlagSet) StringVar ¶

func (f *FlagSet) StringVar(p *string, name string, value string, usage string)

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 (*FlagSet) TextVar ¶ 1.19

func (f *FlagSet) TextVar(p encoding.TextUnmarshaler, name string, value encoding.TextMarshaler, usage string)

func (*FlagSet) Uint ¶

func (f *FlagSet) Uint(name string, value uint, usage string) *uint

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 (*FlagSet) Uint64 ¶

func (f *FlagSet) Uint64(name string, value uint64, usage string) *uint64

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 (*FlagSet) Uint64Var ¶

func (f *FlagSet) Uint64Var(p *uint64, name string, value uint64, usage string)

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 (*FlagSet) UintVar ¶

func (f *FlagSet) UintVar(p *uint, name string, value uint, usage string)

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 (*FlagSet) Var ¶

func (f *FlagSet) Var(value Value, name string, usage string)

func (*FlagSet) Visit ¶

func (f *FlagSet) Visit(fn func(*Flag))

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

func (*FlagSet) VisitAll ¶

func (f *FlagSet) VisitAll(fn func(*Flag))

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

type Getter ¶ 1.2

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() any
}

type Value ¶

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
}

▸ Example

▾ Example

package main

import (
	"flag"
	"fmt"
	"net/url"
)

type URLValue struct {
	URL *url.URL
}

func (v URLValue) String() string {
	if v.URL != nil {
		return v.URL.String()
	}
	return ""
}

func (v URLValue) Set(s string) error {
	if u, err := url.Parse(s); err != nil {
		return err
	} else {
		*v.URL = *u
	}
	return nil
}

var u = &url.URL{}

func main() {
	fs := flag.NewFlagSet("ExampleValue", flag.ExitOnError)
	fs.Var(&URLValue{u}, "url", "URL to parse")

	fs.Parse([]string{"-url", "https://golang.org/pkg/flag/"})
	fmt.Printf(`{scheme: %q, host: %q, path: %q}`, u.Scheme, u.Host, u.Path)

}

{scheme: "https", host: "golang.org", path: "/pkg/flag/"}