...
Run Format

Package flag

import "flag"
Overview
Index
Examples

Overview ▾

Package flag implements command-line flag parsing.

Usage:

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

This declares an integer flag, -flagname, stored in the pointer ip, with type *int.

import "flag"
var ip = flag.Int("flagname", 1234, "help message for flagname")

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:

-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 *

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

Index ▾

Variables
func Arg(i int) string
func Args() []string
func Bool(name string, value bool, usage string) *bool
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 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 NArg() int
func NFlag() int
func Parse()
func Parsed() bool
func PrintDefaults()
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 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) 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) Float64(name string, value float64, usage string) *float64
    func (f *FlagSet) Float64Var(p *float64, name string, value float64, usage string)
    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) 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) 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

Package

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 to standard error a usage message documenting all defined command-line flags. 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.

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

func Arg

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

func Args

Args returns the non-flag command-line arguments.

func Args() []string

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

func BoolVar

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)

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

func DurationVar

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

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

func Float64Var

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)

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

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

func Int64Var

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)

func IntVar

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)

func NArg

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

func NArg() int

func NFlag

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

func NFlag() int

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 Parse()

func Parsed

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

func Parsed() bool

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.
func PrintDefaults()

func Set

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

func Set(name, value string) error

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

func StringVar

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)

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

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

func Uint64Var

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)

func UintVar

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)

func UnquoteUsage

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

func Var

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 Var(value Value, name string, usage string)

func Visit

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))

func VisitAll

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))

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).
        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

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

func Lookup(name string) *Flag

type FlagSet

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()
        // contains filtered or unexported fields
}

func NewFlagSet

NewFlagSet returns a new, empty flag set with the specified name and error handling property.

func NewFlagSet(name string, errorHandling ErrorHandling) *FlagSet

func (*FlagSet) Arg

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

func (*FlagSet) Args

Args returns the non-flag arguments.

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

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

func (*FlagSet) BoolVar

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)

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

func (*FlagSet) DurationVar

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)

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

func (*FlagSet) Float64Var

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)

func (*FlagSet) Init

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)

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

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

func (*FlagSet) Int64Var

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)

func (*FlagSet) IntVar

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)

func (*FlagSet) Lookup

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

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

func (*FlagSet) NArg

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

func (f *FlagSet) NArg() int

func (*FlagSet) NFlag

NFlag returns the number of flags that have been set.

func (f *FlagSet) NFlag() int

func (*FlagSet) Parse

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

func (*FlagSet) Parsed

Parsed reports whether f.Parse has been called.

func (f *FlagSet) Parsed() bool

func (*FlagSet) PrintDefaults

PrintDefaults prints to standard error 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()

func (*FlagSet) Set

Set sets the value of the named flag.

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

func (*FlagSet) SetOutput

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

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

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

func (*FlagSet) StringVar

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)

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

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

func (*FlagSet) Uint64Var

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)

func (*FlagSet) UintVar

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)

func (*FlagSet) Var

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

func (*FlagSet) Visit

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))

func (*FlagSet) VisitAll

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))

type Getter

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.

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

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.

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