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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 after the flag 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

Code:

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

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 Example() {
    // 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 run it here because this is not a main function and
    // the testing suite has already parsed the flags.
}

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 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

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

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

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

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

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.

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.

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 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 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 returns true if the command-line flags have been parsed.

func PrintDefaults

func PrintDefaults()

PrintDefaults prints to standard error the default values of all defined command-line flags.

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 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 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

type ErrorHandling int

ErrorHandling defines how to handle flag parsing errors. 

const (
    ContinueOnError ErrorHandling = iota
    ExitOnError
    PanicOnError
)

type 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
}

A Flag represents the state of a flag.

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

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
}

A FlagSet represents a set of defined flags. The zero value of a FlagSet has no name and has ContinueOnError error handling.

func NewFlagSet

func NewFlagSet(name string, errorHandling ErrorHandling) *FlagSet

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

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.

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) 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

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 (*FlagSet) DurationVar

func (f *FlagSet) 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 (*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) 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) 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 flags in the set.

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) 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)

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 (*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

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

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 Value

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

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.