flag

Imports

Imports # 

"encoding"
"errors"
"fmt"
"io"
"os"
"reflect"
"runtime"
"slices"
"strconv"
"strings"
"time"

Constants & Variables

CommandLine var #

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 *FlagSet

ContinueOnError const #

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

const ContinueOnError ErrorHandling = iota

ErrHelp var #

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

var ErrHelp = *ast.CallExpr

ExitOnError const #

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

const ExitOnError

PanicOnError const #

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

const PanicOnError

Usage var #

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 = *ast.FuncLit

errParse var #

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 = *ast.CallExpr

errRange var #

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 = *ast.CallExpr

Type Aliases

ErrorHandling type #

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

type ErrorHandling int

boolFuncValue type #

-- boolFunc Value

type boolFuncValue func(string) error

boolValue type #

-- bool Value

type boolValue bool

durationValue type #

-- time.Duration Value

type durationValue time.Duration

float64Value type #

-- float64 Value

type float64Value float64

funcValue type #

-- func Value

type funcValue func(string) error

int64Value type #

-- int64 Value

type int64Value int64

intValue type #

-- int Value

type intValue int

stringValue type #

-- string Value

type stringValue string

uint64Value type #

-- uint64 Value

type uint64Value uint64

uintValue type #

-- uint Value

type uintValue uint

Interfaces

Getter 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, except the type used by [Func].

type Getter interface {
Value
Get() any
}

Value interface #

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
}

boolFlag interface #

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

type boolFlag interface {
Value
IsBoolFlag() bool
}

Structs

Flag struct #

A Flag represents the state of a flag.

type Flag struct {
Name string
Usage string
Value Value
DefValue string
}

FlagSet struct #

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 func()
name string
parsed bool
actual map[string]*Flag
formal map[string]*Flag
args []string
errorHandling ErrorHandling
output io.Writer
undef map[string]string
}

textValue struct #

-- encoding.TextUnmarshaler Value

type textValue struct {
p encoding.TextUnmarshaler
}

Functions

Arg method #

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

Arg function #

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

Args function #

Args returns the non-flag command-line arguments.

func Args() []string

Args method #

Args returns the non-flag arguments.

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

Bool method #

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

Bool function #

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

BoolFunc function #

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.

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

BoolFunc method #

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.

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

BoolVar function #

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)

BoolVar method #

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)

Duration function #

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

Duration method #

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

DurationVar function #

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)

DurationVar method #

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)

ErrorHandling method #

ErrorHandling returns the error handling behavior of the flag set.

func (f *FlagSet) ErrorHandling() ErrorHandling

Float64 function #

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

Float64 method #

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

Float64Var function #

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)

Float64Var method #

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 function #

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

Func method #

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

Get method # 

func (i *uint64Value) Get() any

Get method # 

func (v textValue) Get() interface{}

Get method # 

func (s *stringValue) Get() any

Get method # 

func (i *uintValue) Get() any

Get method # 

func (i *intValue) Get() any

Get method # 

func (b *boolValue) Get() any

Get method # 

func (f *float64Value) Get() any

Get method # 

func (d *durationValue) Get() any

Get method # 

func (i *int64Value) Get() any

Init method #

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)

Int function #

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

Int method #

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

Int64 method #

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

Int64 function #

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

Int64Var function #

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)

Int64Var method #

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)

IntVar function #

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)

IntVar method #

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)

IsBoolFlag method # 

func (b *boolValue) IsBoolFlag() bool

IsBoolFlag method # 

func (f boolFuncValue) IsBoolFlag() bool

Lookup method #

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

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

Lookup function #

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

func Lookup(name string) *Flag

NArg method #

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

func (f *FlagSet) NArg() int

NArg function #

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

func NArg() int

NFlag method #

NFlag returns the number of flags that have been set.

func (f *FlagSet) NFlag() int

NFlag function #

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

func NFlag() int

Name method #

Name returns the name of the flag set.

func (f *FlagSet) Name() string

NewFlagSet function #

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

Output method #

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

Parse method #

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

Parse function #

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()

Parsed method #

Parsed reports whether f.Parse has been called.

func (f *FlagSet) Parsed() bool

Parsed function #

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

func Parsed() bool

PrintDefaults function #

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()

PrintDefaults method #

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()

Set method # 

func (d *durationValue) Set(s string) error

Set method # 

func (i *uintValue) Set(s string) error

Set function #

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

func Set(name string, value string) error

Set method #

Set sets the value of the named flag.

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

Set method # 

func (i *uint64Value) Set(s string) error

Set method # 

func (f boolFuncValue) Set(s string) error

Set method # 

func (f funcValue) Set(s string) error

Set method # 

func (v textValue) Set(s string) error

Set method # 

func (i *int64Value) Set(s string) error

Set method # 

func (f *float64Value) Set(s string) error

Set method # 

func (i *intValue) Set(s string) error

Set method # 

func (s *stringValue) Set(val string) error

Set method # 

func (b *boolValue) Set(s string) error

SetOutput method #

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

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

String method # 

func (f *float64Value) String() string

String method # 

func (f funcValue) String() string

String method # 

func (b *boolValue) String() string

String method # 

func (i *intValue) String() string

String method # 

func (i *int64Value) String() string

String method # 

func (i *uintValue) String() string

String method # 

func (i *uint64Value) String() string

String method #

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

String method # 

func (v textValue) String() string

String method # 

func (s *stringValue) String() string

String method # 

func (f boolFuncValue) String() string

String function #

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

String method # 

func (d *durationValue) String() string

StringVar method #

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)

StringVar function #

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)

TextVar method #

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.

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

TextVar function #

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.

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

Uint method #

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

Uint function #

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

Uint64 method #

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

Uint64 function #

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

Uint64Var method #

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)

Uint64Var function #

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)

UintVar method #

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)

UintVar function #

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)

UnquoteUsage function #

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)

Var function #

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)

Var method #

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)

Visit function #

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))

Visit method #

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))

VisitAll method #

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))

VisitAll function #

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))

commandLineUsage function # 

func commandLineUsage()

defaultUsage method #

defaultUsage is the default function to print a usage message.

func (f *FlagSet) defaultUsage()

failf method #

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

func (f *FlagSet) failf(format string, a ...any) error

init function # 

func init()

isZeroValue function #

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

func isZeroValue(flag *Flag, value string) (ok bool, err error)

newBoolValue function # 

func newBoolValue(val bool, p *bool) *boolValue

newDurationValue function # 

func newDurationValue(val time.Duration, p *time.Duration) *durationValue

newFloat64Value function # 

func newFloat64Value(val float64, p *float64) *float64Value

newInt64Value function # 

func newInt64Value(val int64, p *int64) *int64Value

newIntValue function # 

func newIntValue(val int, p *int) *intValue

newStringValue function # 

func newStringValue(val string, p *string) *stringValue

newTextValue function # 

func newTextValue(val encoding.TextMarshaler, p encoding.TextUnmarshaler) textValue

newUint64Value function # 

func newUint64Value(val uint64, p *uint64) *uint64Value

newUintValue function # 

func newUintValue(val uint, p *uint) *uintValue

numError function # 

func numError(err error) error

parseOne method #

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

func (f *FlagSet) parseOne() (bool, error)

set method # 

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

sortFlags function #

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

func sortFlags(flags map[string]*Flag) []*Flag

sprintf method #

sprintf formats the message, prints it to output, and returns it.

func (f *FlagSet) sprintf(format string, a ...any) string

usage method #

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

func (f *FlagSet) usage()

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