Package flag implements command-line flag parsing.
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.
The following forms are permitted:
-flag -flag=x -flag x // non-boolean flags only
One or two minus signs may be used; they are equivalent. The last form is not permitted for boolean flags because the meaning of the command
cmd -x *
where * is a Unix shell wildcard, will change if there is a file called 0, false, etc. You must use the -flag=false form to turn off a boolean flag.
Flag parsing stops just before the first non-flag argument ("-" is a non-flag argument) or after the terminator "--".
Integer flags accept 1234, 0664, 0x1234 and may be negative. Boolean flags may be:
1, 0, t, f, T, F, true, false, TRUE, FALSE, True, False
Duration flags accept any input valid for time.ParseDuration.
The default set of command-line flags is controlled by top-level functions. The FlagSet type allows one to define independent sets of flags, such as to implement subcommands in a command-line interface. The methods of FlagSet are analogous to the top-level functions for the command-line flag set.
// 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 run it here because this is not a main function and
// the testing suite has already parsed the flags.
}
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(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() []string
Args returns the non-flag command-line arguments.
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(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(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(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(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(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(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(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(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(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() int
NArg is the number of arguments remaining after flags have been processed.
func NFlag() int
NFlag returns the number of command-line flags that have been set.
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() bool
Parsed reports whether the command-line flags have been parsed.
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(name, value string) error
Set sets the value of the named command-line flag.
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(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(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(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(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(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(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(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(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(fn func(*Flag))
VisitAll visits the command-line flags in lexicographical order, calling fn for each. It visits all flags, even those not set.
ErrorHandling defines how FlagSet.Parse behaves if the parse fails.
type ErrorHandling int
These constants cause FlagSet.Parse to behave as described if the parse fails.
const (
ContinueOnError ErrorHandling = iota // Return a descriptive error.
ExitOnError // Call os.Exit(2) or for -h/-help Exit(0).
PanicOnError // Call panic with a descriptive error.
) A 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(name string) *Flag
Lookup returns the Flag structure of the named command-line flag, returning nil if none exists.
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
}
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 (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 (f *FlagSet) Args() []string
Args returns the non-flag arguments.
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 (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 (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 (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 (f *FlagSet) ErrorHandling() ErrorHandling
ErrorHandling returns the error handling behavior of the flag set.
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 (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 (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 (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 (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 (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 (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 (f *FlagSet) Lookup(name string) *Flag
Lookup returns the Flag structure of the named flag, returning nil if none exists.
func (f *FlagSet) NArg() int
NArg is the number of arguments remaining after flags have been processed.
func (f *FlagSet) NFlag() int
NFlag returns the number of flags that have been set.
func (f *FlagSet) Name() string
Name returns the name of the flag set.
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 (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 (f *FlagSet) Parsed() bool
Parsed reports whether f.Parse has been called.
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 (f *FlagSet) Set(name, value string) error
Set sets the value of the named flag.
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 (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 (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 (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 (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 (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 (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 (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 (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 (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.
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{}
} 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
} 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)
}
© Google, Inc.
Licensed under the Creative Commons Attribution License 3.0.
https://golang.org/pkg/flag/