package flag

Import Path
	flag (on go.dev)

Dependency Relation
	imports 11 packages, and imported by one package


Involved Source Files

	
		Package flag implements command-line flag parsing.
		
		# Usage
		
		Define flags using flag.String(), Bool(), Int(), etc.
		
		This declares an integer flag, -n, stored in the pointer nFlag, with type *int:
		
			import "flag"
			var nFlag = flag.Int("n", 1234, "help message for flag n")
		
		If you like, you can bind the flag to a variable using the Var() functions.
		
			var flagvar int
			func init() {
				flag.IntVar(&flagvar, "flagname", 1234, "help message for flagname")
			}
		
		Or you can create custom flags that satisfy the Value interface (with
		pointer receivers) and couple them to flag parsing by
		
			flag.Var(&flagVal, "name", "help message for flagname")
		
		For such flags, the default value is just the initial value of the variable.
		
		After all flags are defined, call
		
			flag.Parse()
		
		to parse the command line into the defined flags.
		
		Flags may then be used directly. If you're using the flags themselves,
		they are all pointers; if you bind to variables, they're values.
		
			fmt.Println("ip has value ", *ip)
			fmt.Println("flagvar has value ", flagvar)
		
		After parsing, the arguments following the flags are available as the
		slice flag.Args() or individually as flag.Arg(i).
		The arguments are indexed from 0 through flag.NArg()-1.
		
		# Command line flag syntax
		
		The following forms are permitted:
		
			-flag
			--flag   // double dashes are also permitted
			-flag=x
			-flag x  // non-boolean flags only
		
		One or two dashes may be used; they are equivalent.
		The last form is not permitted for boolean flags because the
		meaning of the command
		
			cmd -x *
		
		where * is a Unix shell wildcard, will change if there is a file
		called 0, false, etc. You must use the -flag=false form to turn
		off a boolean flag.
		
		Flag parsing stops just before the first non-flag argument
		("-" is a non-flag argument) or after the terminator "--".
		
		Integer flags accept 1234, 0664, 0x1234 and may be negative.
		Boolean flags may be:
		
			1, 0, t, f, T, F, true, false, TRUE, FALSE, True, False
		
		Duration flags accept any input valid for time.ParseDuration.
		
		The default set of command-line flags is controlled by
		top-level functions.  The FlagSet type allows one to define
		independent sets of flags, such as to implement subcommands
		in a command-line interface. The methods of FlagSet are
		analogous to the top-level functions for the command-line
		flag set.


Code Examples

	
		// These examples demonstrate more intricate uses of the flag package.
		package main
		
		import (
			"errors"
			"flag"
			"fmt"
			"strings"
			"time"
		)
		
		// Example 1: A single string flag called "species" with default value "gopher".
		var species = flag.String("species", "gopher", "the species we are studying")
		
		// Example 2: Two flags sharing a variable, so we can have a shorthand.
		// The order of initialization is undefined, so make sure both use the
		// same default value. They must be set up with an init function.
		var gopherType string
		
		func init() {
			const (
				defaultGopher = "pocket"
				usage         = "the variety of gopher"
			)
			flag.StringVar(&gopherType, "gopher_type", defaultGopher, usage)
			flag.StringVar(&gopherType, "g", defaultGopher, usage+" (shorthand)")
		}
		
		// Example 3: A user-defined flag type, a slice of durations.
		type interval []time.Duration
		
		// String is the method to format the flag's value, part of the flag.Value interface.
		// The String method's output will be used in diagnostics.
		func (i *interval) String() string {
			return fmt.Sprint(*i)
		}
		
		// Set is the method to set the flag value, part of the flag.Value interface.
		// Set's argument is a string to be parsed to set the flag.
		// It's a comma-separated list, so we split it.
		func (i *interval) Set(value string) error {
			// If we wanted to allow the flag to be set multiple times,
			// accumulating values, we would delete this if statement.
			// That would permit usages such as
			//	-deltaT 10s -deltaT 15s
			// and other combinations.
			if len(*i) > 0 {
				return errors.New("interval flag already set")
			}
			for _, dt := range strings.Split(value, ",") {
				duration, err := time.ParseDuration(dt)
				if err != nil {
					return err
				}
				*i = append(*i, duration)
			}
			return nil
		}
		
		// Define a flag to accumulate durations. Because it has a special type,
		// we need to use the Var function and therefore create the flag during
		// init.
		
		var intervalFlag interval
		
		func init() {
			// Tie the command-line flag to the intervalFlag variable and
			// set a usage message.
			flag.Var(&intervalFlag, "deltaT", "comma-separated list of intervals to use between events")
		}
		
		func main() {
			// All the interesting pieces are with the variables declared above, but
			// to enable the flag package to see the flags defined there, one must
			// execute, typically at the start of main (not init!):
			//	flag.Parse()
			// We don't call it here because this code is a function called "Example"
			// that is part of the testing suite for the package, which has already
			// parsed the flags. When viewed at pkg.go.dev, however, the function is
			// renamed to "main" and it could be run as a standalone example.
		}

	
		package main
		
		import (
			"flag"
			"fmt"
			"os"
		)
		
		func main() {
			fs := flag.NewFlagSet("ExampleBoolFunc", flag.ContinueOnError)
			fs.SetOutput(os.Stdout)
		
			fs.BoolFunc("log", "logs a dummy message", func(s string) error {
				fmt.Println("dummy message:", s)
				return nil
			})
			fs.Parse([]string{"-log"})
			fs.Parse([]string{"-log=0"})
		
		}

	
		package main
		
		import (
			"errors"
			"flag"
			"fmt"
			"net"
			"os"
		)
		
		func main() {
			fs := flag.NewFlagSet("ExampleFunc", flag.ContinueOnError)
			fs.SetOutput(os.Stdout)
			var ip net.IP
			fs.Func("ip", "`IP address` to parse", func(s string) error {
				ip = net.ParseIP(s)
				if ip == nil {
					return errors.New("could not parse IP")
				}
				return nil
			})
			fs.Parse([]string{"-ip", "127.0.0.1"})
			fmt.Printf("{ip: %v, loopback: %t}\n\n", ip, ip.IsLoopback())
		
			// 256 is not a valid IPv4 component
			fs.Parse([]string{"-ip", "256.0.0.1"})
			fmt.Printf("{ip: %v, loopback: %t}\n\n", ip, ip.IsLoopback())
		
		}

	
		package main
		
		import (
			"flag"
			"fmt"
			"net"
			"os"
		)
		
		func main() {
			fs := flag.NewFlagSet("ExampleTextVar", flag.ContinueOnError)
			fs.SetOutput(os.Stdout)
			var ip net.IP
			fs.TextVar(&ip, "ip", net.IPv4(192, 168, 0, 100), "`IP address` to parse")
			fs.Parse([]string{"-ip", "127.0.0.1"})
			fmt.Printf("{ip: %v}\n\n", ip)
		
			// 256 is not a valid IPv4 component
			ip = nil
			fs.Parse([]string{"-ip", "256.0.0.1"})
			fmt.Printf("{ip: %v}\n\n", ip)
		
		}

	
		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)
		
		}




Package-Level Type Names (total 17, in which 5 are exported)


	/* sort exporteds by:  |  */
	
		ErrorHandling defines how FlagSet.Parse behaves if the parse fails.

		
			func (*FlagSet).ErrorHandling() ErrorHandling
		
			func NewFlagSet(name string, errorHandling ErrorHandling) *FlagSet
			func (*FlagSet).Init(name string, errorHandling ErrorHandling)
		
			const ContinueOnError
			const ExitOnError
			const PanicOnError


	
		A Flag represents the state of a flag.

		
			
				// default value (as text); for usage message

			
				// name as it appears on command line

			
				// help message

			
				// value as set

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


	
		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.

		
			
				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.

			
		
			
				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.

			
				Args returns the non-flag arguments.

			
				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.

			
				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.

			
				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.

			
				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.

			
				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.

			
				ErrorHandling returns the error handling behavior of the flag set.

			
				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.

			
				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 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.

			
				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.

			
				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.

			
				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.

			
				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.

			
				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.

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

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

			
				NFlag returns the number of flags that have been set.

			
				Name returns the name of the flag set.

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

			
				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.

			
				Parsed reports whether f.Parse has been called.

			
				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.

			
				Set sets the value of the named flag.

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

			
				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.

			
				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.

			
				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.

			
				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.

			
				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.

			
				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.

			
				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.

			
				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.

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

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

			
		
			func NewFlagSet(name string, errorHandling ErrorHandling) *FlagSet
		
			  var CommandLine *FlagSet


	
		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.

		
			( Getter) Get() any
			( Getter) Set(string) error
			( Getter) String() string
		
			*go.uber.org/zap/zapcore.Level
			
		
			 Getter : Value
			 Getter : fmt.Stringer
			


	
		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.

		
			( Value) Set(string) error
			( Value) String() string
		
			 Getter (interface)
			*go.uber.org/zap/zapcore.Level
			
		
			 Value : fmt.Stringer
			
		
			func Var(value Value, name string, usage string)
			func (*FlagSet).Var(value Value, name string, usage string)


	


Package-Level Functions (total 47, in which 33 are exported)


	
		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.


	
		Args returns the non-flag command-line arguments.


	
		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.


	
		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.


	
		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.


	
		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.


	
		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.


	
		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.


	
		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 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.


	
		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.


	
		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.


	
		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.


	
		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.


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


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


	
		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.


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


	
		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.


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


	
		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.


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


	
		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.


	
		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.


	
		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.


	
		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.


	
		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.


	
		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.


	
		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.


	
		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.


	
		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.


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


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


	


Package-Level Variables (total 5, in which 3 are exported)


	
		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.


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


	
		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.


	


Package-Level Constants (total 3, all are exported)


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


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


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