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Source file src/flag/example_test.go

Documentation: flag

  // Copyright 2012 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  // These examples demonstrate more intricate uses of the flag package.
  package flag_test
  import (
  // 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.

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