flag package trap

Background#

flag.Bool and flag.BoolVar exhibit strange behavior

When the default value is set to true,

// flag.BoolVar
func main() {
	var getBool bool
	flag.BoolVar(&getBool, "get", true, "get a boolean")
	flag.Parse()
	fmt.Println(getBool)
}

# Result
go run main.go -get false
true

// flag.Bool
func main() {
	var getBool *bool
	getBool = flag.Bool("get", true, "get a boolean")
	flag.Parse()
	fmt.Println(*getBool)
}

# Result
go run main.go -get false
true

However, when the default value is set to false and the command line argument is set to true, the code behaves normally (only one case is shown due to consistent results)

func main() {
	var getBool *bool
	getBool = flag.Bool("get", false, "get a boolean")
	flag.Parse()
	fmt.Println(*getBool)
}

# Result
go run main.go -get true
true

Why#

Let's take a look at the code comments

// src/flag/flag.go
// 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
}

// 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
}

The bool flag -name will be parsed by the parser as -name=true instead of parsing the next argument

# For example, false after -b will not be parsed
# The following command is equivalent to go run main.go -b=true
go run main.go -b false

How to Solve#

Use -name=false instead of -name false

# Result
go run main.go -get=true -come=false -num 1
true
false
1