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go-flags: a go library for parsing command line arguments

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This library provides similar functionality to the builtin flag library of go, but provides much more functionality and nicer formatting. From the documentation:

Package flags provides an extensive command line option parser. The flags package is similar in functionality to the go builtin flag package but provides more options and uses reflection to provide a convenient and succinct way of specifying command line options.

Supported features:

The flags package uses structs, reflection and struct field tags to allow users to specify command line options. This results in very simple and concise specification of your application options. For example:

type Options struct {
	Verbose []bool `short:"v" long:"verbose" description:"Show verbose debug information"`
}

This specifies one option with a short name -v and a long name --verbose. When either -v or --verbose is found on the command line, a 'true' value will be appended to the Verbose field. e.g. when specifying -vvv, the resulting value of Verbose will be {[true, true, true]}.

Example:

var opts struct {
	// Slice of bool will append 'true' each time the option
	// is encountered (can be set multiple times, like -vvv)
	Verbose []bool `short:"v" long:"verbose" description:"Show verbose debug information"`

	// Example of automatic marshalling to desired type (uint)
	Offset uint `long:"offset" description:"Offset"`

	// Example of a callback, called each time the option is found.
	Call func(string) `short:"c" description:"Call phone number"`

	// Example of a required flag
	Name string `short:"n" long:"name" description:"A name" required:"true"`

	// Example of a flag restricted to a pre-defined set of strings
	Animal string `long:"animal" choice:"cat" choice:"dog"`

	// Example of a value name
	File string `short:"f" long:"file" description:"A file" value-name:"FILE"`

	// Example of a pointer
	Ptr *int `short:"p" description:"A pointer to an integer"`

	// Example of a slice of strings
	StringSlice []string `short:"s" description:"A slice of strings"`

	// Example of a slice of pointers
	PtrSlice []*string `long:"ptrslice" description:"A slice of pointers to string"`

	// Example of a map
	IntMap map[string]int `long:"intmap" description:"A map from string to int"`

	// Example of env variable
	Thresholds  []int     `long:"thresholds" default:"1" default:"2" env:"THRESHOLD_VALUES"  env-delim:","`
}

// Callback which will invoke callto:<argument> to call a number.
// Note that this works just on OS X (and probably only with
// Skype) but it shows the idea.
opts.Call = func(num string) {
	cmd := exec.Command("open", "callto:"+num)
	cmd.Start()
	cmd.Process.Release()
}

// Make some fake arguments to parse.
args := []string{
	"-vv",
	"--offset=5",
	"-n", "Me",
	"--animal", "dog", // anything other than "cat" or "dog" will raise an error
	"-p", "3",
	"-s", "hello",
	"-s", "world",
	"--ptrslice", "hello",
	"--ptrslice", "world",
	"--intmap", "a:1",
	"--intmap", "b:5",
	"arg1",
	"arg2",
	"arg3",
}

// Parse flags from `args'. Note that here we use flags.ParseArgs for
// the sake of making a working example. Normally, you would simply use
// flags.Parse(&opts) which uses os.Args
args, err := flags.ParseArgs(&opts, args)

if err != nil {
	panic(err)
}

fmt.Printf("Verbosity: %v\n", opts.Verbose)
fmt.Printf("Offset: %d\n", opts.Offset)
fmt.Printf("Name: %s\n", opts.Name)
fmt.Printf("Animal: %s\n", opts.Animal)
fmt.Printf("Ptr: %d\n", *opts.Ptr)
fmt.Printf("StringSlice: %v\n", opts.StringSlice)
fmt.Printf("PtrSlice: [%v %v]\n", *opts.PtrSlice[0], *opts.PtrSlice[1])
fmt.Printf("IntMap: [a:%v b:%v]\n", opts.IntMap["a"], opts.IntMap["b"])
fmt.Printf("Remaining args: %s\n", strings.Join(args, " "))

// Output: Verbosity: [true true]
// Offset: 5
// Name: Me
// Ptr: 3
// StringSlice: [hello world]
// PtrSlice: [hello world]
// IntMap: [a:1 b:5]
// Remaining args: arg1 arg2 arg3

More information can be found in the godocs: http://godoc.org/github.com/jessevdk/go-flags