Structured argument parsing for Go

go get github.com/alexflint/go-arg

Declare command line arguments for your program by defining a struct.

var args struct {
	Foo string
	Bar bool
}
arg.MustParse(&args)
fmt.Println(args.Foo, args.Bar)

$ ./example --foo=hello --bar
hello true

Required arguments

var args struct {
	ID      int `arg:"required"`
	Timeout time.Duration
}
arg.MustParse(&args)

$ ./example
Usage: example --id ID [--timeout TIMEOUT]
error: --id is required

Positional arguments

var args struct {
	Input   string   `arg:"positional"`
	Output  []string `arg:"positional"`
}
arg.MustParse(&args)
fmt.Println("Input:", args.Input)
fmt.Println("Output:", args.Output)

$ ./example src.txt x.out y.out z.out
Input: src.txt
Output: [x.out y.out z.out]

Environment variables

var args struct {
	Workers int `arg:"env"`
}
arg.MustParse(&args)
fmt.Println("Workers:", args.Workers)

$ WORKERS=4 ./example
Workers: 4

$ WORKERS=4 ./example --workers=6
Workers: 6

You can also override the name of the environment variable:

var args struct {
	Workers int `arg:"env:NUM_WORKERS"`
}
arg.MustParse(&args)
fmt.Println("Workers:", args.Workers)

$ NUM_WORKERS=4 ./example
Workers: 4

You can provide multiple values using the CSV (RFC 4180) format:

var args struct {
    Workers []int `arg:"env"`
}
arg.MustParse(&args)
fmt.Println("Workers:", args.Workers)

$ WORKERS='1,99' ./example
Workers: [1 99]

Usage strings

var args struct {
	Input    string   `arg:"positional"`
	Output   []string `arg:"positional"`
	Verbose  bool     `arg:"-v" help:"verbosity level"`
	Dataset  string   `help:"dataset to use"`
	Optimize int      `arg:"-O" help:"optimization level"`
}
arg.MustParse(&args)

$ ./example -h
Usage: [--verbose] [--dataset DATASET] [--optimize OPTIMIZE] [--help] INPUT [OUTPUT [OUTPUT ...]] 

Positional arguments:
  INPUT 
  OUTPUT

Options:
  --verbose, -v            verbosity level
  --dataset DATASET        dataset to use
  --optimize OPTIMIZE, -O OPTIMIZE
                           optimization level
  --help, -h               print this help message

Default values

var args struct {
	Foo string
	Bar bool
}
args.Foo = "default value"
arg.MustParse(&args)

Arguments with multiple values

var args struct {
	Database string
	IDs      []int64
}
arg.MustParse(&args)
fmt.Printf("Fetching the following IDs from %s: %q", args.Database, args.IDs)

./example -database foo -ids 1 2 3
Fetching the following IDs from foo: [1 2 3]

Arguments that can be specified multiple times, mixed with positionals

var args struct {
    Commands  []string `arg:"-c,separate"`
    Files     []string `arg:"-f,separate"`
    Databases []string `arg:"positional"`
}

./example -c cmd1 db1 -f file1 db2 -c cmd2 -f file2 -f file3 db3 -c cmd3
Commands: [cmd1 cmd2 cmd3]
Files [file1 file2 file3]
Databases [db1 db2 db3]

Custom validation

var args struct {
	Foo string
	Bar string
}
p := arg.MustParse(&args)
if args.Foo == "" && args.Bar == "" {
	p.Fail("you must provide either --foo or --bar")
}

./example
Usage: samples [--foo FOO] [--bar BAR]
error: you must provide either --foo or --bar

Version strings

type args struct {
	...
}

func (args) Version() string {
	return "someprogram 4.3.0"
}

func main() {
	var args args
	arg.MustParse(&args)
}

$ ./example --version
someprogram 4.3.0

Embedded structs

The fields of embedded structs are treated just like regular fields:

type DatabaseOptions struct {
	Host     string
	Username string
	Password string
}

type LogOptions struct {
	LogFile string
	Verbose bool
}

func main() {
	var args struct {
		DatabaseOptions
		LogOptions
	}
	arg.MustParse(&args)
}

As usual, any field tagged with arg:"-" is ignored.

Custom parsing

Implement encoding.TextUnmarshaler to define your own parsing logic.

// Accepts command line arguments of the form "head.tail"
type NameDotName struct {
	Head, Tail string
}

func (n *NameDotName) UnmarshalText(b []byte) error {
	s := string(b)
	pos := strings.Index(s, ".")
	if pos == -1 {
		return fmt.Errorf("missing period in %s", s)
	}
	n.Head = s[:pos]
	n.Tail = s[pos+1:]
	return nil
}

func main() {
	var args struct {
		Name NameDotName
	}
	arg.MustParse(&args)
	fmt.Printf("%#v\n", args.Name)
}

$ ./example --name=foo.bar
main.NameDotName{Head:"foo", Tail:"bar"}

$ ./example --name=oops
Usage: example [--name NAME]
error: error processing --name: missing period in "oops"

Custom parsing with default values

Implement encoding.TextMarshaler to define your own default value strings:

// Accepts command line arguments of the form "head.tail"
type NameDotName struct {
	Head, Tail string
}

func (n *NameDotName) UnmarshalText(b []byte) error {
	// same as previous example
}

// this is only needed if you want to display a default value in the usage string
func (n *NameDotName) MarshalText() ([]byte, error) {
	return []byte(fmt.Sprintf("%s.%s", n.Head, n.Tail)), nil
}

func main() {
	var args struct {
		Name NameDotName
	}
	args.Name = NameDotName{"file", "txt"}  // set default value
	arg.MustParse(&args)
	fmt.Printf("%#v\n", args.Name)
}

$ ./example --help
Usage: test [--name NAME]

Options:
  --name NAME [default: file.txt]
  --help, -h             display this help and exit

$ ./example
main.NameDotName{Head:"file", Tail:"txt"}

Description strings

type args struct {
	Foo string
}

func (args) Description() string {
	return "this program does this and that"
}

func main() {
	var args args
	arg.MustParse(&args)
}

$ ./example -h
this program does this and that
Usage: example [--foo FOO]

Options:
  --foo FOO
  --help, -h             display this help and exit

API Documentation

https://godoc.org/github.com/alexflint/go-arg

Rationale

There are many command line argument parsing libraries for Go, including one in the standard library, so why build another?

The flag library that ships in the standard library seems awkward to me. Positional arguments must preceed options, so ./prog x --foo=1 does what you expect but ./prog --foo=1 x does not. It also does not allow arguments to have both long (--foo) and short (-f) forms.

Many third-party argument parsing libraries are great for writing sophisticated command line interfaces, but feel to me like overkill for a simple script with a few flags.

The idea behind go-arg is that Go already has an excellent way to describe data structures using structs, so there is no need to develop additional levels of abstraction. Instead of one API to specify which arguments your program accepts, and then another API to get the values of those arguments, go-arg replaces both with a single struct.

Backward compatibility notes

Earlier versions of this library required the help text to be part of the arg tag. This is still supported but is now deprecated. Instead, you should use a separate help tag, described above, which removes most of the limits on the text you can write. In particular, you will need to use the new help tag if your help text includes any commas.