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Struct-based argument parsing in Go

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Struct-based argument parsing in Go

go-arg

Struct-based argument parsing for Go

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)

bash

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

Installation

bash

go get github.com/alexflint/go-arg

Required arguments

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

bash

$ ./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)

bash

$ ./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)

bash

$ WORKERS=4 ./example
Workers: 4

bash

$ 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)

bash

$ NUM_WORKERS=4 ./example
Workers: 4

You can provide multiple values in environment variables using commas:

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

bash

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

Command line arguments take precedence over environment variables:

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

bash

$ NUM_WORKERS=6 ./example
Workers: 6
$ NUM_WORKERS=6 ./example --count 4
Workers: 4

Configuring a global environment variable name prefix is also possible:

var args struct {
	Workers int `arg:"--count,env:NUM_WORKERS"`
}

p, err := arg.NewParser(arg.Config{
    EnvPrefix: "MYAPP_",
}, &args)

p.MustParse(os.Args[1:])
fmt.Println("Workers:", args.Workers)

bash

$ MYAPP_NUM_WORKERS=6 ./example
Workers: 6

Usage strings

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

bash

$ ./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 `default:"abc"`
	Bar bool
}
arg.MustParse(&args)

Command line arguments take precedence over environment variables, which take precedence over default values. This means that we check whether a certain option was provided on the command line, then if not, we check for an environment variable (only if an env tag was provided), then if none is found, we check for a default tag containing a default value.

var args struct {
    Test  string `arg:"-t,env:TEST" default:"something"`
}
arg.MustParse(&args)

Ignoring environment variables and/or default values

var args struct {
    Test  string `arg:"-t,env:TEST" default:"something"`
}

p, err := arg.NewParser(arg.Config{
    IgnoreEnv: true,
    IgnoreDefault: true,
}, &args)

err = p.Parse(os.Args[1:])

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)

bash

./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"`
}
arg.MustParse(&args)

bash

./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]

Arguments with keys and values

var args struct {
	UserIDs map[string]int
}
arg.MustParse(&args)
fmt.Println(args.UserIDs)

bash

./example --userids john=123 mary=456
map[john:123 mary:456]

Version strings

type args struct {
	...
}

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

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

bash

$ ./example --version
someprogram 4.3.0

Note If a --version flag is defined in args or any subcommand, it overrides the built-in versioning.

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")
}

bash

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

Overriding option names

var args struct {
	Short        string `arg:"-s"`
	Long         string `arg:"--custom-long-option"`
	ShortAndLong string `arg:"-x,--my-option"`
	OnlyShort    string `arg:"-o,--"`
}
arg.MustParse(&args)

bash

$ ./example --help
Usage: example [-o ONLYSHORT] [--short SHORT] [--custom-long-option CUSTOM-LONG-OPTION] [--my-option MY-OPTION]

Options:
  --short SHORT, -s SHORT
  --custom-long-option CUSTOM-LONG-OPTION
  --my-option MY-OPTION, -x MY-OPTION
  -o ONLYSHORT
  --help, -h             display this help and exit

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.

Supported types

The following types may be used as arguments:

built-in integer types: int, int8, int16, int32, int64, byte, rune
built-in floating point types: float32, float64
strings
booleans
URLs represented as url.URL
time durations represented as time.Duration
email addresses represented as mail.Address
MAC addresses represented as net.HardwareAddr
pointers to any of the above
slices of any of the above
maps using any of the above as keys and values
any type that implements encoding.TextUnmarshaler

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)
}

bash

$ ./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 `default:"file.txt"`
	}
	arg.MustParse(&args)
	fmt.Printf("%#v\n", args.Name)
}

bash

$ ./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"}

Custom placeholders

Use the placeholder tag to control which placeholder text is used in the usage text.

var args struct {
	Input    string   `arg:"positional" placeholder:"SRC"`
	Output   []string `arg:"positional" placeholder:"DST"`
	Optimize int      `arg:"-O" help:"optimization level" placeholder:"LEVEL"`
	MaxJobs  int      `arg:"-j" help:"maximum number of simultaneous jobs" placeholder:"N"`
}
arg.MustParse(&args)

bash

$ ./example -h
Usage: example [--optimize LEVEL] [--maxjobs N] SRC [DST [DST ...]]

Positional arguments:
  SRC
  DST

Options:
  --optimize LEVEL, -O LEVEL
                         optimization level
  --maxjobs N, -j N      maximum number of simultaneous jobs
  --help, -h             display this help and exit

Description strings

A descriptive message can be added at the top of the help text by implementing a Description function that returns a string.

type args struct {
	Foo string
}

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

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

bash

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

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

Similarly an epilogue can be added at the end of the help text by implementing the Epilogue function.

type args struct {
	Foo string
}

func (args) Epilogue() string {
	return "For more information visit github.com/alexflint/go-arg"
}

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

bash

$ ./example -h
Usage: example [--foo FOO]

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

For more information visit github.com/alexflint/go-arg

Subcommands

Subcommands are commonly used in tools that wish to group multiple functions into a single program. An example is the git tool:

bash

$ git checkout [arguments specific to checking out code]
$ git commit [arguments specific to committing]
$ git push [arguments specific to pushing]

The strings "checkout", "commit", and "push" are different from simple positional arguments because the options available to the user change depending on which subcommand they choose.

This can be implemented with go-arg as follows:

…

Some additional rules apply when working with subcommands:

The subcommand tag can only be used with fields that are pointers to structs
Any struct that contains a subcommand must not contain any positionals

This package allows to have a program that accepts subcommands, but also does something else when no subcommands are specified. If on the other hand you want the program to terminate when no subcommands are specified, the recommended way is:

p := arg.MustParse(&args)
if p.Subcommand() == nil {
    p.Fail("missing subcommand")
}

Custom handling of --help and --version

The following reproduces the internal logic of MustParse for the simple case where you are not using subcommands or --version. This allows you to respond programati

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PublishedAug 1, 2026

UpdatedSep 17, 2026

Category编程语言

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go-arg

About

go-arg

Struct-based argument parsing for Go

Installation

Required arguments

Positional arguments

Environment variables

Usage strings

Default values

Ignoring environment variables and/or default values

Arguments with multiple values

Arguments that can be specified multiple times, mixed with positionals

Arguments with keys and values

Version strings

Custom validation

Overriding option names

Embedded structs

Supported types

Custom parsing

Custom parsing with default values

Custom placeholders

Description strings

Subcommands

Custom handling of --help and --version

Issues· 0 open

> Tags

> Discussion

> Details

> Related tools