arg

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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

Subcommands

Introduced in v1.1.0

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

$ 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:

type CheckoutCmd struct {
	Branch string `arg:"positional"`
	Track  bool   `arg:"-t"`
}
type CommitCmd struct {
	All     bool   `arg:"-a"`
	Message string `arg:"-m"`
}
type PushCmd struct {
	Remote      string `arg:"positional"`
	Branch      string `arg:"positional"`
	SetUpstream bool   `arg:"-u"`
}
var args struct {
	Checkout *CheckoutCmd `arg:"subcommand:checkout"`
	Commit   *CommitCmd   `arg:"subcommand:commit"`
	Push     *PushCmd     `arg:"subcommand:push"`
	Quiet    bool         `arg:"-q"` // this flag is global to all subcommands
}

arg.MustParse(&args)

switch {
case args.Checkout != nil:
	fmt.Printf("checkout requested for branch %s\n", args.Checkout.Branch)
case args.Commit != nil:
	fmt.Printf("commit requested with message \"%s\"\n", args.Commit.Message)
case args.Push != nil:
	fmt.Printf("push requested from %s to %s\n", args.Push.Branch, args.Push.Remote)
}

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

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.

Documentation

Overview

Package arg parses command line arguments using the fields from a struct.

For example, 

var args struct {
	Iter int
	Debug bool
}
arg.MustParse(&args)

defines two command line arguments, which can be set using any of 

./example --iter=1 --debug  // debug is a boolean flag so its value is set to true
./example -iter 1           // debug defaults to its zero value (false)
./example --debug=true      // iter defaults to its zero value (zero)

The fastest way to see how to use go-arg is to read the examples below.

Fields can be bool, string, any float type, or any signed or unsigned integer type. They can also be slices of any of the above, or slices of pointers to any of the above.

Tags can be specified using the `arg` and `help` tag names: 

var args struct {
	Input string   `arg:"positional"`
	Log string     `arg:"positional,required"`
	Debug bool     `arg:"-d" help:"turn on debug mode"`
	RealMode bool  `arg:"--real"
	Wr io.Writer   `arg:"-"`
}

Any tag string that starts with a single hyphen is the short form for an argument (e.g. `./example -d`), and any tag string that starts with two hyphens is the long form for the argument (instead of the field name).

Other valid tag strings are `positional` and `required`.

Fields can be excluded from processing with `arg:"-"`.

Example

This example demonstrates basic usage 

// These are the args you would pass in on the command line
os.Args = split("./example --foo=hello --bar")

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

Output:

hello true
Example (DefaultValues)

This example demonstrates arguments that have default values 

// These are the args you would pass in on the command line
os.Args = split("./example")

var args struct {
	Foo string
}
args.Foo = "default value"
MustParse(&args)
fmt.Println(args.Foo)

Output:

default value
Example (ErrorText)

This example shows the error string generated by go-arg when an invalid option is provided 

// These are the args you would pass in on the command line
os.Args = split("./example --optimize INVALID")

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"`
}

// This is only necessary when running inside golang's runnable example harness
osExit = func(int) {}
stderr = os.Stdout

MustParse(&args)

Output:

Usage: example [--verbose] [--dataset DATASET] [--optimize OPTIMIZE] INPUT [OUTPUT [OUTPUT ...]]
error: error processing --optimize: strconv.ParseInt: parsing "INVALID": invalid syntax
Example (ErrorTextForSubcommand)

This example shows the error string generated by go-arg when an invalid option is provided 

// These are the args you would pass in on the command line
os.Args = split("./example get --count INVALID")

type getCmd struct {
	Count int
}

var args struct {
	Get *getCmd `arg:"subcommand"`
}

// This is only necessary when running inside golang's runnable example harness
osExit = func(int) {}
stderr = os.Stdout

MustParse(&args)

Output:

Usage: example get [--count COUNT]
error: error processing --count: strconv.ParseInt: parsing "INVALID": invalid syntax
Example (HelpText)

This example shows the usage string generated by go-arg 

// These are the args you would pass in on the command line
os.Args = split("./example --help")

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"`
}

// This is only necessary when running inside golang's runnable example harness
osExit = func(int) {}

MustParse(&args)

Output:

Usage: example [--verbose] [--dataset DATASET] [--optimize OPTIMIZE] 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             display this help and exit
Example (HelpTextForSubcommand)

This example shows the usage string generated by go-arg when using subcommands 

// These are the args you would pass in on the command line
os.Args = split("./example get --help")

type getCmd struct {
	Item string `arg:"positional" help:"item to fetch"`
}

type listCmd struct {
	Format string `help:"output format"`
	Limit  int
}

var args struct {
	Verbose bool
	Get     *getCmd  `arg:"subcommand" help:"fetch an item and print it"`
	List    *listCmd `arg:"subcommand" help:"list available items"`
}

// This is only necessary when running inside golang's runnable example harness
osExit = func(int) {}

MustParse(&args)

Output:

Usage: example get ITEM

Positional arguments:
  ITEM                   item to fetch

Options:
  --help, -h             display this help and exit
Example (HelpTextWithSubcommand)

This example shows the usage string generated by go-arg when using subcommands 

// These are the args you would pass in on the command line
os.Args = split("./example --help")

type getCmd struct {
	Item string `arg:"positional" help:"item to fetch"`
}

type listCmd struct {
	Format string `help:"output format"`
	Limit  int
}

var args struct {
	Verbose bool
	Get     *getCmd  `arg:"subcommand" help:"fetch an item and print it"`
	List    *listCmd `arg:"subcommand" help:"list available items"`
}

// This is only necessary when running inside golang's runnable example harness
osExit = func(int) {}

MustParse(&args)

Output:

Usage: example [--verbose]

Options:
  --verbose
  --help, -h             display this help and exit

Commands:
  get                    fetch an item and print it
  list                   list available items
Example (MultipleMixed)

This eample demonstrates multiple value arguments that can be mixed with other arguments. 

os.Args = split("./example -c cmd1 db1 -f file1 db2 -c cmd2 -f file2 -f file3 db3 -c cmd3")
var args struct {
	Commands  []string `arg:"-c,separate"`
	Files     []string `arg:"-f,separate"`
	Databases []string `arg:"positional"`
}
MustParse(&args)
fmt.Println("Commands:", args.Commands)
fmt.Println("Files:", args.Files)
fmt.Println("Databases:", args.Databases)

Output:

Commands: [cmd1 cmd2 cmd3]
Files: [file1 file2 file3]
Databases: [db1 db2 db3]
Example (MultipleValues)

This example demonstrates arguments that have multiple values 

// The args you would pass in on the command line
os.Args = split("./example --database localhost --ids 1 2 3")

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

Output:

Fetching the following IDs from localhost: [1 2 3]
Example (PositionalArguments)

This example demonstrates positional arguments 

// These are the args you would pass in on the command line
os.Args = split("./example in out1 out2 out3")

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

Output:

In: in
Out: [out1 out2 out3]
Example (RequiredArguments)

This example demonstrates arguments that are required 

// These are the args you would pass in on the command line
os.Args = split("./example --foo=abc --bar")

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

Output:

abc true
Example (Subcommand)

This example demonstrates use of subcommands 

// These are the args you would pass in on the command line
os.Args = split("./example commit -a -m what-this-commit-is-about")

type CheckoutCmd struct {
	Branch string `arg:"positional"`
	Track  bool   `arg:"-t"`
}
type CommitCmd struct {
	All     bool   `arg:"-a"`
	Message string `arg:"-m"`
}
type PushCmd struct {
	Remote      string `arg:"positional"`
	Branch      string `arg:"positional"`
	SetUpstream bool   `arg:"-u"`
}
var args struct {
	Checkout *CheckoutCmd `arg:"subcommand:checkout"`
	Commit   *CommitCmd   `arg:"subcommand:commit"`
	Push     *PushCmd     `arg:"subcommand:push"`
	Quiet    bool         `arg:"-q"` // this flag is global to all subcommands
}

// This is only necessary when running inside golang's runnable example harness
osExit = func(int) {}
stderr = os.Stdout

MustParse(&args)

switch {
case args.Checkout != nil:
	fmt.Printf("checkout requested for branch %s\n", args.Checkout.Branch)
case args.Commit != nil:
	fmt.Printf("commit requested with message \"%s\"\n", args.Commit.Message)
case args.Push != nil:
	fmt.Printf("push requested from %s to %s\n", args.Push.Branch, args.Push.Remote)
}

Output:

commit requested with message "what-this-commit-is-about"

Index

Examples

Constants

This section is empty.

Variables

View Source 
var ErrHelp = errors.New("help requested by user")

ErrHelp indicates that -h or --help were provided

View Source 
var ErrVersion = errors.New("version requested by user")

ErrVersion indicates that --version was provided

Functions

func Parse 

func Parse(dest ...interface{}) error

Parse processes command line arguments and stores them in dest

Types

type ArgUnmarshaler 

type ArgUnmarshaler interface {
	UnmarshalArg(text []byte) error
}

ArgUnmarshaler is TextUnmarshaler but with a different method name

type Config 

type Config struct {
	Program string // Program is the name of the program used in the help text
}

Config represents configuration options for an argument parser

type Described 

type Described interface {
	// Description returns the string that will be printed on a line by itself
	// at the top of the help message.
	Description() string
}

Described is the interface that the destination struct should implement to make a description string appear at the top of the help message.

type Parser 

type Parser struct {
	// contains filtered or unexported fields
}

Parser represents a set of command line options with destination values

func MustParse 

func MustParse(dest ...interface{}) *Parser

MustParse processes command line arguments and exits upon failure

func NewParser 

func NewParser(config Config, dests ...interface{}) (*Parser, error)

NewParser constructs a parser from a list of destination structs

func (*Parser) Fail 

func (p *Parser) Fail(msg string)

Fail prints usage information to stderr and exits with non-zero status

func (*Parser) Parse 

func (p *Parser) Parse(args []string) error

Parse processes the given command line option, storing the results in the field of the structs from which NewParser was constructed

func (*Parser) Subcommand 

func (p *Parser) Subcommand() interface{}

Subcommand returns the user struct for the subcommand selected by the command line arguments most recently processed by the parser. The return value is always a pointer to a struct. If no subcommand was specified then it returns the top-level arguments struct. If no command line arguments have been processed by this parser then it returns nil.

func (*Parser) SubcommandNames 

func (p *Parser) SubcommandNames() []string

SubcommandNames returns the sequence of subcommands specified by the user. If no subcommands were given then it returns an empty slice.

func (*Parser) WriteHelp 

func (p *Parser) WriteHelp(w io.Writer)

WriteHelp writes the usage string followed by the full help string for each option

func (*Parser) WriteUsage 

func (p *Parser) WriteUsage(w io.Writer)

WriteUsage writes usage information to the given writer

type Versioned 

type Versioned interface {
	// Version returns the version string that will be printed on a line by itself
	// at the top of the help message.
	Version() string
}

Versioned is the interface that the destination struct should implement to make a version string appear at the top of the help message.

Source Files

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