README

go-arg
go-arg

Struct-based argument parsing for Go

Sourcegraph Documentation Build Status Coverage Status Go Report Card


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

Installation

go get github.com/alexflint/go-arg

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,--verbose" 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 `default:"abc"`
	Bar bool
}
arg.MustParse(&args)

Default values (before v1.2)

var args struct {
	Foo string
	Bar bool
}
arg.Foo = "abc"
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"`
}
arg.MustParse(&args)
./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)
./example --userids john=123 mary=456
map[john:123 mary:456]

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

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)
$ ./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.

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 `default:"file.txt"`
	}
	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"}

Custom placeholders

Introduced in version 1.3.0

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)
$ ./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

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 version 1.1.0

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

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")
}

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

      Output:
      
      hello true
      
      Example (DefaultValues)

        This example demonstrates arguments that have default values

        Output:
        
        abc
        
        Example (ErrorText)

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

          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

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

              This example shows the usage string generated by go-arg with customized placeholders

              Output:
              
              
              Example (HelpText)

                This example shows the usage string generated by go-arg

                Output:
                
                Usage: example [--verbose] [--dataset DATASET] [--optim OPTIM] INPUT [OUTPUT [OUTPUT ...]]
                
                Positional arguments:
                  INPUT
                  OUTPUT
                
                Options:
                  --verbose, -v          verbosity level
                  --dataset DATASET      dataset to use
                  --optim OPTIM, -O OPTIM
                                         optimization level
                  --help, -h             display this help and exit
                
                Example (HelpTextForSubcommand)

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

                  Output:
                  
                  Usage: example get ITEM
                  
                  Positional arguments:
                    ITEM                   item to fetch
                  
                  Global options:
                    --verbose
                    --help, -h             display this help and exit
                  
                  Example (HelpTextWithSubcommand)

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

                    Output:
                    
                    Usage: example [--verbose] <command> [<args>]
                    
                    Options:
                      --verbose
                      --help, -h             display this help and exit
                    
                    Commands:
                      get                    fetch an item and print it
                      list                   list available items
                    
                    Example (MappingWithCommas)

                      This example demonstrates arguments with keys and values separated by commas

                      Output:
                      
                      map[one:two three:four]
                      
                      Example (Mappings)

                        This example demonstrates arguments with keys and values

                        Output:
                        
                        map[john:123 mary:456]
                        
                        Example (MultipleMixed)

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

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

                            This example demonstrates arguments that have multiple values

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

                              This example demonstrates positional arguments

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

                                This example demonstrates arguments that are required

                                Output:
                                
                                abc true
                                
                                Example (Subcommand)

                                  This example demonstrates use of subcommands

                                  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 Config

                                        type Config struct {
                                        	// Program is the name of the program used in the help text
                                        	Program string
                                        
                                        	// IgnoreEnv instructs the library not to read environment variables
                                        	IgnoreEnv bool
                                        }

                                          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.