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## Structured argument parsing for Go

```shell
go get github.com/alexflint/go-arg
```

Declare the command line arguments your program accepts by defining a struct.

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

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

### Required arguments

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

```shell
$ ./example
usage: example --id ID [--timeout TIMEOUT]
error: --id is required
```

### Positional arguments

```go
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

```go
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:

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

```
$ NUM_WORKERS=4 ./example
Workers: 4
```

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

```shell
$ ./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

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

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

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

### Custom validation
```go
var args struct {
	Foo string
	Bar string
}
p := arg.MustParse(&args)
if args.Foo == "" && args.Bar == "" {
	p.Fail("you must provide one of --foo and --bar")
}
```

```shell
./example
usage: samples [--foo FOO] [--bar BAR]
error: you must provide one of --foo and --bar
```

### Version strings

```go
type args struct {
	...
}

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

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

```shell
$ ./example --version
someprogram 4.3.0
```

### Embedded structs

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

```go

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

You can implement your own argument parser by implementing `encoding.TextUnmarshaler`:

```go
package main

import (
	"fmt"
	"strings"

	"github.com/alexflint/go-arg"
)

// 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)
}
```
```shell
$ ./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"
```

### Description strings

```go
type args struct {
	Foo string
}

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

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

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

options:
  --foo FOO
  --help, -h             display this help and exit
```

### 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 shortcomings of the `flag` library that ships in the standard library are well known. Positional arguments must preceed options, so `./prog x --foo=1` does what you expect but `./prog --foo=1 x` does not. Arguments cannot have both long (`--foo`) and short (`-f`) forms.

Many third-party argument parsing libraries are geared for writing sophisticated command line interfaces. The excellent `codegangsta/cli` is perfect for working with multiple sub-commands and nested flags, but is probably overkill for a simple script with a handful of flags.

The main idea behind `go-arg` is that Go already has an excellent way to describe data structures using Go structs, so there is no need to develop more levels of abstraction on top of this. Instead of one API to specify which arguments your program accepts, and then another API to get the values of those arguments, why not replace both with a single struct?