Structured argument parsing for Go

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

Default values

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

Marking options as required

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

Positional argument

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

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

Usage strings

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)

$ ./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

Options 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]

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. Boolean arguments must have explicit values, so ./prog -debug=1 sets debug to true but ./myprog -debug does not.

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?