This library is inspired on joeshaw/envdecode and this video. Instead of read env variables, flagstruct help you to populate your structs from command line arguments. flagstruct works with plain and nested structs, including pointers to nested structs. But, it will not allocate new points to structs.

Considerations

1. Default values may be provided by appending ",default=value" to the struct tag
2. Required values may be marked by appending ",required" to the struct tag
3. Allowed values may be provided by appending ",allowed=option;option..." to the struct tag
4. flagstruct will ignore every unexported struct field (including one that contains no flag tags at all)
5. You can't use default and required in the same annotation

$ go get github.com/mfuentesg/flagstruct

Define a struct with flag annotation

// main.go file
package main

import (
	"fmt"
	"time"

	"github.com/mfuentesg/flagstruct"
)

type Config struct {
	Server struct {
		Host     string `flag:"server-host,default=localhost"`
		Port     int    `flag:"server-port,allowed=9090;8080"`
	}
	Timeout time.Duration `flag:"timeout,default=1m"`
}

Then, call to flagstruct.Decode function

func main() {
	var c Config
	if err := flagstruct.Decode(&c); err != nil {
		fmt.Println(err)
		return
	}
	fmt.Printf(
		"connecting to %s:%d (timeout %.0f seconds)",
		c.Server.Host,
		c.Server.Port,
		c.Timeout.Seconds(),
	)
}

Running the above example

$ go run main.go
> connecting to localhost:0 (timeout 60 seconds)

$ go run main.go -timeout=2m -server-port=3000
> connecting to localhost:3000 (timeout 120 seconds)

• Structs
• Pointer to structs
• Slices of below defined types, separated by semicolon (;)
• bool
• float32, float64
• int, int8, int16, int32, int64
• uint, uint8, uint16, uint32, uint64
• string
• interface{}
• time.Duration, using the time.ParseDuration() format
• Custom types (those types must implement the flagstruct.Decoder interface)

if you want that a field use a custom decoder, you may implement the Decoder interface.

Decoder is the interface implemented by an object, that can decode an argument string representation of itself.

type Config struct {
  IPAddr IP `flag:"ip"`
}

type IP net.IP

// Decode implements the interface `flagstruct.Decoder`
func (i *IP) Decode(repl string) error {
  *i = net.ParseIP(repl)
  return nil
}