main.go

package main

import (
	"flag"
	"fmt"
	"os"
	"strings"
	"text/tabwriter"
)

const (
	cliName        = "foo"
	cliDescription = "foo, a command that does cool things."
	cliVersion     = "1.0.0"
)

var (
	out           *tabwriter.Writer
	globalFlagset *flag.FlagSet = flag.NewFlagSet(cliName, flag.ExitOnError)

	// set of top-level commands
	commands []*Command

	// flags used by all commands
	globalFlags = struct {
		Verbosity int
		Version   bool
	}{}
)

func init() {
	globalFlagset.BoolVar(&globalFlags.Version, "version", false, "Print the version and exit")
	globalFlagset.IntVar(&globalFlags.Verbosity, "verbosity", 0, "Log at a specified level of verbosity to stderr.")
}

type Command struct {
	Name        string       // Name of the Command and the string to use to invoke it
	Summary     string       // One-sentence summary of what the Command does
	Usage       string       // Usage options/arguments
	Description string       // Detailed description of command
	Flags       flag.FlagSet // Set of flags associated with this command

	Run func(args []string) int // Run a command with the given arguments, return exit status

}

func init() {
	out = new(tabwriter.Writer)
	out.Init(os.Stdout, 0, 8, 1, '\t', 0)
	commands = []*Command{
		cmdHelp,
		cmdVersion,
	}
}

func getAllFlags() (flags []*flag.Flag) {
	return getFlags(globalFlagset)
}

func getFlags(flagset *flag.FlagSet) (flags []*flag.Flag) {
	flags = make([]*flag.Flag, 0)
	flagset.VisitAll(func(f *flag.Flag) {
		flags = append(flags, f)
	})
	return
}

func main() {
	// parse global arguments
	globalFlagset.Parse(os.Args[1:])

	var args = globalFlagset.Args()

	getFlagsFromEnv(cliName, globalFlagset)

	// no command specified - trigger help
	if len(args) < 1 {
		args = append(args, "help")
	}

	// deal specially with --version
	if globalFlags.Version {
		args[0] = "version"
	}

	var cmd *Command

	// determine which Command should be run
	for _, c := range commands {
		if c.Name == args[0] {
			cmd = c
			if err := c.Flags.Parse(args[1:]); err != nil {
				fmt.Println(err.Error())
				os.Exit(2)
			}
			break
		}
	}

	if cmd == nil {
		fmt.Printf("%v: unknown subcommand: %q\n", cliName, args[0])
		fmt.Printf("Run '%v help' for usage.\n", cliName)
		os.Exit(2)
	}

	os.Exit(cmd.Run(cmd.Flags.Args()))

}

// getFlagsFromEnv parses all registered flags in the given flagset,
// and if they are not already set it attempts to set their values from
// environment variables. Environment variables take the name of the flag but
// are UPPERCASE, have the given prefix, and any dashes are replaced by
// underscores - for example: some-flag => PREFIX_SOME_FLAG
func getFlagsFromEnv(prefix string, fs *flag.FlagSet) {
	alreadySet := make(map[string]bool)
	fs.Visit(func(f *flag.Flag) {
		alreadySet[f.Name] = true
	})
	fs.VisitAll(func(f *flag.Flag) {
		if !alreadySet[f.Name] {
			key := strings.ToUpper(prefix + "_" + strings.Replace(f.Name, "-", "_", -1))
			val := os.Getenv(key)
			if val != "" {
				fs.Set(f.Name, val)
			}
		}

	})
}