Command-Line Subcommands #

// Some command-line tools, like the `go` tool or `git`
// have many *subcommands*, each with its own set of
// flags. For example, `go build` and `go get` are two
// different subcommands of the `go` tool.
// The `flag` package lets us easily define simple
// subcommands that have their own flags.

package main

import (
	"flag"
	"fmt"
	"os"
)

func main() {

	// We declare a subcommand using the `NewFlagSet`
	// function, and proceed to define new flags specific
	// for this subcommand.
	fooCmd := flag.NewFlagSet("foo", flag.ExitOnError)
	fooEnable := fooCmd.Bool("enable", false, "enable")
	fooName := fooCmd.String("name", "", "name")

	// For a different subcommand we can define different
	// supported flags.
	barCmd := flag.NewFlagSet("bar", flag.ExitOnError)
	barLevel := barCmd.Int("level", 0, "level")

	// The subcommand is expected as the first argument
	// to the program.
	if len(os.Args) < 2 {
		fmt.Println("expected 'foo' or 'bar' subcommands")
		os.Exit(1)
	}

	// Check which subcommand is invoked.
	switch os.Args[1] {

	// For every subcommand, we parse its own flags and
	// have access to trailing positional arguments.
	case "foo":
		fooCmd.Parse(os.Args[2:])
		fmt.Println("subcommand 'foo'")
		fmt.Println("  enable:", *fooEnable)
		fmt.Println("  name:", *fooName)
		fmt.Println("  tail:", fooCmd.Args())
	case "bar":
		barCmd.Parse(os.Args[2:])
		fmt.Println("subcommand 'bar'")
		fmt.Println("  level:", *barLevel)
		fmt.Println("  tail:", barCmd.Args())
	default:
		fmt.Println("expected 'foo' or 'bar' subcommands")
		os.Exit(1)
	}
}

$ go build command-line-subcommands.go 

# First invoke the foo subcommand.
$ ./command-line-subcommands foo -enable -name=joe a1 a2
subcommand 'foo'
  enable: true
  name: joe
  tail: [a1 a2]

# Now try bar.
$ ./command-line-subcommands bar -level 8 a1
subcommand 'bar'
  level: 8
  tail: [a1]

# But bar won't accept foo's flags.
$ ./command-line-subcommands bar -enable a1
flag provided but not defined: -enable
Usage of bar:
  -level int
    	level

# Next we'll look at environment variables, another common
# way to parameterize programs.