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CONTRIBUTING

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README.md

+Subcommands is a Go package that implements a simple way for a single command to
+have many subcommands, each of which takes arguments and so forth.
+
+This is not an official Google product.

subcommands.go

+/*
+Copyright 2016 Google Inc. All Rights Reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+// Package subcommands implements a simple way for a single command to have many
+// subcommands, each of which takes arguments and so forth.
+package subcommands
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"path"
+	"sort"
+
+	"flag"
+"golang.org/x/net/context"
+)
+
+// A Command represents a single command.
+type Command interface {
+	// Name returns the name of the command.
+	Name() string
+
+	// Synopsis returns a short string (less than one line) describing the command.
+	Synopsis() string
+
+	// Usage returns a long string explaining the command and giving usage
+	// information.
+	Usage() string
+
+	// SetFlags adds the flags for this command to the specified set.
+	SetFlags(*flag.FlagSet)
+
+	// Execute executes the command and returns an ExitStatus.
+	Execute(ctx context.Context, f *flag.FlagSet, args ...interface{}) ExitStatus
+}
+
+// A Commander represents a set of commands.
+type Commander struct {
+	commands  []*commandGroup
+	topFlags  *flag.FlagSet // top-level flags
+	important []string      // important top-level flags
+	name      string        // normally path.Base(os.Args[0])
+}
+
+// A commandGroup represents a set of commands about a common topic.
+type commandGroup struct {
+	name     string
+	commands []Command
+}
+
+// An ExitStatus represents a Posix exit status that a subcommand
+// expects to be returned to the shell.
+type ExitStatus int
+
+const (
+	ExitSuccess ExitStatus = iota
+	ExitFailure
+	ExitUsageError
+)
+
+// NewCommander returns a new commander with the specified top-level
+// flags and command name. The Usage function for the topLevelFlags
+// will be set as well.
+func NewCommander(topLevelFlags *flag.FlagSet, name string) *Commander {
+	cdr := &Commander{
+		topFlags: topLevelFlags,
+		name:     name,
+	}
+	topLevelFlags.Usage = func() { cdr.explain(os.Stderr) }
+	return cdr
+}
+
+// Register adds a subcommand to the supported subcommands in the
+// specified group. (Help output is sorted and arranged by group name.)
+// The empty string is an acceptable group name; such subcommands are
+// explained first before named groups.
+func (cdr *Commander) Register(cmd Command, group string) {
+	for _, g := range cdr.commands {
+		if g.name == group {
+			g.commands = append(g.commands, cmd)
+			return
+		}
+	}
+	cdr.commands = append(cdr.commands, &commandGroup{
+		name:     group,
+		commands: []Command{cmd},
+	})
+}
+
+// ImportantFlag marks a top-level flag as important, which means it
+// will be printed out as part of the output of an ordinary "help"
+// subcommand.  (All flags, important or not, are printed by the
+// "flags" subcommand.)
+func (cdr *Commander) ImportantFlag(name string) {
+	cdr.important = append(cdr.important, name)
+}
+
+// Execute should be called once the top-level-flags on a Commander
+// have been initialized. It finds the correct subcommand and executes
+// it, and returns an ExitStatus with the result. On a usage error, an
+// appropriate message is printed to os.Stderr, and ExitUsageError is
+// returned. The additional args are provided as-is to the Execute method
+// of the selected Command.
+func (cdr *Commander) Execute(ctx context.Context, args ...interface{}) ExitStatus {
+	if cdr.topFlags.NArg() < 1 {
+		cdr.topFlags.Usage()
+		return ExitUsageError
+	}
+
+	name := cdr.topFlags.Arg(0)
+
+	for _, group := range cdr.commands {
+		for _, cmd := range group.commands {
+			if name != cmd.Name() {
+				continue
+			}
+			f := flag.NewFlagSet(name, flag.ContinueOnError)
+			f.Usage = func() { explain(os.Stderr, cmd) }
+			cmd.SetFlags(f)
+			if f.Parse(cdr.topFlags.Args()[1:]) != nil {
+				return ExitUsageError
+			}
+			return cmd.Execute(ctx, f, args...)
+		}
+	}
+
+	// Cannot find this command.
+	cdr.topFlags.Usage()
+	return ExitUsageError
+}
+
+// Sorting of a slice of command groups.
+type byGroupName []*commandGroup
+
+func (p byGroupName) Len() int           { return len(p) }
+func (p byGroupName) Less(i, j int) bool { return p[i].name < p[j].name }
+func (p byGroupName) Swap(i, j int)      { p[i], p[j] = p[j], p[i] }
+
+// explain prints a brief description of all the subcommands and the
+// important top-level flags.
+func (cdr *Commander) explain(w io.Writer) {
+	fmt.Fprintf(w, "Usage: %s <flags> <subcommand> <subcommand args>\n\n", cdr.name)
+	sort.Sort(byGroupName(cdr.commands))
+	for _, group := range cdr.commands {
+		explainGroup(w, group)
+	}
+	if cdr.topFlags == nil {
+		fmt.Fprintln(w, "\nNo top level flags.")
+		return
+	}
+	if len(cdr.important) == 0 {
+		fmt.Fprintf(w, "\nUse \"%s flags\" for a list of top-level flags\n", cdr.name)
+		return
+	}
+
+	fmt.Fprintf(w, "\nTop-level flags (use \"%s flags\" for a full list):\n", cdr.name)
+	for _, name := range cdr.important {
+		f := cdr.topFlags.Lookup(name)
+		if f == nil {
+			panic("Important flag is not defined")
+		}
+		fmt.Fprintf(w, "  -%s=%s: %s\n", f.Name, f.DefValue, f.Usage)
+	}
+}
+
+// Sorting of the commands within a group.
+func (g commandGroup) Len() int           { return len(g.commands) }
+func (g commandGroup) Less(i, j int) bool { return g.commands[i].Name() < g.commands[j].Name() }
+func (g commandGroup) Swap(i, j int)      { g.commands[i], g.commands[j] = g.commands[j], g.commands[i] }
+
+// explainGroup explains all the subcommands for a particular group.
+func explainGroup(w io.Writer, group *commandGroup) {
+	if len(group.commands) == 0 {
+		return
+	}
+	if group.name == "" {
+		fmt.Fprintf(w, "Subcommands:\n")
+	} else {
+		fmt.Fprintf(w, "Subcommands for %s:\n", group.name)
+	}
+	sort.Sort(group)
+	for _, cmd := range group.commands {
+		fmt.Fprintf(w, "\t%-15s  %s\n", cmd.Name(), cmd.Synopsis())
+	}
+	fmt.Fprintln(w)
+}
+
+// explainCmd prints a brief description of a single command.
+func explain(w io.Writer, cmd Command) {
+	fmt.Fprintf(w, "%s", cmd.Usage())
+	subflags := flag.NewFlagSet(cmd.Name(), flag.PanicOnError)
+	subflags.SetOutput(w)
+	cmd.SetFlags(subflags)
+	subflags.PrintDefaults()
+}
+
+// A helper is a Command implementing a "help" command for
+// a given Commander.
+type helper Commander
+
+func (h *helper) Name() string           { return "help" }
+func (h *helper) Synopsis() string       { return "describe subcommands and their syntax" }
+func (h *helper) SetFlags(*flag.FlagSet) {}
+func (h *helper) Usage() string {
+	return `help [<subcommand>]:
+	With an argument, prints detailed information on the use of
+	the specified subcommand. With no argument, print a list of
+	all commands and a brief description of each.
+`
+}
+func (h *helper) Execute(_ context.Context, f *flag.FlagSet, args ...interface{}) ExitStatus {
+	switch f.NArg() {
+	case 0:
+		(*Commander)(h).explain(os.Stdout)
+		return ExitSuccess
+
+	case 1:
+		for _, group := range h.commands {
+			for _, cmd := range group.commands {
+				if f.Arg(0) != cmd.Name() {
+					continue
+				}
+				explain(os.Stdout, cmd)
+				return ExitSuccess
+			}
+		}
+		fmt.Fprintf(os.Stderr, "Subcommand %s not understood\n", f.Arg(0))
+	}
+
+	f.Usage()
+	return ExitUsageError
+}
+
+// HelpCommand returns a Command which implements a "help" subcommand.
+func (cdr *Commander) HelpCommand() Command {
+	return (*helper)(cdr)
+}
+
+// A flagger is a Command implementing a "flags" command for a given Commander.
+type flagger Commander
+
+func (flg *flagger) Name() string           { return "flags" }
+func (flg *flagger) Synopsis() string       { return "describe all known top-level flags" }
+func (flg *flagger) SetFlags(*flag.FlagSet) {}
+func (flg *flagger) Usage() string {
+	return `flags [<subcommand>]:
+	With an argument, print all flags of <subcommand>. Else,
+	print a description of all known top-level flags.  (The basic
+	help information only discusses the most generally important
+	top-level flags.)
+`
+}
+func (flg *flagger) Execute(_ context.Context, f *flag.FlagSet, _ ...interface{}) ExitStatus {
+	if f.NArg() > 1 {
+		f.Usage()
+		return ExitUsageError
+	}
+
+	if f.NArg() == 0 {
+		if flg.topFlags == nil {
+			fmt.Println("No top-level flags are defined.")
+		} else {
+			flg.topFlags.PrintDefaults()
+		}
+		return ExitSuccess
+	}
+
+	for _, group := range flg.commands {
+		for _, cmd := range group.commands {
+			if f.Arg(0) != cmd.Name() {
+				continue
+			}
+			subflags := flag.NewFlagSet(cmd.Name(), flag.PanicOnError)
+			subflags.SetOutput(os.Stdout)
+			cmd.SetFlags(subflags)
+			subflags.PrintDefaults()
+			return ExitSuccess
+		}
+	}
+	fmt.Fprintf(os.Stderr, "Subcommand %s not understood\n", f.Arg(0))
+	return ExitFailure
+}
+
+// FlagsCommand returns a Command which implements a "flags" subcommand.
+func (cdr *Commander) FlagsCommand() Command {
+	return (*flagger)(cdr)
+}
+
+// A lister is a Command implementing a "commands" command for a given Commander.
+type lister Commander
+
+func (l *lister) Name() string           { return "commands" }
+func (l *lister) Synopsis() string       { return "list all command names" }
+func (l *lister) SetFlags(*flag.FlagSet) {}
+func (l *lister) Usage() string {
+	return `commands:
+	Print a list of all commands.
+`
+}
+func (l *lister) Execute(_ context.Context, f *flag.FlagSet, _ ...interface{}) ExitStatus {
+	if f.NArg() != 0 {
+		f.Usage()
+		return ExitUsageError
+	}
+
+	for _, group := range l.commands {
+		for _, cmd := range group.commands {
+			fmt.Printf("%s\n", cmd.Name())
+		}
+	}
+	return ExitSuccess
+}
+
+// CommandsCommand returns Command which implements a "commands" subcommand.
+func (cdr *Commander) CommandsCommand() Command {
+	return (*lister)(cdr)
+}
+
+// DefaultCommander is the default commander using flag.CommandLine for flags
+// and os.Args[0] for the command name.
+var DefaultCommander *Commander
+
+func init() {
+	DefaultCommander = NewCommander(flag.CommandLine, path.Base(os.Args[0]))
+}
+
+// Register adds a subcommand to the supported subcommands in the
+// specified group. (Help output is sorted and arranged by group
+// name.)  The empty string is an acceptable group name; such
+// subcommands are explained first before named groups. It is a
+// wrapper around DefaultCommander.Register.
+func Register(cmd Command, group string) {
+	DefaultCommander.Register(cmd, group)
+}
+
+// ImportantFlag marks a top-level flag as important, which means it
+// will be printed out as part of the output of an ordinary "help"
+// subcommand.  (All flags, important or not, are printed by the
+// "flags" subcommand.) It is a wrapper around
+// DefaultCommander.ImportantFlag.
+func ImportantFlag(name string) {
+	DefaultCommander.ImportantFlag(name)
+}
+
+// Execute should be called once the default flags have been
+// initialized by flag.Parse. It finds the correct subcommand and
+// executes it, and returns an ExitStatus with the result. On a usage
+// error, an appropriate message is printed to os.Stderr, and
+// ExitUsageError is returned. The additional args are provided as-is
+// to the Execute method of the selected Command. It is a wrapper
+// around DefaultCommander.Execute.
+func Execute(ctx context.Context, args ...interface{}) ExitStatus {
+	return DefaultCommander.Execute(ctx, args...)
+}
+
+// HelpCommand returns a Command which implements "help" for the
+// DefaultCommander. Use Register(HelpCommand(), <group>) for it to be
+// recognized.
+func HelpCommand() Command {
+	return DefaultCommander.HelpCommand()
+}
+
+// FlagsCommand returns a Command which implements "flags" for the
+// DefaultCommander. Use Register(FlagsCommand(), <group>) for it to be
+// recognized.
+func FlagsCommand() Command {
+	return DefaultCommander.FlagsCommand()
+}
+
+// CommandsCommand returns Command which implements a "commands" subcommand.
+func CommandsCommand() Command {
+	return DefaultCommander.CommandsCommand()
+}