Exit non-zero if a unknown subcommand is given

Currently it just prints the help message and exits 0.

We do this by modifying the helpCommand and helpSubcommand cli.Commands
to return an error if they are called with an unknown subcommand. This
propogates up to the app which exits with 3 and prints the error.

Thanks to @danslimmon for the initial approach!

Fixes #276

CHANGELOG.md

@@ -13,6 +13,10 @@ quoted in help text output.
   default value can be (reasonably) detected.
 - `IntSliceFlag` and `StringSliceFlag` usage strings are now more consistent
   with non-slice flag types
+- Apps now exit with a code of 3 if an unknown subcommand is specified
+  (previously they printed "No help topic for...", but still exited 0. This
+  makes it easier to script around apps built using `cli` since they can trust
+  that a 0 exit code indicated a successful execution.
 
 ## [1.16.0] - 2016-05-02
 ### Added

help.go

@@ -81,10 +81,10 @@ var helpCommand = Command{
 	Action: func(c *Context) error {
 		args := c.Args()
 		if args.Present() {
-			ShowCommandHelp(c, args.First())
-		} else {
-			ShowAppHelp(c)
+			return ShowCommandHelp(c, args.First())
 		}
+
+		ShowAppHelp(c)
 		return nil
 	},
 }
@@ -97,11 +97,10 @@ var helpSubcommand = Command{
 	Action: func(c *Context) error {
 		args := c.Args()
 		if args.Present() {
-			ShowCommandHelp(c, args.First())
-		} else {
-			ShowSubcommandHelp(c)
+			return ShowCommandHelp(c, args.First())
 		}
-		return nil
+
+		return ShowSubcommandHelp(c)
 	},
 }
 
@@ -127,30 +126,31 @@ func DefaultAppComplete(c *Context) {
 }
 
 // Prints help for the given command
-func ShowCommandHelp(ctx *Context, command string) {
+func ShowCommandHelp(ctx *Context, command string) error {
 	// show the subcommand help for a command with subcommands
 	if command == "" {
 		HelpPrinter(ctx.App.Writer, SubcommandHelpTemplate, ctx.App)
-		return
+		return nil
 	}
 
 	for _, c := range ctx.App.Commands {
 		if c.HasName(command) {
 			HelpPrinter(ctx.App.Writer, CommandHelpTemplate, c)
-			return
+			return nil
 		}
 	}
 
-	if ctx.App.CommandNotFound != nil {
-		ctx.App.CommandNotFound(ctx, command)
-	} else {
-		fmt.Fprintf(ctx.App.Writer, "No help topic for '%v'\n", command)
+	if ctx.App.CommandNotFound == nil {
+		return NewExitError(fmt.Sprintf("No help topic for '%v'", command), 3)
 	}
+
+	ctx.App.CommandNotFound(ctx, command)
+	return nil
 }
 
 // Prints help for the given subcommand
-func ShowSubcommandHelp(c *Context) {
-	ShowCommandHelp(c, c.Command.Name)
+func ShowSubcommandHelp(c *Context) error {
+	return ShowCommandHelp(c, c.Command.Name)
 }
 
 // Prints the version number of the App

help_test.go

@@ -2,6 +2,8 @@ package cli
 
 import (
 	"bytes"
+	"flag"
+	"strings"
 	"testing"
 )
 
@@ -110,3 +112,59 @@ func Test_Version_Custom_Flags(t *testing.T) {
 		t.Errorf("unexpected output: %s", output.String())
 	}
 }
+
+func Test_helpCommand_Action_ErrorIfNoTopic(t *testing.T) {
+	app := NewApp()
+
+	set := flag.NewFlagSet("test", 0)
+	set.Parse([]string{"foo"})
+
+	c := NewContext(app, set, nil)
+
+	err := helpCommand.Action.(func(*Context) error)(c)
+
+	if err == nil {
+		t.Fatalf("expected error from helpCommand.Action(), but got nil")
+	}
+
+	exitErr, ok := err.(*ExitError)
+	if !ok {
+		t.Fatalf("expected ExitError from helpCommand.Action(), but instead got: %v", err.Error())
+	}
+
+	if !strings.HasPrefix(exitErr.Error(), "No help topic for") {
+		t.Fatalf("expected an unknown help topic error, but got: %v", exitErr.Error())
+	}
+
+	if exitErr.exitCode != 3 {
+		t.Fatalf("expected exit value = 3, got %d instead", exitErr.exitCode)
+	}
+}
+
+func Test_helpSubcommand_Action_ErrorIfNoTopic(t *testing.T) {
+	app := NewApp()
+
+	set := flag.NewFlagSet("test", 0)
+	set.Parse([]string{"foo"})
+
+	c := NewContext(app, set, nil)
+
+	err := helpSubcommand.Action.(func(*Context) error)(c)
+
+	if err == nil {
+		t.Fatalf("expected error from helpCommand.Action(), but got nil")
+	}
+
+	exitErr, ok := err.(*ExitError)
+	if !ok {
+		t.Fatalf("expected ExitError from helpCommand.Action(), but instead got: %v", err.Error())
+	}
+
+	if !strings.HasPrefix(exitErr.Error(), "No help topic for") {
+		t.Fatalf("expected an unknown help topic error, but got: %v", exitErr.Error())
+	}
+
+	if exitErr.exitCode != 3 {
+		t.Fatalf("expected exit value = 3, got %d instead", exitErr.exitCode)
+	}
+}