Return an error in the case of unrunnable subcommand

* credit to @chriswhelix for initial commit

command.go

@@ -17,6 +17,7 @@ package cobra
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"io"
 	"os"
@@ -27,6 +28,9 @@ import (
 	flag "github.com/spf13/pflag"
 )
 
+// NotRunnable defines subcommand error
+var NotRunnable = errors.New("subcommand is required")
+
 // FParseErrWhitelist configures Flag parse errors to be ignored
 type FParseErrWhitelist flag.ParseErrorsWhitelist
 
@@ -369,7 +373,7 @@ func (c *Command) HelpFunc() func(*Command, []string) {
 	}
 	return func(c *Command, a []string) {
 		c.mergePersistentFlags()
-		err := tmpl(c.OutOrStdout(), c.HelpTemplate(), c)
+		err := tmpl(c.OutOrStderr(), c.HelpTemplate(), c)
 		if err != nil {
 			c.Println(err)
 		}
@@ -786,7 +790,7 @@ func (c *Command) execute(a []string) (err error) {
 	}
 
 	if !c.Runnable() {
-		return flag.ErrHelp
+		return NotRunnable
 	}
 
 	c.preRun()
@@ -920,6 +924,14 @@ func (c *Command) ExecuteC() (cmd *Command, err error) {
 			return cmd, nil
 		}
 
+		// If command wasn't runnable, show full help, but do return the error.
+		// This will result in apps by default returning a non-success exit code, but also gives them the option to
+		// handle specially.
+		if err == NotRunnable {
+			cmd.HelpFunc()(cmd, args)
+			return cmd, err
+		}
+
 		// If root command has SilentErrors flagged,
 		// all subcommands should respect it
 		if !cmd.SilenceErrors && !c.SilenceErrors {

command_test.go

@@ -836,8 +836,8 @@ func TestHelpExecutedOnNonRunnableChild(t *testing.T) {
 	rootCmd.AddCommand(childCmd)
 
 	output, err := executeCommand(rootCmd, "child")
-	if err != nil {
-		t.Errorf("Unexpected error: %v", err)
+	if err != NotRunnable {
+		t.Errorf("Expected error")
 	}
 
 	checkStringContains(t, output, childCmd.Long)