From 3e07cbd8ba54b574e35e763467111d229f199b35 Mon Sep 17 00:00:00 2001
From: Casey McTaggart <cmctaggart@pivotallabs.com>
Date: Wed, 4 Sep 2013 13:58:31 -0600
Subject: [PATCH] Show usage and exit with error whenever arguments parsing
 fails

Signed-off-by: Damien Le Berrigaud <damien@pivotallabs.com>
---
 app.go     |  9 ++++++++-
 command.go | 16 +++++++++++++++-
 2 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/app.go b/app.go
index 50a13e9..2655b6f 100644
--- a/app.go
+++ b/app.go
@@ -2,6 +2,8 @@ package cli
 
 import (
 	"os"
+	"io/ioutil"
+	"fmt"
 )
 
 type App struct {
@@ -40,12 +42,17 @@ func (a *App) Run(arguments []string) {
 
 	// parse flags
 	set := flagSet(a.Name, a.Flags)
+	set.SetOutput(ioutil.Discard)
 	err := set.Parse(arguments[1:])
+	context := NewContext(a, set, set)
+
 	if err != nil {
+		fmt.Println("Incorrect Usage.\n")
+		ShowAppHelp(context)
+		fmt.Println("")
 		os.Exit(1)
 	}
 
-	context := NewContext(a, set, set)
 	checkHelp(context)
 	checkVersion(context)
 
diff --git a/command.go b/command.go
index 6844855..f2ad467 100644
--- a/command.go
+++ b/command.go
@@ -1,5 +1,11 @@
 package cli
 
+import (
+	"io/ioutil"
+	"os"
+	"fmt"
+)
+
 type Command struct {
 	Name        string
 	ShortName   string
@@ -17,7 +23,15 @@ func (c Command) Run(ctx *Context) {
 	)
 
 	set := flagSet(c.Name, c.Flags)
-	set.Parse(ctx.Args()[1:])
+	set.SetOutput(ioutil.Discard)
+	err := set.Parse(ctx.Args()[1:])
+
+	if err != nil {
+		fmt.Println("Incorrect Usage.\n")
+		ShowCommandHelp(ctx, c.Name)
+		fmt.Println("")
+		os.Exit(1)
+	}
 
 	context := NewContext(ctx.App, set, ctx.globalSet)
 	checkCommandHelp(context, c.Name)