* Added ability to customize usage error messages

app.go

@@ -44,6 +44,10 @@ type App struct {
 	Action func(context *Context)
 	// Execute this function if the proper command cannot be found
 	CommandNotFound func(context *Context, command string)
+	// Execute this function if an usage error occurs. This is useful for displaying customized usage error messages.
+	// This function is able to manipulate the original error in another.
+	// If this function is not set the "Incorrect usage" is displayed and the execution is interrupted.
+	OnUsageError func(context *Context, err error, isSubcommand bool) error
 	// Compilation date
 	Compiled time.Time
 	// List of all authors who contributed
@@ -132,11 +136,16 @@ func (a *App) Run(arguments []string) (err error) {
 	}
 
 	if err != nil {
+		if a.OnUsageError != nil {
+			err := a.OnUsageError(context, err, false)
+			return err
+		} else {
 			fmt.Fprintln(a.Writer, "Incorrect Usage.")
 			fmt.Fprintln(a.Writer)
 			ShowAppHelp(context)
 			return err
 		}
+	}
 
 	if !a.HideHelp && checkHelp(context) {
 		ShowAppHelp(context)
@@ -242,11 +251,16 @@ func (a *App) RunAsSubcommand(ctx *Context) (err error) {
 	}
 
 	if err != nil {
+		if a.OnUsageError != nil {
+			err = a.OnUsageError(context, err, true)
+			return err
+		} else {
 			fmt.Fprintln(a.Writer, "Incorrect Usage.")
 			fmt.Fprintln(a.Writer)
 			ShowSubcommandHelp(context)
 			return err
 		}
+	}
 
 	if len(a.Commands) > 0 {
 		if checkSubcommandHelp(context) {

app_test.go

@@ -9,6 +9,7 @@ import (
 	"os"
 	"strings"
 	"testing"
+"errors"
 )
 
 func ExampleApp_Run() {
@@ -965,3 +966,63 @@ func TestApp_Run_SubcommandDoesNotOverwriteErrorFromBefore(t *testing.T) {
 		t.Errorf("expected text of error from After method, but got none in \"%v\"", err)
 	}
 }
+
+func TestApp_OnUsageError_WithWrongFlagValue(t *testing.T) {
+	app := NewApp()
+	app.Flags = []Flag{
+		IntFlag{Name: "flag"},
+	}
+	app.OnUsageError = func(c *Context, err error, isSubcommand bool) error {
+		if isSubcommand {
+			t.Errorf("Expect no subcommand")
+		}
+		if !strings.HasPrefix(err.Error(), "invalid value \"wrong\"") {
+			t.Errorf("Expect an invalid value error, but got \"%v\"", err)
+		}
+		return errors.New("intercepted: " + err.Error())
+	}
+	app.Commands = []Command{
+		Command{
+			Name: "bar",
+		},
+	}
+
+	err := app.Run([]string{"foo", "--flag=wrong"})
+	if err == nil {
+		t.Fatalf("expected to receive error from Run, got none")
+	}
+
+	if !strings.HasPrefix(err.Error(), "intercepted: invalid value") {
+		t.Errorf("Expect an intercepted error, but got \"%v\"", err)
+	}
+}
+
+func TestApp_OnUsageError_WithWrongFlagValue_ForSubcommand(t *testing.T) {
+	app := NewApp()
+	app.Flags = []Flag{
+		IntFlag{Name: "flag"},
+	}
+	app.OnUsageError = func(c *Context, err error, isSubcommand bool) error {
+		if isSubcommand {
+			t.Errorf("Expect subcommand")
+		}
+		if !strings.HasPrefix(err.Error(), "invalid value \"wrong\"") {
+			t.Errorf("Expect an invalid value error, but got \"%v\"", err)
+		}
+		return errors.New("intercepted: " + err.Error())
+	}
+	app.Commands = []Command{
+		Command{
+			Name: "bar",
+		},
+	}
+
+	err := app.Run([]string{"foo", "--flag=wrong", "bar"})
+	if err == nil {
+		t.Fatalf("expected to receive error from Run, got none")
+	}
+
+	if !strings.HasPrefix(err.Error(), "intercepted: invalid value") {
+		t.Errorf("Expect an intercepted error, but got \"%v\"", err)
+	}
+}

command.go

@@ -30,6 +30,10 @@ type Command struct {
 	After func(context *Context) error
 	// The function to call when this command is invoked
 	Action func(context *Context)
+	// Execute this function if an usage error occurs. This is useful for displaying customized usage error messages.
+	// This function is able to manipulate the original error in another.
+	// If this function is not set the "Incorrect usage" is displayed and the execution is interrupted.
+	OnUsageError func(context *Context, err error) error
 	// List of child commands
 	Subcommands []Command
 	// List of flags to parse
@@ -110,11 +114,16 @@ func (c Command) Run(ctx *Context) (err error) {
 	}
 
 	if err != nil {
+		if c.OnUsageError != nil {
+			err := c.OnUsageError(ctx, err)
+			return err
+		} else {
 			fmt.Fprintln(ctx.App.Writer, "Incorrect Usage.")
 			fmt.Fprintln(ctx.App.Writer)
 			ShowCommandHelp(ctx, c.Name)
 			return err
 		}
+	}
 
 	nerr := normalizeFlags(c.Flags, set)
 	if nerr != nil {

command_test.go

@@ -68,3 +68,30 @@ func TestCommand_Run_DoesNotOverwriteErrorFromBefore(t *testing.T) {
 		t.Errorf("expected text of error from After method, but got none in \"%v\"", err)
 	}
 }
+
+func TestCommand_OnUsageError_WithWrongFlagValue(t *testing.T) {
+	app := NewApp()
+	app.Commands = []Command{
+		Command{
+			Name:   "bar",
+			Flags: []Flag{
+				IntFlag{Name: "flag"},
+			},
+			OnUsageError: func(c *Context, err error) error {
+				if !strings.HasPrefix(err.Error(), "invalid value \"wrong\"") {
+					t.Errorf("Expect an invalid value error, but got \"%v\"", err)
+				}
+				return errors.New("intercepted: " + err.Error())
+			},
+		},
+	}
+
+	err := app.Run([]string{"foo", "bar", "--flag=wrong"})
+	if err == nil {
+		t.Fatalf("expected to receive error from Run, got none")
+	}
+
+	if !strings.HasPrefix(err.Error(), "intercepted: invalid value") {
+		t.Errorf("Expect an intercepted error, but got \"%v\"", err)
+	}
+}