Fix:(issue_1623) Send error to stderr instead of stdout

app.go

@@ -109,6 +109,9 @@ type App struct {
 	didSetup bool
 
 	rootCommand *Command
+
+	// if the app is in error mode
+	isInError bool
 }
 
 // Setup runs initialization code to ensure all data structures are ready for
@@ -420,6 +423,17 @@ func runFlagActions(c *Context, fs []Flag) error {
 	return nil
 }
 
+func (a *App) writer() io.Writer {
+	if a.isInError {
+		// this can happen in test but not in normal usage
+		if a.ErrWriter == nil {
+			return os.Stderr
+		}
+		return a.ErrWriter
+	}
+	return a.Writer
+}
+
 func checkStringSliceIncludes(want string, sSlice []string) bool {
 	found := false
 	for _, s := range sSlice {

command.go

@@ -157,15 +157,16 @@ func (c *Command) Run(cCtx *Context, arguments ...string) (err error) {
 	}
 
 	if err != nil {
+		cCtx.App.isInError = true
 		if c.OnUsageError != nil {
 			err = c.OnUsageError(cCtx, err, !c.isRoot)
 			cCtx.App.handleExitCoder(cCtx, err)
 			return err
 		}
-		_, _ = fmt.Fprintf(cCtx.App.Writer, "%s %s\n\n", "Incorrect Usage:", err.Error())
+		_, _ = fmt.Fprintf(cCtx.App.writer(), "%s %s\n\n", "Incorrect Usage:", err.Error())
 		if cCtx.App.Suggest {
 			if suggestion, err := c.suggestFlagFromError(err, ""); err == nil {
-				fmt.Fprintf(cCtx.App.Writer, "%s", suggestion)
+				fmt.Fprintf(cCtx.App.writer(), "%s", suggestion)
 			}
 		}
 		if !c.HideHelp {
@@ -203,6 +204,7 @@ func (c *Command) Run(cCtx *Context, arguments ...string) (err error) {
 
 	cerr := cCtx.checkRequiredFlags(c.Flags)
 	if cerr != nil {
+		cCtx.App.isInError = true
 		_ = ShowSubcommandHelp(cCtx)
 		return cerr
 	}

help.go

@@ -76,7 +76,7 @@ var helpCommand = &Command{
 			if templ == "" {
 				templ = CommandHelpTemplate
 			}
-			HelpPrinter(cCtx.App.Writer, templ, cCtx.Command)
+			HelpPrinter(cCtx.App.writer(), templ, cCtx.Command)
 			return nil
 		}
 		return ShowSubcommandHelp(cCtx)
@@ -125,7 +125,7 @@ func ShowAppHelp(cCtx *Context) error {
 	}
 
 	if cCtx.App.ExtraInfo == nil {
-		HelpPrinter(cCtx.App.Writer, tpl, cCtx.App)
+		HelpPrinter(cCtx.App.writer(), tpl, cCtx.App)
 		return nil
 	}
 
@@ -134,7 +134,7 @@ func ShowAppHelp(cCtx *Context) error {
 			"ExtraInfo": cCtx.App.ExtraInfo,
 		}
 	}
-	HelpPrinterCustom(cCtx.App.Writer, tpl, cCtx.App, customAppData())
+	HelpPrinterCustom(cCtx.App.writer(), tpl, cCtx.App, customAppData())
 
 	return nil
 }
@@ -213,23 +213,23 @@ func DefaultCompleteWithFlags(cmd *Command) func(cCtx *Context) {
 
 			if strings.HasPrefix(lastArg, "-") {
 				if cmd != nil {
-					printFlagSuggestions(lastArg, cmd.Flags, cCtx.App.Writer)
+					printFlagSuggestions(lastArg, cmd.Flags, cCtx.App.writer())
 
 					return
 				}
 
-				printFlagSuggestions(lastArg, cCtx.App.Flags, cCtx.App.Writer)
+				printFlagSuggestions(lastArg, cCtx.App.Flags, cCtx.App.writer())
 
 				return
 			}
 		}
 
 		if cmd != nil {
-			printCommandSuggestions(cmd.Commands, cCtx.App.Writer)
+			printCommandSuggestions(cmd.Commands, cCtx.App.writer())
 			return
 		}
 
-		printCommandSuggestions(cCtx.App.Commands, cCtx.App.Writer)
+		printCommandSuggestions(cCtx.App.Commands, cCtx.App.writer())
 	}
 }
 
@@ -264,7 +264,7 @@ func ShowCommandHelp(ctx *Context, command string) error {
 				}
 			}
 
-			HelpPrinter(ctx.App.Writer, templ, c)
+			HelpPrinter(ctx.App.writer(), templ, c)
 
 			return nil
 		}
@@ -296,7 +296,7 @@ func ShowSubcommandHelp(cCtx *Context) error {
 		return nil
 	}
 
-	HelpPrinter(cCtx.App.Writer, SubcommandHelpTemplate, cCtx.Command)
+	HelpPrinter(cCtx.App.writer(), SubcommandHelpTemplate, cCtx.Command)
 	return nil
 }
 
@@ -306,7 +306,7 @@ func ShowVersion(cCtx *Context) {
 }
 
 func printVersion(cCtx *Context) {
-	_, _ = fmt.Fprintf(cCtx.App.Writer, "%v version %v\n", cCtx.App.Name, cCtx.App.Version)
+	_, _ = fmt.Fprintf(cCtx.App.writer(), "%v version %v\n", cCtx.App.Name, cCtx.App.Version)
 }
 
 // ShowCompletions prints the lists of commands within a given context

suggestions_test.go

@@ -3,6 +3,7 @@ package cli
 import (
 	"errors"
 	"fmt"
+	"os"
 	"testing"
 )
 
@@ -123,6 +124,7 @@ func TestSuggestCommand(t *testing.T) {
 func ExampleApp_Suggest() {
 	app := &App{
 		Name:                  "greet",
+		ErrWriter:             os.Stdout,
 		Suggest:               true,
 		HideHelp:              false,
 		HideHelpCommand:       true,
@@ -148,6 +150,7 @@ func ExampleApp_Suggest() {
 func ExampleApp_Suggest_command() {
 	app := &App{
 		Name:                  "greet",
+		ErrWriter:             os.Stdout,
 		Suggest:               true,
 		HideHelpCommand:       true,
 		CustomAppHelpTemplate: "(this space intentionally left blank)\n",