Allow a writer to be set that represents Stdout so that redirection of App output may occur.

app.go

@@ -2,8 +2,11 @@ package cli
 
 import (
 	"fmt"
+	"io"
 	"io/ioutil"
 	"os"
+	"text/tabwriter"
+	"text/template"
 	"time"
 )
 
@@ -37,6 +40,8 @@ type App struct {
 	Author string
 	// Author e-mail
 	Email string
+	// Stdout writer to write output to
+	Stdout io.Writer
 }
 
 // Tries to find out when this binary was compiled.
@@ -60,11 +65,28 @@ func NewApp() *App {
 		Compiled:     compileTime(),
 		Author:       "Author",
 		Email:        "unknown@email",
+		Stdout:       os.Stdout,
 	}
 }
 
 // Entry point to the cli app. Parses the arguments slice and routes to the proper flag/args combination
 func (a *App) Run(arguments []string) error {
+	if HelpPrinter == nil {
+		defer func() {
+			HelpPrinter = nil
+		}()
+
+		HelpPrinter = func(templ string, data interface{}) {
+			w := tabwriter.NewWriter(a.Stdout, 0, 8, 1, '\t', 0)
+			t := template.Must(template.New("help").Parse(templ))
+			err := t.Execute(w, data)
+			if err != nil {
+				panic(err)
+			}
+			w.Flush()
+		}
+	}
+
 	// append help to commands
 	if a.Command(helpCommand.Name) == nil {
 		a.Commands = append(a.Commands, helpCommand)
@@ -83,18 +105,18 @@ func (a *App) Run(arguments []string) error {
 	err := set.Parse(arguments[1:])
 	nerr := normalizeFlags(a.Flags, set)
 	if nerr != nil {
-		fmt.Println(nerr)
+		io.WriteString(a.Stdout, fmt.Sprintln(nerr))
 		context := NewContext(a, set, set)
 		ShowAppHelp(context)
-		fmt.Println("")
+		io.WriteString(a.Stdout, fmt.Sprintln(""))
 		return nerr
 	}
 	context := NewContext(a, set, set)
 
 	if err != nil {
-		fmt.Printf("Incorrect Usage.\n\n")
+		io.WriteString(a.Stdout, fmt.Sprintf("Incorrect Usage.\n\n"))
 		ShowAppHelp(context)
-		fmt.Println("")
+		io.WriteString(a.Stdout, fmt.Sprintln(""))
 		return err
 	}
 
@@ -154,18 +176,18 @@ func (a *App) RunAsSubcommand(ctx *Context) error {
 	context := NewContext(a, set, set)
 
 	if nerr != nil {
-		fmt.Println(nerr)
+		io.WriteString(a.Stdout, fmt.Sprintln(nerr))
 		if len(a.Commands) > 0 {
 			ShowSubcommandHelp(context)
 		} else {
 			ShowCommandHelp(ctx, context.Args().First())
 		}
-		fmt.Println("")
+		io.WriteString(a.Stdout, fmt.Sprintln(""))
 		return nerr
 	}
 
 	if err != nil {
-		fmt.Printf("Incorrect Usage.\n\n")
+		io.WriteString(a.Stdout, fmt.Sprintf("Incorrect Usage.\n\n"))
 		ShowSubcommandHelp(context)
 		return err
 	}

app_test.go

@@ -262,6 +262,50 @@ func TestApp_ParseSliceFlags(t *testing.T) {
 	}
 }
 
+func TestApp_DefaultStdout(t *testing.T) {
+	app := cli.NewApp()
+
+	if app.Stdout != os.Stdout {
+		t.Error("Default output writer not set.")
+	}
+}
+
+type fakeWriter struct {
+	written []byte
+}
+
+func (fw *fakeWriter) Write(p []byte) (n int, err error) {
+	if fw.written == nil {
+		fw.written = p
+	} else {
+		fw.written = append(fw.written, p...)
+	}
+
+	return len(p), nil
+}
+
+func (fw *fakeWriter) GetWritten() (b []byte) {
+	return fw.written
+}
+
+func TestApp_SetStdout(t *testing.T) {
+	mockWriter := &fakeWriter{}
+
+	app := cli.NewApp()
+	app.Name = "test"
+	app.Stdout = mockWriter
+
+	err := app.Run([]string{"help"})
+
+	if err != nil {
+		t.Fatalf("Run error: %s", err)
+	}
+
+	if len(mockWriter.written) == 0 {
+		t.Error("App did not write output to desired writer.")
+	}
+}
+
 func TestApp_BeforeFunc(t *testing.T) {
 	beforeRun, subcommandRun := false, false
 	beforeError := fmt.Errorf("fail")

command.go

@@ -2,6 +2,7 @@ package cli
 
 import (
 	"fmt"
+	"io"
 	"io/ioutil"
 	"strings"
 )
@@ -70,18 +71,18 @@ func (c Command) Run(ctx *Context) error {
 	}
 
 	if err != nil {
-		fmt.Printf("Incorrect Usage.\n\n")
+		io.WriteString(ctx.App.Stdout, fmt.Sprintf("Incorrect Usage.\n\n"))
 		ShowCommandHelp(ctx, c.Name)
-		fmt.Println("")
+		io.WriteString(ctx.App.Stdout, fmt.Sprintln(""))
 		return err
 	}
 
 	nerr := normalizeFlags(c.Flags, set)
 	if nerr != nil {
-		fmt.Println(nerr)
-		fmt.Println("")
+		io.WriteString(ctx.App.Stdout, fmt.Sprintln(nerr))
+		io.WriteString(ctx.App.Stdout, fmt.Sprintln(""))
 		ShowCommandHelp(ctx, c.Name)
-		fmt.Println("")
+		io.WriteString(ctx.App.Stdout, fmt.Sprintln(""))
 		return nerr
 	}
 	context := NewContext(ctx.App, set, ctx.globalSet)

help.go

@@ -2,9 +2,7 @@ package cli
 
 import (
 	"fmt"
-	"os"
-	"text/tabwriter"
-	"text/template"
+	"io"
 )
 
 // The text template for the Default help topic.
@@ -90,7 +88,9 @@ var helpSubcommand = Command{
 }
 
 // Prints help for the App
-var HelpPrinter = printHelp
+type helpPrinter func(templ string, data interface{})
+
+var HelpPrinter helpPrinter = nil
 
 func ShowAppHelp(c *Context) {
 	HelpPrinter(AppHelpTemplate, c.App)
@@ -99,9 +99,9 @@ func ShowAppHelp(c *Context) {
 // Prints the list of subcommands as the default app completion method
 func DefaultAppComplete(c *Context) {
 	for _, command := range c.App.Commands {
-		fmt.Println(command.Name)
+		io.WriteString(c.App.Stdout, fmt.Sprintln(command.Name))
 		if command.ShortName != "" {
-			fmt.Println(command.ShortName)
+			io.WriteString(c.App.Stdout, fmt.Sprintln(command.ShortName))
 		}
 	}
 }
@@ -118,7 +118,7 @@ func ShowCommandHelp(c *Context, command string) {
 	if c.App.CommandNotFound != nil {
 		c.App.CommandNotFound(c, command)
 	} else {
-		fmt.Printf("No help topic for '%v'\n", command)
+		io.WriteString(c.App.Stdout, fmt.Sprintf("No help topic for '%v'\n", command))
 	}
 }
 
@@ -129,7 +129,7 @@ func ShowSubcommandHelp(c *Context) {
 
 // Prints the version number of the App
 func ShowVersion(c *Context) {
-	fmt.Printf("%v version %v\n", c.App.Name, c.App.Version)
+	io.WriteString(c.App.Stdout, fmt.Sprintf("%v version %v\n", c.App.Name, c.App.Version))
 }
 
 // Prints the lists of commands within a given context
@@ -148,16 +148,6 @@ func ShowCommandCompletions(ctx *Context, command string) {
 	}
 }
 
-func printHelp(templ string, data interface{}) {
-	w := tabwriter.NewWriter(os.Stdout, 0, 8, 1, '\t', 0)
-	t := template.Must(template.New("help").Parse(templ))
-	err := t.Execute(w, data)
-	if err != nil {
-		panic(err)
-	}
-	w.Flush()
-}
-
 func checkVersion(c *Context) bool {
 	if c.GlobalBool("version") {
 		ShowVersion(c)