inherit Reader/Writer/ErrWriter from parent on subcommand setup

command_setup.go

@@ -46,18 +46,33 @@ func (cmd *Command) setupDefaults(osArgs []string) {
 	}
 
 	if cmd.Reader == nil {
-		tracef("setting default Reader as os.Stdin (cmd=%[1]q)", cmd.Name)
-		cmd.Reader = os.Stdin
+		if cmd.parent != nil && cmd.parent.Reader != nil {
+			tracef("inheriting Reader from parent (cmd=%[1]q)", cmd.Name)
+			cmd.Reader = cmd.parent.Reader
+		} else {
+			tracef("setting default Reader as os.Stdin (cmd=%[1]q)", cmd.Name)
+			cmd.Reader = os.Stdin
+		}
 	}
 
 	if cmd.Writer == nil {
-		tracef("setting default Writer as os.Stdout (cmd=%[1]q)", cmd.Name)
-		cmd.Writer = os.Stdout
+		if cmd.parent != nil && cmd.parent.Writer != nil {
+			tracef("inheriting Writer from parent (cmd=%[1]q)", cmd.Name)
+			cmd.Writer = cmd.parent.Writer
+		} else {
+			tracef("setting default Writer as os.Stdout (cmd=%[1]q)", cmd.Name)
+			cmd.Writer = os.Stdout
+		}
 	}
 
 	if cmd.ErrWriter == nil {
-		tracef("setting default ErrWriter as os.Stderr (cmd=%[1]q)", cmd.Name)
-		cmd.ErrWriter = os.Stderr
+		if cmd.parent != nil && cmd.parent.ErrWriter != nil {
+			tracef("inheriting ErrWriter from parent (cmd=%[1]q)", cmd.Name)
+			cmd.ErrWriter = cmd.parent.ErrWriter
+		} else {
+			tracef("setting default ErrWriter as os.Stderr (cmd=%[1]q)", cmd.Name)
+			cmd.ErrWriter = os.Stderr
+		}
 	}
 
 	if cmd.AllowExtFlags {

command_test.go

@@ -2833,6 +2833,32 @@ func TestSetupInitializesOnlyNilWriters(t *testing.T) {
 	assert.Equal(t, cmd.Writer, os.Stdout, "expected a.Writer to be os.Stdout")
 }
 
+// Regression for #2325. A Writer set on the root command should reach
+// the subcommand's Action via c.Writer, not get silently replaced by
+// os.Stdout the first time the subcommand runs setupDefaults.
+func TestSubcommandInheritsRootWriters(t *testing.T) {
+	var out, errOut bytes.Buffer
+	root := &Command{
+		Name:      "demo",
+		Writer:    &out,
+		ErrWriter: &errOut,
+		Commands: []*Command{
+			{
+				Name: "sub",
+				Action: func(_ context.Context, c *Command) error {
+					_, _ = fmt.Fprintln(c.Writer, "from sub")
+					_, _ = fmt.Fprintln(c.ErrWriter, "errors from sub")
+					return nil
+				},
+			},
+		},
+	}
+
+	assert.NoError(t, root.Run(buildTestContext(t), []string{"demo", "sub"}))
+	assert.Equal(t, "from sub\n", out.String())
+	assert.Equal(t, "errors from sub\n", errOut.String())
+}
+
 func TestFlagAction(t *testing.T) {
 	now := time.Now().UTC().Truncate(time.Minute)
 	testCases := []struct {