add customizable UnknownCommandErrorFunc

args.go

@@ -19,7 +19,7 @@ func legacyArgs(cmd *Command, args []string) error {
 
 	// root command with subcommands, do subcommand checking.
 	if !cmd.HasParent() && len(args) > 0 {
-		return fmt.Errorf("unknown command %q for %q%s", args[0], cmd.CommandPath(), cmd.findSuggestions(args[0]))
+		return cmd.UnknownCommandErrorFunc()(cmd, args[0])
 	}
 	return nil
 }

command.go

@@ -192,6 +192,8 @@ type Command struct {
 	// flagErrorFunc is func defined by user and it's called when the parsing of
 	// flags returns an error.
 	flagErrorFunc func(*Command, error) error
+	// unknownCommandErrorFunc is func defined by user, called when the typed command cannot be found
+	unknownCommandErrorFunc func(*Command, string) error
 	// helpTemplate is help template defined by user.
 	helpTemplate string
 	// helpFunc is help func defined by user.
@@ -264,6 +266,12 @@ func (c *Command) SetFlagErrorFunc(f func(*Command, error) error) {
 	c.flagErrorFunc = f
 }
 
+// SetUnknownCommandErrorFunc sets a function to generate an error when flag parsing
+// fails.
+func (c *Command) SetUnknownCommandErrorFunc(f func(*Command, string) error) {
+	c.unknownCommandErrorFunc = f
+}
+
 // SetHelpFunc sets help function. Can be defined by Application.
 func (c *Command) SetHelpFunc(f func(*Command, []string)) {
 	c.helpFunc = f
@@ -435,6 +443,22 @@ func (c *Command) FlagErrorFunc() (f func(*Command, error) error) {
 	}
 }
 
+// UnknownCommandErrorFunc returns either the function set by SetUnknownCommandErrorFunc for this
+// command or a parent, or it returns a function which returns the original
+// error.
+func (c *Command) UnknownCommandErrorFunc() (f func(*Command, string) error) {
+	if c.unknownCommandErrorFunc != nil {
+		return c.unknownCommandErrorFunc
+	}
+
+	if c.HasParent() {
+		return c.parent.unknownCommandErrorFunc
+	}
+	return func(c *Command, arg string) error {
+		return fmt.Errorf("unknown command %q for %q%s", arg, c.CommandPath(), c.findSuggestions(arg))
+	}
+}
+
 var minUsagePadding = 25
 
 // UsagePadding return padding for the usage.

command_test.go

@@ -1653,6 +1653,28 @@ func TestFlagErrorFunc(t *testing.T) {
 	}
 }
 
+func TestUnknownCommandErrorFunc(t *testing.T) {
+	rootCmd := &Command{Use: "root", Run: emptyRun}
+	subCmd := &Command{
+		Use: "type",
+		Run: emptyRun,
+	}
+	rootCmd.AddCommand(subCmd)
+
+	expectedFmt := "unknown command: %v"
+	rootCmd.SetUnknownCommandErrorFunc(func(_ *Command, arg string) error {
+		return fmt.Errorf(expectedFmt, arg)
+	})
+
+	_, err := executeCommand(rootCmd, "typo")
+
+	got := err.Error()
+	expected := "unknown command: typo"
+	if got != expected {
+		t.Errorf("Expected %v, got %v", expected, got)
+	}
+}
+
 // TestSortedFlags checks,
 // if cmd.LocalFlags() is unsorted when cmd.Flags().SortFlags set to false.
 // Related to https://github.com/spf13/cobra/issues/404.