Added ability to do custom stuff when command is not found

command.go

@@ -114,6 +114,8 @@ type Command struct {
 	PersistentPostRun func(cmd *Command, args []string)
 	// PersistentPostRunE: PersistentPostRun but returns an error.
 	PersistentPostRunE func(cmd *Command, args []string) error
+	// OnCommandNotFound: custom CommandNotFound handler
+	OnCommandNotFound func (cmd *Command, args []string)
 
 	// SilenceErrors is an option to quiet errors down stream.
 	SilenceErrors bool
@@ -903,8 +905,13 @@ func (c *Command) ExecuteC() (cmd *Command, err error) {
 			c = cmd
 		}
 		if !c.SilenceErrors {
-			c.Println("Error:", err.Error())
+			if c.OnCommandNotFound != nil {
-			c.Printf("Run '%v --help' for usage.\n", c.CommandPath())
+				c.OnCommandNotFound(c, args)
+				return c, nil
+			} else {
+				c.Println("Error:", err.Error())
+				c.Printf("Run '%v --help' for usage.\n", c.CommandPath())
+			}
 		}
 		return c, err
 	}

command_test.go

@@ -147,6 +147,19 @@ func TestRootUnknownCommandSilenced(t *testing.T) {
 	}
 }
 
+func TestRootUnknownCommandCustomHandler(t *testing.T) {
+	rootCmd := &Command{Use: "root", Run: emptyRun}
+	rootCmd.OnCommandNotFound = func(cmd *Command, args []string) {
+		cmd.Println("Command not found. Sorry buddy :(")
+	}
+	rootCmd.AddCommand(&Command{Use: "child", Run: emptyRun})
+
+	output, _ := executeCommand(rootCmd, "unknown")
+	if output != "Command not found. Sorry buddy :(\n" {
+		t.Errorf("Expected custom output message, because of custom CommandNotFound handler.\nGot:\n %q\n", output)
+	}
+}
+
 func TestCommandAlias(t *testing.T) {
 	var timesCmdArgs []string
 	rootCmd := &Command{Use: "root", Args: NoArgs, Run: emptyRun}