- Update README

- Fix issues / crashes with sub-commands handling

.vscode/launch.json

@@ -37,7 +37,7 @@
             "mode": "auto",
             "program": "${workspaceFolder}/sample/subcmd-demo",
             "args": [
-                "migrate",
+                // "migrate",
             ]
         }
     ]

README.md

@@ -229,6 +229,56 @@ completion machinery can be disabled by setting `cmd.DisableCompletion` on the
 root command.
 
 
+## Sub-command support
+
+The `go-cli` package also provides support for sub-commands definition, to build
+a command-line interface similar to `git`, `go` and `kubectl`,  where a single
+binary exposes a number of related actions.
+
+Sub-commands are defined by adding multiple named commands to the `SubCommands`
+field of the main command definition:
+
+```
+func main() {
+    cli.Run(&cli.Command{
+        SubCommands: []cli.Command{
+            {
+                Name:        "migrate",
+                Description: "run DB migration",
+                Handler:     &MigrateCmd{},
+            },
+            ....
+        },
+    })
+}
+```
+
+### No prefix argument
+
+When defining sub-commands with `go-cli`, the sequence of sub-commands verbs and
+noun are expected to appear at the beginning of the command-line, before any
+option or argument. This is in contrast to e.g. `git` which allows a subset of
+global arguments to appear before the sub-command.
+
+With `go-cli`, each sub-command has its own set of command-line options and
+arguments, defined in the command struct. Options that are shared between some
+or all sub-commands can be defined on a shared nested struct.
+
+
+### Bare command with sub-commands
+
+In general, command-line utilities either have no sub-commands or only
+sub-commands. With `gp-cli`, you can have both a default behavior and a list of
+sub-command to provide additional related functionalities.
+
+
+### Help
+
+In general, utilities with sub-commands provide help through an `help`
+sub-command that precede the specific command the inquiry is about. With
+`go-cli`, help is by default requested with the `--help` option.
+
+
 ## Enum support
 
 To help with command-line handling of enumerated types, the `go-cli` package

cli.go

@@ -49,7 +49,7 @@ func Run(cmd *Command) {
 
 	} else if errors.Is(err, cli.ErrHelpRequested) {
 		fmt.Print(cmd.Usage())
-
+		os.Exit(1)
 	} else if errors.Is(err, cli.ErrVersionRequested) {
 		var version = cmd.Version()
 		if version != "" {

pkg/cli/cmd.go

@@ -123,6 +123,9 @@ func (cmd *Command) Run() error {
 		return err
 	}
 
+	if cmd.Handler == nil {
+		return ErrHelpRequested
+	}
 	if err := cmd.Handler.Run(); err != nil {
 		return err
 	}

pkg/option/optionset.go

@@ -23,6 +23,9 @@ type Set struct {
 // that can be set through commandline arguments
 func NewOptionSet(v interface{}) (*Set, error) {
 	var pv = reflect.ValueOf(v)
+	if !pv.IsValid() {
+		return &Set{}, nil
+	}
 	if pv.Kind() != reflect.Ptr || pv.IsNil() || pv.Elem().Kind() != reflect.Struct {
 		return nil, fmt.Errorf("invalid argument of type '%v', non-null pointer to struct expected", pv.Type())
 	}

sample/subcmd-demo/main.go

@@ -9,7 +9,7 @@ import (
 
 func main() {
 	cli.Run(&cli.Command{
-		Handler:     &ServerCmd{},
+		// Handler:     &ServerCmd{},
 		Description: "API server for demo service",
 
 		SubCommands: []cli.Command{