Added command test.

- cmd/micromdm now has a simple test.
- changed flagset defaults to support testing and remove default side
effects. Go flags write to os.Stderr and call os.Exit(2) when parsing
usage.

cmd/micromdm/micromdm.go

@@ -33,11 +33,15 @@ func micromdm(args []string, stdin io.Reader, stdout, stderr io.Writer) int {
 		ctx       = context.Background()
 		logger    = log.New()
 		ffOptions = []ff.Option{ff.WithConfigFileParser(ff.PlainParser), ff.WithConfigFileFlag("config")}
-		rootfs    = flag.NewFlagSet("micromdm", flag.ExitOnError)
+		rootfs    = flag.NewFlagSet("micromdm", flag.ContinueOnError)
 		pidfile   = rootfs.String("pidfile", "/tmp/micromdm.pid", "Path to server pidfile")
 		_         = rootfs.String("config", "", "Path to config file (optional)")
 	)
 
+	// default output is os.Stderr.
+	// setting the output and flag.ContinueOnError overrides allows testing usage.
+	rootfs.SetOutput(stderr)
+
 	version := &ffcli.Command{
 		Name:       "version",
 		ShortUsage: "version [<arg> ...]",
@@ -48,11 +52,21 @@ func micromdm(args []string, stdin io.Reader, stdout, stderr io.Writer) int {
 		},
 	}
 
+	// add a help subcommand to make usage more discoverable.
+	helpCmd := &ffcli.Command{
+		Name:      "help",
+		UsageFunc: func(c *ffcli.Command) string { return "" },
+		Exec: func(_ context.Context, args []string) error {
+			rootfs.Usage()
+			return flag.ErrHelp
+		},
+	}
+
 	root := &ffcli.Command{
 		ShortUsage:  "micromdm [flags] <subcommand>",
 		FlagSet:     rootfs,
 		Options:     ffOptions,
-		Subcommands: []*ffcli.Command{version},
+		Subcommands: []*ffcli.Command{helpCmd, version},
 		Exec: func(context.Context, []string) error {
 			if err := writePID(*pidfile); err != nil {
 				return err
@@ -107,8 +121,10 @@ func micromdm(args []string, stdin io.Reader, stdout, stderr io.Writer) int {
 	}
 
 	switch err := root.ParseAndRun(ctx, args[1:]); {
-	case err == nil, errors.Is(err, flag.ErrHelp):
+	case err == nil:
 		return 0
+	case errors.Is(err, flag.ErrHelp):
+		return 2
 	default:
 		fmt.Fprintln(stderr) // when Ctrl+C is used, avoid messing up the logger line
 		log.Info(logger).Log("exit", err)

cmd/micromdm/micromdm_test.go

@@ -0,0 +1,72 @@
+package main
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+)
+
+type mainIOFunc func(t *testing.T, stdin, stdout, stderr *bytes.Buffer)
+
+func mainUsage(t *testing.T, stdin, stdout, stderr *bytes.Buffer) {
+	output := stderr.String()
+	words := []string{"USAGE", "pidfile"}
+	for _, word := range words {
+		if !strings.Contains(output, word) {
+			t.Errorf("expected %q in output, got:\n%s", word, output)
+		}
+	}
+}
+
+func TestMain(t *testing.T) {
+	tests := []struct {
+		name   string
+		stdin  *bytes.Buffer
+		stdout *bytes.Buffer
+		stderr *bytes.Buffer
+		args   []string
+		exit   int
+		check  mainIOFunc
+	}{
+		{
+			name:   "short usage",
+			stdin:  new(bytes.Buffer),
+			stdout: new(bytes.Buffer),
+			stderr: new(bytes.Buffer),
+			args:   []string{"micromdm", "-h"},
+			exit:   2,
+			check:  mainUsage,
+		},
+		{
+			name:   "long usage",
+			stdin:  new(bytes.Buffer),
+			stdout: new(bytes.Buffer),
+			stderr: new(bytes.Buffer),
+			args:   []string{"micromdm", "-help"},
+			exit:   2,
+			check:  mainUsage,
+		},
+		{
+			name:   "subcommand help usage",
+			stdin:  new(bytes.Buffer),
+			stdout: new(bytes.Buffer),
+			stderr: new(bytes.Buffer),
+			args:   []string{"micromdm", "help"},
+			exit:   2,
+			check:  mainUsage,
+		},
+	}
+
+	for _, tt := range tests {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			if got, want := micromdm(tt.args, tt.stdin, tt.stdout, tt.stderr), tt.exit; got != want {
+				t.Fatalf("exit code: got %d, want %d", got, want)
+			}
+
+			tt.check(t, tt.stdin, tt.stdout, tt.stderr)
+		})
+	}
+}