cmd: switch to --log and --verbose

Add custom logging levels, defaulting to the logging level of warning.
This makes the output much less spammy.

Signed-off-by: Aleksa Sarai <asarai@suse.de>

cmd/umoci/main.go

@@ -44,9 +44,6 @@ const (
 )
 
 func main() {
-	// TODO: Add some form of --verbose flag.
-	log.SetHandler(logcli.New(os.Stderr))
-
 	app := cli.NewApp()
 	app.Name = "umoci"
 	app.Usage = usage
@@ -69,14 +66,34 @@ func main() {
 
 	app.Flags = []cli.Flag{
 		cli.BoolFlag{
-			Name:  "debug",
-			Usage: "set log level to debug",
+			Name:  "verbose",
+			Usage: "alias for --log=info",
+		},
+		cli.StringFlag{
+			Name:  "log",
+			Usage: "set the log level (debug, info, [warn], error, fatal)",
+			Value: "warn",
 		},
 	}
 
 	app.Before = func(ctx *cli.Context) error {
-		if ctx.GlobalBool("debug") {
-			log.SetLevel(log.DebugLevel)
+		log.SetHandler(logcli.New(os.Stderr))
+
+		if ctx.GlobalBool("verbose") {
+			if ctx.GlobalIsSet("log") {
+				return errors.New("--log=* and --verbose are mutually exclusive")
+			}
+			ctx.GlobalSet("log", "info")
+		}
+
+		level, err := log.ParseLevel(ctx.GlobalString("log"))
+		if err != nil {
+			return errors.Wrap(err, "parsing log level")
+		}
+
+		log.SetLevel(level)
+
+		if level == log.DebugLevel {
 			errors.Debug(true)
 		}
 		return nil