code: func isCmdLabelSpec(spec string) (services.CommandLabel, error) {
  	// command spec? (surrounded by brackets?)
  	if len(spec) > 5 && spec[0] == '[' && spec[len(spec)-1] == ']' {
  		invalidSpecError := trace.BadParameter(
  			"invalid command label spec: '%s'", spec)
  		spec = strings.Trim(spec, "[]")
  		idx := strings.IndexRune(spec, ':')
  		if idx < 0 {
  			return nil, trace.Wrap(invalidSpecError)
  		}
  		periodSpec := spec[:idx]
  		period, err := time.ParseDuration(periodSpec)
  		if err != nil {
  			return nil, trace.Wrap(invalidSpecError)
  		}
  		cmdSpec := spec[idx+1:]
  		if len(cmdSpec) < 1 {
  			return nil, trace.Wrap(invalidSpecError)
  		}
  		var openQuote bool = false
  		return &services.CommandLabelV2{
  			Period: services.NewDuration(period),
  			Command: strings.FieldsFunc(cmdSpec, func(c rune) bool {
  				if c == '"' {
  					openQuote = !openQuote
  				}
  				return unicode.IsSpace(c) && !openQuote
  			}),
  		}, nil
  	}
  	// not a valid spec
  	return nil, nil
  }
docs: // isCmdLabelSpec tries to interpret a given string as a "command label" spec.
  // A command label spec looks like [time_duration:command param1 param2 ...] where
  // time_duration is in "1h2m1s" form.
  //
  // Example of a valid spec: "[1h:/bin/uname -m]"