only read from stdin when no other files/folders given

acceptance.bats

@@ -170,7 +170,7 @@ resetCacheFolder() {
 }
 
 @test "Pass when parsing a valid Kubernetes config YAML file explicitly on stdin" {
-  run bash -c "cat fixtures/valid.yaml | bin/kubeconform -summary"
+  run bash -c "cat fixtures/valid.yaml | bin/kubeconform -summary -"
   [ "$status" -eq 0 ]
   [ "$output" = "Summary: 1 resource found parsing stdin - Valid: 1, Invalid: 0, Errors: 0, Skipped: 0" ]
 }
@@ -180,6 +180,16 @@ resetCacheFolder() {
   [ "$status" -eq 1 ]
 }
 
+@test "Fail when not passing data to stdin, when implicitly configured to read from stdin" {
+  run bash -c "bin/kubeconform -summary"
+  [ "$status" -eq 1 ]
+}
+
+@test "Fail when not passing data to stdin, when explicitly configured to read from stdin" {
+  run bash -c "bin/kubeconform -summary -"
+  [ "$status" -eq 1 ]
+}
+
 @test "Skip when parsing a resource from a kind to skip" {
   run bin/kubeconform -verbose -skip ReplicationController fixtures/valid.yaml
   [ "$status" -eq 0 ]

cmd/kubeconform/main.go

@@ -68,15 +68,17 @@ func realMain() int {
 		defer pprof.StopCPUProfile()
 	}
 
-	// Detect whether we have data being piped through stdin
-	stat, _ := os.Stdin.Stat()
-	isStdin := (stat.Mode() & os.ModeCharDevice) == 0
-	if len(cfg.Files) == 1 && cfg.Files[0] == "-" {
-		isStdin = true
+	useStdin := false
+	if len(cfg.Files) == 0 || (len(cfg.Files) == 1 && cfg.Files[0] == "-") {
+		stat, _ := os.Stdin.Stat()
+		if (stat.Mode() & os.ModeCharDevice) != 0 {
+			log.Fatalf("failing to read data from stdin")
+		}
+		useStdin = true
 	}
 
 	var o output.Output
-	if o, err = output.New(cfg.OutputFormat, cfg.Summary, isStdin, cfg.Verbose); err != nil {
+	if o, err = output.New(cfg.OutputFormat, cfg.Summary, useStdin, cfg.Verbose); err != nil {
 		fmt.Fprintln(os.Stderr, err)
 		return 1
 	}
@@ -101,7 +103,7 @@ func realMain() int {
 
 	var resourcesChan <-chan resource.Resource
 	var errors <-chan error
-	if isStdin {
+	if useStdin {
 		resourcesChan, errors = resource.FromStream(ctx, "stdin", os.Stdin)
 	} else {
 		resourcesChan, errors = resource.FromFiles(ctx, cfg.Files, cfg.IgnoreFilenamePatterns)