diff --git a/mdtest/lib/src/commands/auto.dart b/mdtest/lib/src/commands/auto.dart
index 88323fe..088d8c2 100644
--- a/mdtest/lib/src/commands/auto.dart
+++ b/mdtest/lib/src/commands/auto.dart
@@ -33,6 +33,10 @@
     printInfo('Running "mdtest auto command" ...');
 
     this._specs = await loadSpecs(argResults);
+    if (sanityCheckSpecs(_specs, argResults['spec']) != 0) {
+      printError('Test spec does not meet requirements.');
+      return 1;
+    }
 
     this._devices = await getDevices();
     if (_devices.isEmpty) {
diff --git a/mdtest/lib/src/commands/run.dart b/mdtest/lib/src/commands/run.dart
index 07d332e..e6e9890 100644
--- a/mdtest/lib/src/commands/run.dart
+++ b/mdtest/lib/src/commands/run.dart
@@ -31,6 +31,10 @@
 
     this._specs = await loadSpecs(argResults);
     printTrace(_specs.toString());
+    if (sanityCheckSpecs(_specs, argResults['specs']) != 0) {
+      printError('Test spec does not meet requirements.');
+      return 1;
+    }
 
     this._devices = await getDevices();
     if (_devices.isEmpty) {
diff --git a/mdtest/lib/src/mobile/device.dart b/mdtest/lib/src/mobile/device.dart
index 574114f..6629eec 100644
--- a/mdtest/lib/src/mobile/device.dart
+++ b/mdtest/lib/src/mobile/device.dart
@@ -47,9 +47,9 @@
                              .transform(new Utf8Decoder())
                              .transform(new LineSplitter());
   bool startReading = false;
-  RegExp startPattern = new RegExp(r'List of devices attached');
-  RegExp deviceIDPattern = new RegExp(r'\s+(\w+)\s+.*');
-  RegExp stopPattern = new RegExp(r'\d+ connected devices?|No devices detected');
+  RegExp startPattern = new RegExp(r'\d+ connected device|No devices detected');
+  RegExp deviceIDPattern = new RegExp(r'\d+ ms •.*•\s+(\S+)\s+•.*');
+  RegExp stopPattern = new RegExp(r"'flutter devices' took \d+ms; exiting with code");
   await for (var line in lineStream) {
     if (!startReading && startPattern.hasMatch(line.toString())) {
       startReading = true;
diff --git a/mdtest/lib/src/mobile/device_spec.dart b/mdtest/lib/src/mobile/device_spec.dart
index 8522159..2f7e924 100644
--- a/mdtest/lib/src/mobile/device_spec.dart
+++ b/mdtest/lib/src/mobile/device_spec.dart
@@ -75,16 +75,16 @@
     // from the command line argument
     List<String> testPathsFromSpec
       = listFilePathsFromGlobPatterns(rootPath, newSpecs['test-paths']);
-    print('Test paths from spec: $testPathsFromSpec');
+    printTrace('Test paths from spec: $testPathsFromSpec');
     List<String> testPathsFromCommandLine
       = listFilePathsFromGlobPatterns(Directory.current.path, argResults.rest);
-    print('Test paths from command line: $testPathsFromCommandLine');
+    printTrace('Test paths from command line: $testPathsFromCommandLine');
     newSpecs['test-paths'] = mergeWithoutDuplicate(
       testPathsFromSpec,
       testPathsFromCommandLine
     );
     // Normalize the 'app-path' in the specs file
-    newSpecs['devices'].forEach((String name, Map<String, String> map) {
+    newSpecs['devices']?.forEach((String name, Map<String, String> map) {
       map['app-path'] = normalizePath(rootPath, map['app-path']);
       map['app-root'] = normalizePath(rootPath, map['app-root']);
     });
@@ -101,6 +101,66 @@
   }
 }
 
+/// Check if test spec meets the requirements.  If user does not specify any
+/// valid test paths neither from the test spec nor from the command line,
+/// report error.  If 'devices' property is not specified, report error.  If
+/// no device spec is specified, report error.  If screen size property is not
+/// one of 'small', 'normal', 'large' and 'xlarge', report error.  If app-root
+/// is not specified or is not a directory, report error.  If appPath is not
+/// specified or is not a file, report error.
+///
+/// Note: If a test path does not exist, it will be ignored and thus does not
+/// count as a valid test path.  If device nickname is not unique, json decoder
+/// will overwrite the previous device spec associated with the same nickname,
+/// thus only the last nickname to device spec pair is used.
+int sanityCheckSpecs(dynamic spec, String specsPath) {
+  if (spec['test-paths'].isEmpty) {
+    printError(
+      'No test paths found.  '
+      'You must specify at least one test path.'
+    );
+    return 1;
+  }
+  dynamic deviceSpecs = spec['devices'];
+  if (deviceSpecs == null) {
+    printError('"devices" property is not specified in $specsPath');
+    return 1;
+  }
+  if (deviceSpecs.isEmpty) {
+    printError('No device spec is found in $specsPath');
+    return 1;
+  }
+  for (String nickname in deviceSpecs.keys) {
+    dynamic individualDeviceSpec = deviceSpecs[nickname];
+    List<String> screenSizes = <String>['small', 'normal', 'large', 'xlarge'];
+    if (individualDeviceSpec['screen-size'] != null
+        &&
+        !screenSizes.contains(individualDeviceSpec['screen-size'])) {
+      printError('Screen size must be one of $screenSizes');
+      return 1;
+    }
+    String appRootPath = individualDeviceSpec['app-root'];
+    if (appRootPath == null) {
+      printError('Application root path is not specified.');
+      return 1;
+    }
+    if (!FileSystemEntity.isDirectorySync(appRootPath)) {
+      printError('Application root path is not a directory.');
+      return 1;
+    }
+    String appPath = individualDeviceSpec['app-path'];
+    if (appPath == null) {
+      printError('Application path is not specified.');
+      return 1;
+    }
+    if (!FileSystemEntity.isFileSync(appPath)) {
+      printError('Application path is not a file.');
+      return 1;
+    }
+  }
+  return 0;
+}
+
 /// Build a list of device specs from mappings loaded from JSON .spec file
 Future<List<DeviceSpec>> constructAllDeviceSpecs(dynamic allSpecs) async {
   List<DeviceSpec> deviceSpecs = <DeviceSpec>[];
diff --git a/mdtest/lib/src/util.dart b/mdtest/lib/src/util.dart
index 2840353..f9f6402 100644
--- a/mdtest/lib/src/util.dart
+++ b/mdtest/lib/src/util.dart
@@ -39,6 +39,9 @@
 }
 
 String normalizePath(String rootPath, String relativePath) {
+  if (rootPath == null || relativePath == null) {
+    return null;
+  }
   return path.normalize(
     path.join(rootPath, relativePath)
   );