Here is another drawable xml for those who want to add all together gradient background, corner radius and ripple effect:
<?xml version="1.0" encoding="utf-8"?>
<ripple xmlns:android="http://schemas.android.com/apk/res/android"
android:color="@color/colorPrimaryDark">
<item android:id="@android:id/mask">
<shape android:shape="rectangle">
<solid android:color="@color/colorPrimaryDark" />
<corners android:radius="@dimen/button_radius_large" />
</shape>
</item>
<item android:id="@android:id/background">
<shape android:shape="rectangle">
<gradient
android:angle="90"
android:endColor="@color/colorPrimaryLight"
android:startColor="@color/colorPrimary"
android:type="linear" />
<corners android:radius="@dimen/button_radius_large" />
</shape>
</item>
</ripple>
Add this to the background of your button.
<Button
...
android:background="@drawable/button_background" />
PS: this answer works for android api 21 and above.