android button with half circle edges

You must specify every corner radius like this:

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle">
    <solid android:color="@color/green" />
    <size android:height="30dp" />
    <corners
        android:bottomRightRadius="15dp"
        android:bottomLeftRadius="15dp"
        android:topRightRadius="15dp"
        android:topLeftRadius="15dp"
        />
</shape>

You should not specify the width, so that it can adjust the width as background drawable.