public static boolean checkAvailable() {

    // Retrieving the external storage state
    String state = Environment.getExternalStorageState();

    // Check if available
    if (Environment.MEDIA_MOUNTED.equals(state)) {
        return true;
    }
    return false;
}

public static boolean checkWritable() {
    // Retrieving the external storage state
    String state = Environment.getExternalStorageState();

    // Check if writable
    if (Environment.MEDIA_MOUNTED.equals(state){
        if(Environment.MEDIA_MOUNTED_READ_ONLY.equals(state)) {
            return false;
        }else{
            return true;
        }
    }
    return false;
}

<provider
android:name="android.support.v4.content.FileProvider"
android:authorities="${applicationId}.fileprovider"
android:grantUriPermissions="true"
android:exported="false">
<meta-data
android:name="android.support.FILE_PROVIDER_PATHS"
android:resource="@xml/file_path" />
</provider>

<?xml version="1.0" encoding="utf-8"?>
<paths xmlns:android="http://schemas.android.com/apk/res/android">
    <paths>
        <files-path path="/" name="default" />
    </paths>
</paths>

File file = new File(activity.getFilesDir(), "/exported_data.zip");
Uri uri = FileProvider.getUriForFile(context, activity.getPackageName()+".fileprovider", file);
Intent i = new Intent(Intent.ACTION_SEND);
i.setFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION);
i.setType("message/rfc822");
i.putExtra(Intent.EXTRA_SUBJECT, "Gaitspeedometer data");
i.putExtra(Intent.EXTRA_STREAM, uri);

try {
    startActivity(Intent.createChooser(i, "Send file..."));
} catch (Exception e) {
    Toast.makeText(activity, "No activity found for export", Toast.LENGTH_SHORT).show();
}

sudo apt-get install firmware-b43-installer
sudo modprobe -r b43
sudo modprobe b43

sudo apt-get update
sudo apt-get install --reinstall bcmwl-kernel-source

sudo apt-get update
sudo apt-get install gnome-tweak-tool

gnome-tweak-tool

<uses-permission android:name="android.permission.BLUETOOTH_ADMIN" />
<uses-permission android:name="android.permission.BLUETOOTH" />
<uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />
<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />

IntentFilter stateFilter = new IntentFilter();
stateFilter.addAction(BluetoothAdapter.ACTION_STATE_CHANGED); //BluetoothAdapter.ACTION_STATE_CHANGED : 블루투스 상태변화 액션
stateFilter.addAction(BluetoothAdapter.ACTION_CONNECTION_STATE_CHANGED);
stateFilter.addAction(BluetoothDevice.ACTION_ACL_CONNECTED); //연결 확인
stateFilter.addAction(BluetoothDevice.ACTION_ACL_DISCONNECTED); //연결 끊김 확인
stateFilter.addAction(BluetoothDevice.ACTION_BOND_STATE_CHANGED);
stateFilter.addAction(BluetoothDevice.ACTION_FOUND);    //기기 검색됨
stateFilter.addAction(BluetoothAdapter.ACTION_DISCOVERY_STARTED);   //기기 검색 시작
stateFilter.addAction(BluetoothAdapter.ACTION_DISCOVERY_FINISHED);  //기기 검색 종료
stateFilter.addAction(BluetoothDevice.ACTION_PAIRING_REQUEST);
registerReceiver(mBluetoothStateReceiver, stateFilter);

unregisterReceiver(bluetoothRequest.mBluetoothStateReceiver);

BroadcastReceiver mBluetoothStateReceiver = new BroadcastReceiver() {
    @Override
    public void onReceive(Context context, Intent intent) {
        final String action = intent.getAction();   //입력된 action
        Toast.makeText(context, "받은 액션 : "+action , Toast.LENGTH_SHORT).show();
        Log.d("Bluetooth action", action);
        final BluetoothDevice device =   intent.getParcelableExtra(BluetoothDevice.EXTRA_DEVICE);
        String name = null;
        if (device != null) {
            name = device.getName();    //broadcast를 보낸 기기의 이름을 가져온다.
        }
        //입력된 action에 따라서 함수를 처리한다
        switch (action){
            case BluetoothAdapter.ACTION_STATE_CHANGED: //블루투스의 연결 상태 변경
                final int state = intent.getIntExtra(BluetoothAdapter.EXTRA_STATE, BluetoothAdapter.ERROR);
                switch(state) {
                    case BluetoothAdapter.STATE_OFF:

                        break;
                    case BluetoothAdapter.STATE_TURNING_OFF:

                        break;
                    case BluetoothAdapter.STATE_ON:

                        break;
                    case BluetoothAdapter.STATE_TURNING_ON:

                        break;
                }

                break;
            case BluetoothDevice.ACTION_ACL_CONNECTED:  //블루투스 기기 연결

                break;
            case BluetoothDevice.ACTION_BOND_STATE_CHANGED:

                break;
            case BluetoothDevice.ACTION_ACL_DISCONNECTED:   //블루투스 기기 끊어짐
                
                break;
                
            case BluetoothAdapter.ACTION_DISCOVERY_STARTED: //블루투스 기기 검색 시작
                
                break;
            case BluetoothDevice.ACTION_FOUND:  //블루투스 기기 검색 됨, 블루투스 기기가 근처에서 검색될 때마다 수행됨
                String device_name = device.getName();
                String device_Address = device.getAddress();
                //본 함수는 블루투스 기기 이름의 앞글자가 "GSM"으로 시작하는 기기만을 검색하는 코드이다
                if(device_name != null && device_name.length() > 4){
                    Log.d("Bluetooth Name: ", device_name);
                    Log.d("Bluetooth Mac Address: ", device_Address);
                    if(device_name.substring(0,3).equals("GSM")){
                        bluetooth_device.add(device);
                    }
                }
                break;
            case BluetoothDevice.ACTION_DISCOVERY_FINISHED:    //블루투스 기기 검색 종료
                Log.d("Bluetooth", "Call Discovery finished");
                StartBluetoothDeviceConnection();   //원하는 기기에 연결
                break;
            case BluetoothDevice.ACTION_PAIRING_REQUEST:

                break;
            }

        }
    };

private static final int REQUEST_ENABLE_BT = 3;
public BluetoothAdapter mBluetoothAdapter = null;
Set<BluetoothDevice> mDevices;
int mPairedDeviceCount;
BluetoothDevice mRemoteDevice;
BluetoothSocket mSocket;
InputStream mInputStream;
OutputStream mOutputStream;
Thread mWorkerThread;
int readBufferPositon;      //버퍼 내 수신 문자 저장 위치
byte[] readBuffer;      //수신 버퍼
byte mDelimiter = 10;

mBluetoothAdapter = BluetoothAdapter.getDefaultAdapter();   //블루투스 adapter 획득
mBluetoothAdapter.startDiscovery(); //블루투스 기기 검색 시작

public void StartBluetoothDeviceConnection(){

//Bluetooth connection start

    if(bluetooth_device.size() == 0){
        Toast.makeText(activity,"There is no device", Toast.LENGTH_SHORT).show();
    }
   
    AlertDialog.Builder builder = new AlertDialog.Builder(activity);
    builder.setTitle(R.string.select_device_title);
 
    // 페어링 된 블루투스 장치의 이름 목록 작성
    List<String> listItems = new ArrayList<String>();
    for (BluetoothDevice bt_device : bluetooth_device) {
    listItems.add(bt_device.getName());
    }
    listItems.add("Cancel");    // 취소 항목 추가

    final CharSequence[] items = listItems.toArray(new CharSequence[listItems.size()]);

    builder.setItems(items, new DialogInterface.OnClickListener() {
    @Override
    public void onClick(DialogInterface dialog, int which) {
        Dialog dialog_ = (Dialog) dialog;
        
        if (which == bluetooth_device.size()) {
            // 연결할 장치를 선택하지 않고 '취소'를 누른 경우

        } else {
            // 기기 이름을 선택한 경우 선택한 기기 이름과 같은 블루투스 객체를 찾아서 연결을 시도한다
            for (BluetoothDevice bt_device : bluetooth_device) {
                if (bt_device.getName().equals(items[which].toString())) {
                    Log.d("Bluetooth Connect", bt_device.getName());
                    ConnectBluetoothDevice(bt_device);  //해당하는 블루투스 객체를 이용하여 연결 시도
                    Log.d("Bluetooth Connect", "ConnectBluetoothDevice");
                    break;
                }
            }
        }

        }
    });
    builder.setCancelable(false);    // 뒤로 가기 버튼 사용 금지
    AlertDialog alert = builder.create();
    alert.show();
    Log.d("Bluetooth Connect", "alert start");
}

public void ConnectBluetoothDevice(final BluetoothDevice device){

    mDevices = mBluetoothAdapter.getBondedDevices();
    mPairedDeviceCount = mDevices.size();

    //pairing되어 있는 기기의 목록을 가져와서 연결하고자 하는 기기가 이전 기기 목록에 있는지 확인
    boolean already_bonded_flag = false;
    if(mPairedDeviceCount > 0){
        for (BluetoothDevice bonded_device : mDevices) {
            if(activity.bluetooth_device_name.equals(bonded_device.getName())){
                already_bonded_flag = true;
            }
        }
    }
    //pairing process
    //만약 pairing기록이 있으면 바로 연결을 수행하며, 없으면 createBond()함수를 통해서 pairing을 수행한다.
    if(!already_bonded_flag){
        try {
            //pairing수행
            device.createBond();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }else{
        connectToSelectedDevice(device);
    }
}