wifi password

Because wpa_supplicant.conf contains readable wifi passwords, it can be replaced by the psk unreadable codes. 

cf: https://raspberrypi.stackexchange.com/questions/91616/wpa-supplicant-conf-in-raspberry-pi-3

1. Run wpa_passphrase "chiao-dlink842"
2. Enter password, e.g. aaa123456, Press enter

1. Some lines come out. e.g.
network={
    ssid="chiao-dlink842"
    #psk="aaa123456"
         key_mgmt=WPA-PSK
    psk=<codes>
}
3. Delete the line with #psk="aaa123456", Then, add the rest lines to  /etc/wpa_supplicant/wpa_supplicant.conf
4. Done.
5. Also, for one (or some) of the AP not connected automatically, one can add this:
disabled=1