Configure the WiFi interface on Raspberry Pi 2/3.

config_wifi.sh

#!/bin/bash

if [ "$(id -u)" != "0" ]; then
  exec sudo "$0" "$@"
fi

ssid="" # Enter SSID of WiFi
passwd="" # Enter Password of Given WiFi SSID
interface="wlan0"
wpa_conf="/etc/wpa_supplicant/wpa_supplicant.conf"

systemctl stop network-manager && systemctl disable network-manager
ifconfig "$interface" up
wpa_passphrase "$ssid" "$passwd" > "$wpa_conf"

x="/etc/network/interfaces"

if ! [ -f  "$x".bak ]; then
	cp "$x" "$x".bak
fi

cat > "$x" <<EOF
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet dhcp

allow-hotplug "$interface"
iface "$interface" inet dhcp
wpa-conf "$wpa_conf"
EOF

wpa_supplicant -D wext -i "$interface" -c "$wpa_conf" &
dhclient -v "$interface"