How to Connect to a WPA2-PSK Wireless Network on Ubuntu

Network Manager (nm) may not always be there, this how you connect to a WPA2-PSK wireless network from the command line using wpa_supplicant on Ubnutu:

/etc/wireless-wpa.conf

ctrl_interface=/var/run/wpa_supplicant
network={
ssid="ssid"
scan_ssid=1
key_mgmt=WPA-PSK
psk="key"
}

config_wireless.sh

#!/bin/sh
iface=wlan0
ifdown $iface
ifup $iface
wpa_supplicant -B -Dwext -i $iface -c /etc/wireless-wpa.conf -dd
dhclient $iface