AddWifi.sh - joins a wifi network at the login window.

AddWifi.sh

#!/bin/sh

# Set paths to our utilities
networksetup=/usr/sbin/networksetup
airport=/System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport

# Determines which OS the script is running on
osvers=$(sw_vers -productVersion | awk -F. '{print $2}')

# On 10.7 and higher, the Wi-Fi interface needs to be identified.
# On 10.5 and 10.6, the Wi-Fi interface should be named as "AirPort"

if [[ ${osvers} -ge 7 ]]; then
    wifiDevice=`/usr/sbin/networksetup -listallhardwareports | awk '/^Hardware Port: Wi-Fi/,/^Ethernet Address/' | head -2 | tail -1 | cut -c 9-`
  /usr/sbin/networksetup -setnetworkserviceenabled Wi-Fi on
else
    wifiDevice=`/usr/sbin/networksetup -listallhardwareports | awk '/^Hardware Port: AirPort/,/^Ethernet Address/' | head -2 | tail -1 | cut -c 9-`
	/usr/sbin/networksetup -setnetworkserviceenabled AirPort on
fi

# Set the SSID variable to your wireless network name
# to set the network name you want to connect to.
# Note: Wireless network name cannot contain spaces.
SSID=ssid

# Set the INDEX variable to the index number you'd like
# it to be assigned to (leave it as "0" if you do not know
# what index number to use.)
INDEX=0

# Set the SECURITY variable to the security type of the
# wireless network (NONE, WEP, WPA, WPA2, WPAE or
# WPA2E) Setting it to NONE means that it's an open
# network with no encryption.
SECURITY=WPA2

# Set the password here. For example, if you are using WPA
# encryption with a password of "thedrisin", set the PASSWORD
# variable to "thedrisin" (no quotes.)
PASSWORD=password
  
# Once the running OS is determined, the settings for the specified
# wireless network are created and set as the first preferred network listed

if [[ $wifiDevice == "" ]]; then
	echo "No Wi-Fi device found!"
	exit 0;
fi
 
# Run our tests first
airportScan=`/System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport -s`
preferredList=`/usr/sbin/networksetup -listpreferredwirelessnetworks $wifiDevice`

# First: turn on wifi
/usr/sbin/networksetup -setairportpower $wifiDevice on


# Does the wifi even exist?
if [[ -n "$airportScan | grep $SSID" ]]; then
	# Yes, the wifi exists.  Is it already on the preferred list?
	if [[ -z `echo $preferredList | grep $SSID` ]]; then
		# No, it isn't in the preferred list, so we set the connection right now.
		/usr/sbin/networksetup -setairportnetwork $wifiDevice $SSID $PASSWORD
		echo "Setting network now."
		exit 0;
 	fi
	# If it gets here, it is on the preferred list and the wifi exists, so it should join automatically (or have another SSID to join).
fi 	

# If it gets here, the wifi doesn't exist, so we shouldn't try to join it right now.  Add it to the preferred list.
echo "Setting network later."
/usr/sbin/networksetup -addpreferredwirelessnetworkatindex $wifiDevice $SSID $INDEX $SECURITY

This is exactly what I have been trying to do ... thanks for posting! I am going to try it this weekend.