Guide to the output of the builtin airport command line utility
There’s not much documentation available, but these are the basics:


agrCtlRSSI - Signal strength. The higher, the better.
agrCtlNoise - Noise level.  The lower the better.
state - operating state (running or not)
op mode - station or repeater
lastTxRate - last rate of transmission speed 
maxRate 867 - maximum transmission rate
802.11 auth - authentication type (open, shared, EAP, etc.)
link auth: - encryption (WEP, WPA, WPA2, etc)
BSSID - Basic Service Set Identifier (usually the MAC address of the WAP)
SSID - Service Set Identifier (name of your WiFi network)



  I am trying to figure out if there is any way to programmatically get the "Link Speed"


The item that you’d be looking for is “lastTxRate”.  So, to get that, you can issue the command

$ airport -I | grep lastTxRate