How to determine the WiFi channel from Mac command line?

There is an airport info utility that comes with Apple's 802.11 framework. It is a bit hidden though. If you want to call it without specifying the path every time, enter the following into the Terminal to link this utility somewhere you can call it from the PATH:

sudo ln -s /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport /usr/bin/airport

Then you call it from anywhere. Just enter airport for a list of options.

airport -I

This would output:

charon:Resources werner$ airport -I
     agrCtlRSSI: -56
     agrExtRSSI: 0
    agrCtlNoise: -86
    agrExtNoise: 0
          state: running
        op mode: station 
     lastTxRate: 78
        maxRate: 144
lastAssocStatus: 0
    802.11 auth: open
      link auth: wpa2-psk
          BSSID: 0:23:69:14:ad:5c
           SSID: Supersaurus
            MCS: 12
        channel: 1

To get the channel only, pipe into grep:

 airport -I | grep channel