Add check for presence of gawk and update README to describe gawk as …

…a dependency. Close brannondorsey#3.

README.md

@@ -27,7 +27,7 @@ IFACE=wlan0 ./sniff-probes.sh
 00:01:13 -87dBm 38:63:bb:d1:6a:b7 "offline"
 00:01:25 -83dBm 5c:aa:fd:20:23:41 "Sonos_pZkIex0zatRvhdJTAifLzmatdh"
 ```
-Requires tcpdump and awk (installed on many *nix systems by default). Wireless device must support monitor mode.
+Requires **tcpdump** and **gawk** (GNU awk). Both of these packages are installed on many *nix systems by default, but if they aren't you will have to install them manually. Your wireless device must also support monitor mode. Here is [a list of WiFi cards that support monitor mode](https://www.wirelesshack.org/best-kali-linux-compatible-usb-adapter-dongles-2016.html) (2018).
 
 Prints `timetamp`, `signal strength`, `sender MAC address` and `SSID` to screen. Saves output as a space-delimeted "csv" to `probes.txt` by default.
 

sniff-probes.sh

@@ -21,6 +21,11 @@ channel_hop() {
 	done
 }
 
+if ! [ -x "$(command -v gawk)" ]; then
+  echo 'gawk (GNU awk) is not installed. Please install gawk.' >&2
+  exit 1
+fi
+
 if [ -z "$IFACE" ] ; then
 	echo "IFACE env variable must be set. Type \"ifconfig\" to view network interaces."
 	exit 1