Monitor signal strength (RSSI) of a Wi-Fi network on Linux

rssi.sh

#!/bin/sh

# SPDX-FileCopyrightText: 2019 Kirill Elagin <https://kir.elagin.me/>
# SPDX-License-Identifier: MPL-2.0

###
#
# Monitor signal strength (RSSI) of a Wi-Fi network on Linux.
#
# Given an SSID, the script outputs the signal strength of the
# corresponding access point every second. On the first scan it
# remembers the frequence of the SSID to be able to rescan rapidly.
#
# This script was tested on an OpenWRT router, but should work
# on any wireless-capable Linux machine. The dependencies are:
#
#   * POSIX shell (tested with Busybox ash)
#   * standard Unix tools or shell builtins: cut, echo, grep, head, true, sleep
#   * iw
#
#
# Usage:
#   rssi.sh <wdev> <SSID>
#
# Use `iw dev` to find a suitable wireless device to pass as `wdev` (it will
# probably be something like `wlan0`, at least on OpenWRT).
#
#
# https://gist.github.com/kirelagin/a3e6f0114dafbff912616b3bcc70c8b3
###

[ -z "$1" -o -z "$2" -o -n "$3" ] && {
  echo "Usage: $0 <wdev> <ssid>"
  exit 1
}
wdev="$1"
ssid="$2"

# Check the device exists
iw dev "$wdev" info >/dev/null || exit 2


cleanup() {
  # Show cursor
  echo -en '\e[?25h'
}
trap 'cleanup; trap - INT; kill -s INT "$$"' INT
trap 'cleanup' EXIT

# Hide cursor
echo -en '\e[?25l'

get_ssid_info() {
  local args
  # Only scan a specific frequency, if given, to speed up
  [ -n "$freq" ] && args="freq $freq"
  iw dev "$wdev" scan $args | grep 'freq:\|signal:\|SSID:' | grep -B2 "SSID: $ssid"
}

get_field() {
  grep "$1:" | cut -f 2 | cut -d ' ' -f 2
}

# Get the frequency of the SSID
echo -n "Locking on '$ssid'... "
ssid_info=$(get_ssid_info)
[ "$?" -ne 0 ] && {
  echo ""
  echo "SSID '$ssid' not found"
  exit 2
}
echo "done"
freq=$(echo "$ssid_info" | get_field 'freq')

while true; do
  sig=$(get_ssid_info | get_field 'signal')
  echo -ne "$sig\r"
  sleep 1
done

I visited a big school and they had eduroam WAPs everywhere. That meant duplicate freq values which broke the script. Also, maybe it's a new feature of iw dev <device> scan, or maybe because it's on a laptop instead of OpenWRT, but the freq values also have ".0" appended to them, which also break the script.

I've appropriately doctored this script to prevent the errors above for my own use. Thanks a lot!

@aureateAnatidae Glad that it (mostly) worked for you! Given that it’s been 6 years, it’s almost a miracle that you were able to update it relatively easily :).