blomarchy/bin/blob-network-band

#!/bin/bash

# blob:summary=Show or pin the Wi-Fi band for the active connection
# blob:group=network
# blob:args=[auto|2.4|5|6]
# blob:examples=blob network band | blob network band 5 | blob network band auto

set -euo pipefail

# NetworkManager 1.44+ accepts a third band value, so 5GHz and 6GHz can be
# pinned apart. Pinning the band rather than a BSSID keeps working when the AP
# rotates BSSIDs, and leaves roaming between APs intact.
nm_band_for() {
  case "$1" in
    2.4) echo "bg" ;;
    5) echo "a" ;;
    6) echo "6GHz" ;;
    *) return 1 ;;
  esac
}

band_from_nm() {
  case "$1" in
    bg) echo "2.4" ;;
    a) echo "5" ;;
    6GHz) echo "6" ;;
    *) echo "auto" ;;
  esac
}

# Accepts either nmcli's "2412 MHz" or iw's "5745.0": keep the leading digits
# and ignore whatever unit or fraction follows. The boundaries mirror Model.js
# formatHeaderFreq so the panel's label and this command cannot disagree.
band_for_freq() {
  local mhz=${1%%[!0-9]*}
  [[ -n $mhz ]] || return 1

  if ((mhz >= 2400 && mhz < 2500)); then
    echo "2.4"
  elif ((mhz >= 4900 && mhz < 5925)); then
    echo "5"
  elif ((mhz >= 5925 && mhz < 7125)); then
    echo "6"
  else
    return 1
  fi
}

# Every value read back from nmcli goes through here. LC_ALL=C because nmcli
# translates state words such as "connected", which would silently stop matching
# under a non-English session; `-e no` because `-g` otherwise escapes ':' and
# '\' in values, and an escaped SSID would never match iw's raw one.
nm_get() {
  LC_ALL=C nmcli -e no -g "$@" 2>/dev/null
}

wifi_device() {
  nm_get DEVICE,TYPE,STATE device status |
    awk -F: '$2 == "wifi" && $3 == "connected" { print $1; exit }'
}

wifi_profile() {
  nm_get GENERAL.CONNECTION device show "$1"
}

selected_band() {
  band_from_nm "$(nm_get 802-11-wireless.band connection show "$1")"
}

# Sets $ssid and $freq from one `iw dev <device> link`. iw prints the SSID as
# the rest of its line, so an SSID containing ':' needs no special handling.
read_link() {
  local link
  link=$(iw dev "$1" link 2>/dev/null)
  ssid=$(awk '/SSID:/ { sub(/.*SSID: /, ""); print; exit }' <<<"$link")
  freq=$(awk '/freq:/ { print $2; exit }' <<<"$link")
}

# Every band the SSID is reachable on, low to high, always including the one
# already in use -- a weak radio gets missed by plenty of scans, and the band we
# are sitting on must never be absent from its own list of options. A band the
# AP does not answer on is never offered: pinning to it would drop the
# connection with nothing to reassociate to.
#
# --rescan no reads NetworkManager's cache, which the panel's own scanner keeps
# warm; forcing a scan here would stall every poll. The kernel's own cache (iw
# scan dump) is pruned far harder and would make bands flicker.
available_bands() {
  local device=$1 ssid=$2 current=$3

  {
    if [[ -n $current ]]; then echo "$current"; fi

    # SSID is queried last so one containing ':' can be reassembled verbatim.
    # It reaches awk through the environment, not -v, which would expand
    # backslash escapes in an SSID that contains one.
    nm_get FREQ,SSID dev wifi list ifname "$device" --rescan no |
      want="$ssid" awk -F: '
        BEGIN { want = ENVIRON["want"] }
        {
          name = $2
          for (i = 3; i <= NF; i++) name = name ":" $i
          if (name == want) print $1
        }' |
      while read -r freq; do band_for_freq "$freq" || true; done
  } | sort -u -g | tr '\n' ' ' | sed 's/ $//'
}

print_status() {
  local device profile band available

  device=$(wifi_device)
  [[ -n $device ]] || return 0

  read_link "$device"
  [[ -n $ssid ]] || return 0

  band=$(band_for_freq "$freq" || true)
  available=$(available_bands "$device" "$ssid" "$band")
  profile=$(wifi_profile "$device")

  printf 'band\t%s\n' "$band"
  printf 'available\t%s\n' "$available"
  if [[ -n $profile ]]; then printf 'selected\t%s\n' "$(selected_band "$profile")"; fi
}

set_band() {
  local target=$1
  local device profile previous desired

  device=$(wifi_device)
  if [[ -z $device ]]; then
    echo "Error: no connected Wi-Fi device." >&2
    exit 1
  fi

  profile=$(wifi_profile "$device")
  if [[ -z $profile ]]; then
    echo "Error: no active Wi-Fi connection profile." >&2
    exit 1
  fi

  if [[ $target == "auto" ]]; then
    desired=""
  else
    read_link "$device"
    if [[ " $(available_bands "$device" "$ssid" "$(band_for_freq "$freq" || true)") " != *" $target "* ]]; then
      echo "Error: ${target}GHz is not available on this network." >&2
      exit 1
    fi
    desired=$(nm_band_for "$target")
  fi

  previous=$(nm_get 802-11-wireless.band connection show "$profile")
  [[ $previous == "$desired" ]] && exit 0

  nmcli connection modify "$profile" 802-11-wireless.band "$desired" >/dev/null

  # A band change only takes effect on reassociation. If the radio cannot come
  # back up on the requested band, put the previous setting back and reconnect
  # rather than leaving the machine stranded offline.
  if ! nmcli connection up "$profile" >/dev/null 2>&1; then
    nmcli connection modify "$profile" 802-11-wireless.band "$previous" >/dev/null
    nmcli connection up "$profile" >/dev/null 2>&1 || true
    echo "Error: could not connect on ${target}; reverted to previous band." >&2
    exit 1
  fi
}

usage() {
  echo "Usage: blob-network-band [auto|2.4|5|6]" >&2
}

if (($# == 0)); then
  print_status
  exit 0
fi

if (($# > 1)); then
  usage
  exit 1
fi

case "$1" in
  auto | 2.4 | 5 | 6)
    set_band "$1"
    ;;
  *)
    usage
    exit 1
    ;;
esac