i3bar

path_exists channel {
        format = ""
        format_down = "Upgrade!"
        path = "/var/lib/is-nix-channel-up-to-date/up-to-date"
}

is-nix-up-to-date.service.nix

{ config, pkgs, ... }:
{
  users = {
    users.is-nix-channel-up-to-date = {
      description = "Is-Nix-Channel-Up-To-Date";
      home = "/var/lib/is-nix-channel-up-to-date";
      createHome = true;
      group = "is-nix-channel-up-to-date";
      uid = 400;
    };

    groups.is-nix-channel-up-to-date.gid = 400;
  };

  systemd.services.is-nix-channel-up-to-date = {
    enable = true;
    after = [ "network.target" "network-online.target" ];
    wants = [ "network-online.target" ];
    wantedBy = [ "multi-user.target" ];

    environment.VERSION_LOCAL = config.system.nixosRevision;
    path = with pkgs; [ curl coreutils ];

    serviceConfig = {
      User = "is-nix-channel-up-to-date";
      Group = "is-nix-channel-up-to-date";
      PrivateTmp = true;
      WorkingDirectory = "/var/lib/is-nix-channel-up-to-date";
    };

    preStart = ''
      chmod 755 /var/lib/is-nix-channel-up-to-date
    '';
    script = "${./is-nix-up-to-date.sh}";
  };
}

is-nix-up-to-date.sh

#!/bin/bash

set -eu

CHAN=17.09
DELAY=675
SENTINAL=/var/lib/is-nix-channel-up-to-date/up-to-date

fetch_channel_version() {
    curl "https://channels.nix.gsc.io/nixos-$CHAN/latest" \
        | cut -d' ' -f1
}

remove_sentinal() {
    echo "Removing sentinal from $SENTINAL"
    rm "$SENTINAL"
    exit 0
}

VERSION_CHAN=$(fetch_channel_version)
echo "Channel starts at $VERSION_CHAN"
echo "System is at $VERSION_LOCAL"

if [ "$VERSION_CHAN" == "$VERSION_LOCAL" ]; then
    touch "$SENTINAL"
else
    remove_sentinal
fi

while true; do
    sleep "$DELAY"

    VERSION_CHAN=$(fetch_channel_version)
    if [ "$VERSION_CHAN" != "$VERSION_LOCAL" ]; then
        printf "Channel changed to %s\n" "$VERSION_CHAN"
        remove_sentinal
    fi
done