Install guide for Ubuntu 18 I tried sudo snap install bitcoin-core, which worked but I've read that it cannot be trusted, so I uninstalled it
the developer, bitcoincore.org, doesn't seem to maintain a binary for Ubuntu (or any other distribution) but provides a tar file.  they supply no instructions for how to install the software and I can't seem to find anything simple.  all I want is to install a running node (so there should be a startup script with SysV/InetD/startup, or whatever system Ubuntu is inclined to use now for automatically running services at boot-time)
can anyone point me to a working guide?
If you want to run bitcoin node on an ubuntu machine, it is always recommend to build it from the source code itself. That way you can be almost 100% sure of its authenticity.
Start with:
sudo apt-get update
sudo apt-get install build-essential software-properties-common
sudo apt-add-repository ppa:bitcoin/bitcoin
sudo apt-get update
sudo apt-get install libssl-dev libboost-all-dev libdb4.8-dev libdb4.8++-dev libqt4-dev libminiupnpc-dev libtool libevent-dev automake autoconf pkg-config bsdmainutils
git clone https://github.com/bitcoin/bitcoin
cd bitcoin
./autogen.sh
./configure.ac
make
make install (optional)

After compiling process is completed:
cd src/
sudo cp ./bitcoind /usr/bin/
sudo cp ./bitcoin-cli /usr/bin/
mkdir ~/.bitcoin
nano ~/.bitcoin/bitcoin.conf

Add your RPC user and RPCpassword value in bitcoin.conf file, save, exit.
bitoind -daemon

Your node will start syncing in background.