Install guide for Ubuntu 18
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.