Google tells me the command below is used in the Ubuntu Terminal to install an application:
sudo apt-get install
That is if you are installing it through a PPA (which you prefer not). You would need to add Bitcoin Core's PPA repo and use "sudo apt-get install bitcoin-qt" to install it.
To install through the tar.gz file, follow these instructions:
https://bitcoin.org/en/full-node#linux-instructionsBasically:
tar xzf bitcoin-0.18.0-x86_64-linux-gnu.tar.gz
sudo install -m 0755 -o root -g root -t /usr/local/bin bitcoin-0.18.1/bin/*
Then run
bitcoin-qt to open the GUI.