How do I install Signal in Ubuntu?

For all releases of Ubuntu 18.04 and newer:

sudo snap install signal-desktop

Open your terminal and copy-paste the below command one by one

sudo apt install curl
curl -s https://updates.signal.org/desktop/apt/keys.asc | sudo apt-key add -
echo "deb [arch=amd64] https://updates.signal.org/desktop/apt xenial main" | sudo tee -a /etc/apt/sources.list.d/signal-xenial.list
sudo apt update 
sudo apt install signal-desktop

All credit goes to https://websiteforstudents.com/how-to-install-signal-desktop-on-ubuntu-16-04-17-10-18-04/

Apart from a deb package from the Signal PPA and snap package (sudo snap install signal-desktop), you can install it as flatpak*:

flatpak install flathub org.signal.Signal

Once installed, you can run it from the menu or use the following command in the terminal:

flatpak run org.signal.Signal

*PS: Make sure to follow the setup guide before installing.

Source: How to Install Signal on Ubuntu & Other Linux Distributions