Step-by-step Ubuntu Add Cosmic Repository Easy & Safe Guide

Setting Up Repo

2. 2. Adding Repo

   Then to Add Ubuntu Cosmic Repository
   To Specify the Differents Ubuntu Repos:

   • Main – Canonical-supported free and open-source software.
   • Universe – Community-maintained free and open-source software.
   • Multiverse – Software restricted by copyright or legal issues.
   • Restricted – Proprietary drivers for devices.

   Simply Copy and Execute this Command for the essential Main Repo:
   (And in case of “su: Authentication failure” try instead with “sudo su“)

   su -c "echo 'deb http://us.archive.ubuntu.com/ubuntu/ cosmic main' >> /etc/apt/sources.list.d/sid.list"

   Again to Add also the Universe Repo:

   (And in case of “su: Authentication failure” try instead with “sudo su“)

   su -c "echo 'deb http://us.archive.ubuntu.com/ubuntu/ cosmic universe' >> /etc/apt/sources.list.d/sid.list"

   Again to Add also the Multiverse:
   (And in case of “su: Authentication failure” try instead with “sudo su“)

   su -c "echo 'deb http://us.archive.ubuntu.com/ubuntu/ cosmic multiverse' >> /etc/apt/sources.list.d/sid.list"

   Finally, to Summ also the Restricted Stuff:
   (And in case of “su: Authentication failure” try instead with “sudo su“)

   su -c "echo 'deb http://us.archive.ubuntu.com/ubuntu/ cosmic restricted' >> /etc/apt/sources.list.d/sid.list"

   Ctrl+x to Save & Exit from nano Editor :)
   When in Doubt Start by just Adding the Main and Universe and then Searching for the Needed Squeezes.

3. 3. Setting Up Default

   Then to Set the Default Release
   And this is Indispensable to Preserve the Stability of the System in Case of Updates/Upgrades!
   Make or Edit the Apt Config File:

   sudo nano /etc/apt/apt.conf

   And Depending of your Release Append
   For a Bionic Based OS:

   APT::Default-Release "bionic";

   Rather for Xenial one:

   APT::Default-Release "xenial";

   Possibly if needed to Find out What’s your actual Repo Base
   Use the cat Command like:

   cat /etc/apt/sources.list

   Or again look into the sources Directory to find out eventual others Repos:

   cat /etc/apt/sources.list.d/*.list

   Thanks to dracae

4. 4. Enabling Repo

   Finally, to Re-Load Repositories
   Do:

   sudo apt update

5. 5. Installing Single Package

   Finally, to Install One Package from Cosmic Repo
   Execute:

   sudo apt -t cosmic install [MYCOOLPACKAGE]

   So for instance:

   sudo apt -t cosmic install rclone-browser