It's pretty common to find instructions for adding an deb repository source as a shell script. They normally append stuff to /etc/apt/sources.list, while I prefer creating a separate file for each repo on /etc/apt/sources.list.d.

Here's how I do:

1. Create a /etc/apt/sources.list.d/{repo name}.list file with its content being the repository source (some at the end of this post):

sudo vim /etc/apt/sources.list.d/{repo name}.list

2. Download the repo GPG key (as text):

curl {gpg key url} > {repo name}

3. Convert it to a .gpg (binary):

gpg --dearmor {repo name}

4. Delete the original file and move the .gpg one to /etc/apt/trusted.gpg.d/:

rm {repo name} && sudo mv {repo name}.gpg /etc/apt/trusted.gpg.d/

Now you can sudo apt update and sudo apt install {your package}.

Here's some of my repos (Ubuntu 20.04):

• Dropbox:
  • deb [arch=i386,amd64] http://linux.dropbox.com/ubuntu disco main
  • https://linux.dropbox.com/ubuntu/dists/disco/Release.gpg
• Microsoft (for code and dotnet):
  • deb [arch=amd64,arm64,armhf] https://packages.microsoft.com/ubuntu/22.04/prod jammy main
  • https://packages.microsoft.com/keys/microsoft.asc
• NodeSource (for node and npm):
  • deb https://deb.nodesource.com/node_14.x focal main
  • https://deb.nodesource.com/gpgkey/nodesource.gpg.key
• Libcontainers (for buildah, podman, and skopeo):
  • deb https://download.opensuse.org/repositories/devel:/kubic:/libcontainers:/stable/xUbuntu_20.04/ /
  • https://download.opensuse.org/repositories/devel:/kubic:/libcontainers:/stable/xUbuntu_20.04/Release.gpg
• Libretro (for RetroArch):
  • deb http://ppa.launchpad.net/libretro/stable/ubuntu jammy main
  • See "Signing key" in "Technical details about this PPA" for the GPG key.
• AppImageLauncher:
  • deb https://ppa.launchpadcontent.net/appimagelauncher-team/stable/ubuntu jammy main
  • See "Signing key" in "Technical details about this PPA" for the GPG key.