Occasionally I want to install packages located at an Ubuntu PPA repository on my Debian stretch machine. There’s a bit of a trick to it, thanks to chrisjean.com for outlining what needs to be done.
Step 1 is the same as in Ubuntu, add the PPA with add-apt-repository (install if it’s not already there)
sudo add-apt-repository ppa:<contributor>/<ppa name>
This will appear to work but when you do an apt update you may get something similar to this
W: GPG error: http://ppa.launchpad.net/jonathonf/gcc-7.1/ubuntu xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8CF63AD3F06FC659 W: The repository 'http://ppa.launchpad.net/jonathonf/gcc-7.1/ubuntu xenial InRelease' is not signed. N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use. N: See apt-secure(8) manpage for repository creation and user configuration details.
The last step is to manually import the key with the following command:
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys <KEY_OF_PPA>
The PPA key will be listed on the PPA’s page. Once I ran that second command everything worked swimmingly.