Fix GPG key error when adding a PPA with apt-key

Since Ubuntu 22.04 and derivative distributions such as Linux Mint 21, adding a PPA using the command sudo add-apt-repository ppa:lutris-team/lutris triggers the following error: Warning: apt-key is deprecated. Manage keyring files in trusted.gpg.d instead (see apt-key(8)). during each sudo apt update.

Here are two methods to fix the problem:

Method for EACH problematic PPA

For this example, I will use the Lutris PPA.

1. Open the terminal
2. List the GPG keys:

   sudo apt-key list

   Which gives, for Lutris:

   pub rsa4096 2019-03-23 [SC]
   82D9 6E43 0A1F 1C0F 0502  747E 37B9 0EDD 4E3E FAE4
   uid [ unknown] Launchpad PPA for lutris

3. Export the GPG key (using only the last 8 characters of the ID):

   sudo apt-key export 4E3EFAE4 | sudo gpg --dearmour -o ~/Desktop/lutris-keyring.gpg

4. Remove all keys related to the Lutris PPA to avoid duplicates:

   sudo apt-key del 4E3EFAE4

5. Copy the file ~/Desktop/lutris-keyring.gpg to the correct location:

   sudo cp ~/Desktop/lutris-keyring.gpg /etc/apt/trusted.gpg.d

6. sudo apt update should no longer display any errors.

Global method (to do once)

1. Open the terminal and type the following:

   cd /etc/apt
       sudo cp trusted.gpg trusted.gpg.d
       sudo apt update

2. The issue is fixed 😊