notpushkin/apt-key but it is bash one-liner.md

## apt-key but it is bash one-liner.md

      
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              apt-key but it is bash one-liner.md
            
          
    Do not use apt-key add.

apt-key add [filename]
Note: Instead of using this command a keyring should be placed directly in the /etc/apt/trusted.gpg.d/ directory
with a descriptive name and either "gpg" or "asc" as file extension.

    — apt-key(8) manpage

There's a good reason for this deprecation: it adds keys to /etc/apt/trusted.gpg, which APT will use for all repositories. This means any third-party repository you add would be able to pretend to be your system repository too (if they manage to MITM it).
Update (2023): Current best practice is to put key in /usr/share/keyrings instead and add pin it in the source file with [signed-by=...]. So instead of curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add - you should use something like this:
sudo mkdir -p /etc/apt/keyrings
curl -fsSL https://download.docker.com/linux/ubuntu/gpg \
  | gpg --dearmor \
  | sudo tee /etc/apt/keyrings/docker.gpg \
  > /dev/null
echo "deb [arch=amd64 signed-by=/etc/apt/keyrings/docker.gpg] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable" | sudo tee /etc/apt/sources.list.d/docker.list > /dev/null
(You have to use gpg --dearmor because sometimes APT doesn't recognize ASCII-armoured keys, which kinda sucks but works for us so whatever)
Once again, in a single line:
curl -fsSL https://download.docker.com/linux/ubuntu/gpg | gpg --dearmor | sudo tee /etc/apt/keyrings/docker.gpg > /dev/null
Further reading

Connect to a third-party repository on Debian Wiki
Update (2023)

Most repos I see in the wild have adopted this (or similar) syntax 🎉
(that includes Docker, I just used it as an example)
I've removed the snippets in the first comment: I don't feel like updating them, as it would mostly just be a copy of official instructions.