“curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -” are we supposed to add some key here?

Question

I am trying to install docker on ubuntu and following installation guide I came across this command.

"curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -"

And in its responce I get OK and nothing really downloads. Am I suppose to provide a key or what is the purpose of this curl command.

Answer 1

The apt-key add adds a trusted key to for the docker repository. The curl command is downloading this key and piping it to the apt-key add command which adds it as a trusted key.

>man apt-key

COMMANDS
Add filename
Add a new key to the list of trusted keys. The key is read from the filename 
given with the parameter filename or if the filename is - from standard 
input.

It is critical that keys added manually via apt-key are verified to belong 
to the owner of the repositories they claim to be for otherwise the apt-
secure(8) infrastructure is completely undermined.

In short, This command downloads the key as adds it as a trusted key.

Answer 2

I done this in a alternative way to make to understand !

1. stored key to a file

#curl -fsSL https://download.docker.com/linux/debian/gpg > key_stored2file

2. Add a new key to the list of trusted keys.

#apt-key add key_stored2file
OK

If - given instead of filename( in above example key_stored2file ) then it will take from standard input so it can be run as a single command as follows

#curl -fsSL https://download.docker.com/linux/debian/gpg | sudo apt-key add -