To generate and output an SSH key on Linux or macOS, such as for authentication with GitHub, SSH sessions, etc., you can use the following two commands:
To generate the key:
ssh-keygen -t rsa -b 4096 -C "mail@example.com"
To output the key:
cat ~/.ssh/id_rsa.pub
These commands will generate an RSA key with a length of 4096 bits and an email address comment, and then output the public key (located at ~/.ssh/id_rsa.pub).