ssh -T

Recap on ssh, what it is and why it exists

Mastering ssh is essential to connecting into remote servers.

ssh is used to connect to a remote server using the syntax:

ssh [username]@[hostname or IP address]

Explanation of -T flag, and why it makes sense for testing GitHub logins

When you do:

ssh -T 

it means do not allocate a terminal (TTY), so you are asking to connect, but without the interactive shell. 

Connecting to some external services like GitHub operate in such a way that they do not provide shell access. You can still check/test SSH authentication:

ssh -T git@github.com

GitHub should respond with text like:

Hi your_username!  You've successfully authenticated, but GitHub does not provide shell access.

This proves your SSH key was accepted , GitHub recognised your account, future Git operations over SSH should work.

Some Technical Notes on TCP Ports for ssh

Technical note 1: Port 22 is the default TCP port used by SSH for encrypted remote login, command execution and file transfer, as registered by IANA and used by all major SSH implementations.

Technical note 2: IANA - Internet Assigned Numbers Authority