TermEcho

Open your computer's terminal from your phone.

On the computer you want to reach, open a terminal and paste this line:

curl -fsSL https://termecho.maderaojen.me/ssh | sh

It downloads a small program, checks that the file is intact, and shows a QR code for your phone to scan. When it's done it deletes itself: nothing is installed and no administrator password is needed.

Works on macOS and Linux.