How do I use the Aegir user?

As the root user, you can type

su -s /bin/sh aegir

And you will be dropped into your server's default shell as the aegir user.

If you want to specify the type of shell (say, Bash), and if that shell isn't already the default '/bin/sh' shell, you can type

su -s /bin/bash aegir

If you don't run these commands as root, but as a different user, you must prefix them with sudo.

Good luck!