Why does this Autologin command work?

Question

I installed minimal Ubuntu 12.10. I want this environment to take as little disk space as possible, so I didn't install a desktop and am doing everything via console. I want to auto-login to the system when it is booted. I discovered a method for auto-login, but I do not fully understand it.

I edited the file /etc/init/tty1.conf and changed the last line from

exec /sbin/getty -8 38400 tty1

to

exec /bin/login -f username < /dev/tty1 > /dev/tty1 2>&1

...where "username" is the name of the user to login. It works perfectly, but why? I understand "login -f username" and redirection, but I don't see where the auto-login magic is happening.

Thanks.

Answer 1

Most of the answer is found in the man page: man login.

In your original command, getty does some setup (see the man page) prompts for a login name, and invokes login. In your new command, you're bypassing getty and invoking login directly. The key is the -f option. The manual says of it:

Do not perform authentication, user is authenticated. Note: In that case, username is mandatory.

Since login is the program that handles logins, it can easily bypass authentication if necessary.