https://help.ubuntu.com/9.04/serverguide/C/automatic-updates.html

The unattended-upgrades package can be used to automatically install updated packages, and can be configured to update all packages or just install security updates. First, install the package by entering the following in a terminal:

sudo apt-get install unattended-upgrades

To configure unattended-upgrades, edit /etc/apt/apt.conf.d/50unattended-upgrades and adjust the following to fit your needs:

Unattended-Upgrade::Allowed-Origins {
        "Ubuntu jaunty-security";
//      "Ubuntu jaunty-updates";
};

Certain packages can also be blacklisted and therefore will not be automatically updated. To blacklist a package, add it to the list:

Unattended-Upgrade::Package-Blacklist {
//      "vim";
//      "libc6";
//      "libc6-dev";
//      "libc6-i686";
};

[Note] The double “” serve as comments, so whatever follows “” will not be evaluated.

The results of unattended-upgrades will be logged to /var/log/unattended-upgrades.

Configuring Unattended-Upgrade::Mail in /etc/apt/apt.conf.d/50unattended-upgrades will enable unattended-upgrades to email an administrator detailing any packages that need upgrading or have problems.