Ubuntu: How can I install just security updates from the command line?

The package unattended-upgrades provides functionality to install security updates automatically.
You could use this, but instead of configuring the automatic part you could call it manually. For this case, the following should do it:

sudo unattended-upgrade

This assumes that the package is installed by default, which it probably is. If not, just do:

sudo apt-get install unattended-upgrades

(watch out: the package is with final "s", the first command, not)
See also /usr/share/doc/unattended-upgrades/README.

http://askubuntu.com/questions/194/how-can-i-install-just-security-updates-from-the-command-line