> sudo softwareupdate -i -r
Password:
Software Update Tool
Copyright 2002-2012 Apple Inc.

Finding available software

Downloaded OS X Update
Installing OS X Update
Done.

Which breaks down as

sudo
needs to run as admin (you will have to enter a password after you type return)
softwareupdate
this is the command line utility that does the job
-i
install - you can specify what to install, all, or just...
-r
...the recommended updates
This is all there is to it.

Slightly more complex usage of softwareupdate

You can ask for a list of available software before you install:

> softwareupdate -l
Software Update Tool
Copyright 2002-2012 Apple Inc.

Finding available software
Software Update found the following new or updated software:
    * MacBookAirEFIUpdate2.4-2.4
        MacBook Air EFI Firmware Update (2.4), 3817K [recommended] [restart]
    * ProAppsQTCodecs-1.0
        ProApps QuickTime codecs (1.0), 968K [recommended]
    * JavaForOSX-1.0
        Java for OS X 2012-005 (1.0), 65288K [recommended]

You can then install individual packages by simply naming them

> sudo softwareupdate -i JavaForOSX-1.0

Or you can ignore some packages and then install everything else (-a, all)

> softwareupdate --ignore JavaForOSX-1.0
> sudo softwareupdate -i -a

Note that the –ignore flag will persist until it is reset with --reset-ignored (which clears all flags)

> softwareupdate --reset-ignored

More options can be viewed with softwareupdate -h or man softwareupdate