This will check for new and updated versions of your software based on information about your computer and current software.

Common Options
-l –list  List all available updates.
-i –install
-a all available updates
-r all recommended updates

EXAMPLES
The following examples are shown as given to the shell:

softwareupdate –list
! iPhoto-2.0
iPhoto, 2.0, 34100K [required]
! iSync-1.0
iSync, 1.0, 4840K [required]
! StuffItExpander7-7.0
StuffIt Expander Security Update, 7.0, 4420K [required]

softwareupdate –install iCal-1.0.2
Software Update Tool
Copyright 2002-2003 Apple Computer, Inc.

Installing iCal:
0…10…20…30…40…50…60…70…80…90…100 Done.

via http://www.ss64.com/osx/softwareupdate.html