To install software updates from the command line, you'll use the aptly named 'softwareupdate' command. Here are the most relevant usages of the command for most users:
softwareupdate -l
will give you a list of all updates available
softwareupdate -i [package name]
will install the specified package
softwareupdate -i -a
will install all updates available
softwareupdate -i -r
will install only the recommended updates
softwareupdate --ignore [package name]
will ignore specified update
softwareupdate --help
for more info on the softwareupdate command
You'll likely need to use the sudo command to actually install the updates on a Mac, simply place sudo in front of one of the above commands to run it with full permissions as root.
For example:
sudo softwareupdate -i -r