If you want to do an automatic install of all the updates for your particular Mac, first log in as an administrative user. Then open Terminal.app (located in /Applications/Utilities), and type (or copy/paste) the following command:
sudo softwareupdate -i -a
and hit Return. Next, you'll be prompted to enter your password; do so, and then hit Return. The application will then look for available updates, download them, and then install them. You will see the progress in Terminal. If no updates are available, Terminal will let you know.
This command replicates the functionality of the normal Software Update utility. However, you can use it on other machines in the background via SSH remote login, or you could apply a specific update rather than the full list of available patches. To see all the command options, type
man softwareupdateat the Terminal prompt.
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