To update macOS using the command line software update tool, you can use the softwareupdate command. This command is part of the Apple Software Update utility and can be used to install system software updates, security updates, and other updates from the Apple servers.
To check for available updates, you can run the following command:
softwareupdate -l
To install available updates, you can run the following command:
softwareupdate -i -a
The -i option tells the command to install the updates, and the -a option tells it to install all available updates. You can also specify specific updates to install by their identifier, which you can see with the softwareupdate -l command.
Note that in order to run the softwareupdate command, you must have administrative privileges on the Mac.