MacOS – accidentally deleted the admin account on the MacBook

There are several problems with the script you posted.

Don't remove /Users/Shared - iTunes needs it and will fail on certain store and sharing operations.

dscl on Lion will still whine if you start it (it's looking for the missing plist) - even if you launch /System/Library/LaunchDaemons/com.apple.opendirectoryd.plist as Ingmar Hupp has suggested.

I've never been one to worry about cleaning up the groups since I'm deleting user 501. The system will just add UID 501 back into the Admin and Staff groups the next time the Setup Assistant boots, so I do the following: (assume the short name of the user 501 is test)

1. ⌘+S
2. /sbin/mount -uw /
3. cd /var/db/dslocal/nodes/Default/users/
4. rm test.plist
5. rm -rf /Users/test
6. rm /var/db/.AppleSetupDone
7. halt

I do use the official Users & Groups Preference Pane to delete all users except 501 and/or create the test user as 501 as needed since launchd can have jobs stuck if the users you are clearing aren't trivial users set up for simple updates as your case allows.

I would avoid saving a WiFi password or setting up WiFi outside the user account or use ethernet for your updates if you want to be fastidious about leaving as little of your settings behind (especially if the SSID/name isn't generic like Apple Network or has WEP/WPA keys).

As long as it is ok to forcibly log-out the user, you just need to to logon as the local admin using Apple Remote Desktop (available in the App Store) or ssh (through a terminal), and kill that user's loginwindow process. You must login to the computer and not simply "Share screen" from Finder. Then you will be able to login.

Here's a really easy-to-read article about this.

Of course, for either of those options to work, you have to enable Remote Login and Remote Management in "Sharing" under the System Preferences. The Sharing menu is shown below:

If you would rather use fast-user-switching, which I personally don't like because it can often really slow down your computer, then you can set up an "logout user after x minutes of activity" option in the System Preferences -> Security & Privacy, which will return you to a login screen. There may also be nice hacks if you search around, that allow you to get the user-switching icon or a keyboard shortcut setup to do a fast-switch, or trigger a switch or logout by tweaking a call to CGSession, although that may have been possible only in older OSX versions. There is a huge discussion of many other ideas in that last link as well.