MacOS – How to backup with Time Machine in Single User Mode

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).

Although this does not answer the original question, as it is about an earlier mac model, this is a somewhat common issue for newer macs, and I feel that this answer may be helpful from the people that come here from Google, where this page is one of the top results.

If, when entering single user mode, you encounter the issue shown in this question's GIF (the mac shows the single user mode white text, but never prompts a terminal and just contiunes booting anyway), the answer is that Single User Mode via CMD+S does not work on T2 macs (if your mac's model appears on this list, then it has a T2 chip).

Apple's official guidance (which strangely now leads to a 404 page, but is still available on the Wayback Machine) is to:

1. Turn on your Mac, then immediately press and hold CMD+R to start up from macOS Recovery.
2. Select Disk Utility from the Utilities window, then click Continue.
3. From the Disk Utility sidebar, select the volume that you're using, then choose File > Mount from the menu bar. (If the volume is already mounted, this option is dimmed.) Then enter your administrator password when prompted.
4. Quit Disk Utility.
5. Choose Terminal from the Utilities menu in the menu bar.
6. You can now enter UNIX commands. When done, choose Apple () menu > Restart.

These steps replace the CMD+S method, which doesn't work with the recommended settings on current Mac computers.