Admin group missing

Question

Hi,
I'm a new Mac user and i used to dev on Linux.
I'm french too so i apologize for my poor knowledge in English practice ..
Anyway, i recently wanted to add the admin group to another user (Apache) just for a temporary test and when i tried to revert this change it seems that anyone on the system was impacted.
Here are the commands i entered :
> add the admin group :
sudo dseditgroup -o edit -a www -t user admin
> drop the group :
sudo dseditgroup -o delete -a www -t user admin
OR
dscl . -append /Groups/admin GroupMembership monusername (with root user)
These commands gave me an error :
> first : Group not found
> second :
append: Invalid Path
<dscl_cmd> DS Error: -14009 (eDSUnknownNodeName)
Any idea ?

Answer 1

Please take these steps to restore administrator privileges to your account. This somewhat tedious procedure is only necessary if you've confirmed that no adminstrator account exists on the system.
If you don't already have a current backup of all your data, you must back up before taking any of the steps below. Ask if you need guidance. You won't need the backup unless something goes wrong—which is always possible.
Step 1
Start up in Recovery mode. The OS X Utilities screen will appear.
Step 2
Take this step only if you use FileVault 2. Launch Disk Utility, then select the icon of the FileVault startup volume ("Macintosh HD," unless you gave it a different name.) It will be nested below another icon with the same name. Click the  Unlock button in the toolbar and enter your login password when prompted. Then quit Disk Utility to be returned to the main screen.
Step 3
Select
          Utilities ▹ Terminal
from the menu bar. In the window that opens, type this:
res
Press the tab key. The partial command you typed will automatically be completed to this:
resetpassword
Press return. A Reset Password window opens. Select your startup volume if not already selected. Pull down the menu labeled
          Select the user account
and select
          System Administrator (root)
Follow the prompts to set a password. It's safest to choose a password that includes only the characters a-z, A-Z, and 0-9. I suggest you write down the password. If you don't write it down and forget it, you'll have to start over from Step 1.
Select
           ▹ Restart
from the menu bar.
Step 4
This step, like Step 2, applies only if you use FileVault. Log in as usual, then select
           ▹ Log Out...
from the menu bar, or press the key combination shift-command-Q. Don't restart. You'll be returned to the login screen.
Step 5
At the login screen, click Other... Enter "root" (without the quotes) in the Name field, and enter the password you set in Step 3 in the Password field. You should now be logged in as root. This is a potentially dangerous condition. Do nothing while logged in as root except as indicated below. You'll be fine as long as you don't deviate from the plan.
Open the Users & Groups preference pane. Select your usual administrator account in the list of users and check the box marked
          Allow user to administer this computer
You'll be prompted to restart. Do that and log in as yourself—not as root. Your administrator status should now be restored.
Step 6 (optional, but recommended)
Follow the instructions in this support article under the heading "Disable the root user." You must authenticate in Directory Utility as "root" with the password you set in Step 3. Authenticating as another administrator won't work.
Credit for this idea to ASC member wessongroup.