Ubuntu: How do you modify group membership in Ubuntu Core


Running Ubuntu Core 16 on RP3. Attempting to add supplementary group 'test' to my username, but unable to. Created the group using:

sudo groupadd --system --extrausers test  

The flags are required for the software I am using (basically trying to get lxd group membership added to my account, but trying to at least do it with a test group first).

Attempting with 'adduser':

alexk1@localhost:~$ sudo adduser --extrausers alexk1 lxd  Adding user `alexk1' to group `lxd' ...  gpasswd: group 'lxd' does not exist in /etc/group  adduser: `/usr/bin/gpasswd -a alexk1 lxd' returned error code 3. Exiting.  

Attempting with 'usermod':

alexk1@localhost:~$ sudo usermod -a -G test alexk1  usermod: /etc/group.3813: Read-only file system  usermod: cannot lock /etc/group; try again later.  

It's true that the group is not listed in '/etc/group', however 'getent' confirms it's there:

alexk1@localhost:~$ getent group  ...  test:x:998:  

As listed here, there is a snap command 'create-user' that has the ability to add membership to certain groups upon account creation (the link refers to members of sudo group), so I'm not sure if there is a snap command for modifying group membership (I can't use 'create-user' as I have only one Ubuntu Core install and one Ubuntu SSO account). Ubuntu Core documentation is shockingly bleak and I only discovered the 'create-user' command through bug reports and not any actual documentation.


Ubuntu core manages group membership in /var/lib/extrausers

I edited the groups file and added my username to the group like this:


I'm assuming the flag '--extrausers' in the "sudo groupadd --system --extrausers test" command allowed me to do this.

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