How to add user to system group

The usermod command would normally be used to add a user to a group provided by the OS packages.

e.g. adding ruby to docker:

sudo usermod -aG docker ruby

However this works by adding ruby to the list of users in the group, where the groups are defined in /etc/group

In the Atomic Desktop based OS such as Fedora Silverblue/Kinoite the system groups are stored in /usr/lib/group and /etc/group file essentially provides additions and overrides to the system groups. Trying to do the above usermod command therefore fails, because the docker group isn't in /etc/group, so it can't be edited to add ruby.

To fix:

cat /usr/lib/group

Find the line for the system group you want to add a user to, for example the docker group looks like this:

docker:x:954:

Now edit the normal group file:

sudo vim /etc/group

Add the line you copied, add ruby to the group while you're at it:

docker:x:954:ruby

Now reboot

systemctl reboot

Verify you're now in the group

groups ruby

Should be working :)