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 acts as an overlay. Trying to do the above usermod command 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

Copy the docker line

sudo vim /etc/group

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

docker:x:954:ruby

Now reboot

systemctl reboot

Verify you're in the docker group

groups ruby

Should be working :)