`usermod` - modify a user account

usermod [options] LOGIN

Options you will use

-aG GROUPS: APPEND to the supplementary groups. This is almost always what you want.
-G GROUPS: Without -a: REPLACE the supplementary groups with exactly this list. usermod -G docker learner removes learner from sudo.
-g GROUP: change the primary group
-s SHELL: change the login shell
-L / -U: lock / unlock the password (prefixes ! in /etc/shadow)
-e, --expiredate EXPIRE_DATE: The date the account is disabled (YYYY-MM-DD); -e 1 disables it now.

$ sudo usermod -aG ops appuser

add to ops, keep everything else

Group changes apply to NEW logins. A shell that is already running keeps the groups it started with: log out and in, or newgrp GROUP.

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