shell
usermod

ajouter un utilisateur à un groupe

Disons que vous avez deux utilisateurs avec le nom d'utilisateur chyingp et casper , et que vous voulez les ajouter tous les deux à la root groupe.

usermod chyingp -a -G root
usermod casper -a -G root

voici l'explication de -a , -G

-a, --append
Add the user to the supplementary group(s). Use only with the -G option.
-G, --groups GROUP1[,GROUP2,...[,GROUPN]]]
A list of supplementary groups which the user is also a member of. Each group is separated from the next by
a comma, with no intervening whitespace. The groups are subject to the same restrictions as the group given
with the -g option.

If the user is currently a member of a group which is not listed, the user will be removed from the group.
This behaviour can be changed via the -a option, which appends the user to the current supplementary group
list.