Linux add Users (user) to user groups (group)

Add a user to a user group and never use it directly:

Usermod-g Groupa

Doing so will allow you to leave the other user groups, just as members of this user group Groupa.
Should be added with the-a option:

Usermod-a-G Groupa User

(fc4:usermod-g GROUPA,GROUPB,GROUPC user)
-A represents append, which is to add itself to the user group Groupa without having to leave another user group.

All the options for the command, and what it means:
Options:

• -c,–comment comment New value of the GECOS field
• -d,–home home_dir New Home directory for the user account
• -e,–expiredate expire_date Set account expiration DATE to Expire_date
• -f,–inactive inactive Set Password inactive after expiration
• To INACTIVE
• -g,–gid Group Force use Group as new primary group
• -g,–groups groups new List of supplementary groups
• -a,–append append the user to the supplemental GROUPS
• mentioned by the-g option without removing
• Him/Her from other groups
• -H,–HELP Display this help message and exit
• -l,–login New_login NEW value of the login name
• -l,–lock Lock the user account
• -m,–move-home move contents of the home directory to the new
• Location (with only with-d)
• -o,–non-unique allow using duplicate (Non-unique) UID
• -p,–password Password use encrypted password for the new password
• -s,–shell Shell New Login shell for the user account
• -u,–uid UID New UID for the user account
• -u,–unlock Unlock the user account

To view the group usage commands that the user belongs to:

Groups User

or view files:

Cat /etc/group

Linux add Users (user) to user groups (group)