將一個用戶添加到用戶組中，千萬不能直接用：
usermod -G groupA
這樣做會使你離開其他用戶組，僅僅做為 這個用戶組 groupA 的成員。
應該用 加上 -a 選項：
usermod -a -G groupA user
(FC4: usermod -G groupA,groupB,groupC user)
-a 代表 append， 也就是 將自己添加到 用戶組groupA 中，而不必離開 其他用戶組。
命令的所有的選項，及其含義：
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 (use 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
查看用戶所屬的組使用命令：$ groups user
或者查看文件：$ cat /etc/group