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