将一个用户添加到用户组中，千万不能直接用：
usermod -G groupA
这样做会使你离开其他用户组，仅仅做为 这个用户组 groupA 的成员。
应该用 加上 -a 选项：
usermod -a -G groupA user
(FC4: usermod -G groupA,groupB,groupC user)
-a 代表 append， 也就是 将自己添加到 用户组groupA 中，而不必离开 其他用户组。
命令的所有的选项，及其含义：
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
查看用户所属的组使用命令：
$ groups user
或者查看文件：
$ cat /etc/group