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 expirationto 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 GROUPSmentioned by the -G option without removinghim/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 newlocation (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