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