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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。

usermod -a -G groupA user

(FC4: usermod -G groupA,groupB,groupC user)

• -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

$ groups user

或者查看文件：

$ cat /etc/group