本篇讲述 Linux 用户组别权限的理解，希望给大家带来帮助。

先看操作历史记录。Last login: Thu Aug 3 03:31:18 2017 from 202.101.22.42

root@may:~# groups root

root : root

root@may:~# groups sudo

groups: sudo: no such user

root@may:~# groups www-data

www-data : www-data

root@may:~# usermod -G www-data root

You have new mail in /var/mail/root

root@may:~# groups www-data

www-data : www-data

root@may:~# exit

logout

Connection to lmlphp.com closed.

Last login: Thu Aug 3 04:29:46 2017 from 202.101.22.42

root@may:~# groups www-data

www-data : www-data

root@may:~# groups root

root : root www-data

root@may:~# usermod --help

Usage: usermod [options] LOGIN

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

-R, --root CHROOT_DIR directory to chroot into

-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

-v, --add-subuids FIRST-LAST add range of subordinate uids

-V, --del-subuids FIRST-LAST remvoe range of subordinate uids

-w, --add-subgids FIRST-LAST add range of subordinate gids

-W, --del-subgids FIRST-LAST remvoe range of subordinate gids

-Z, --selinux-user SEUSER new SELinux user mapping for the user account

root@may:~# group

groupadd groupdel groupmod groups

root@may:~# groupdel --help

Usage: groupdel [options] GROUP

Options:

-h, --help display this help message and exit

-R, --root CHROOT_DIR directory to chroot into

You have new mail in /var/mail/root

root@may:~# man groupdel

-bash: man: command not found

root@may:~# vim /etc/passwd

You have new mail in /var/mail/root

root@may:~# groups

root www-data

root@may:~# vim /etc/group

root@may:~# usermod -G lin root

root@may:~# groups

root www-data

root@may:~# groups root

root : root lin

root@may:~# groups www-data

www-data : www-data

root@may:~# groups lin

lin : lin sudo

root@may:~# groups

root www-data

root@may:~# groups root

root : root lin

root@may:~# exit

logout

Connection to lmlphp.com closed.

Welcome to Ubuntu 14.04 LTS (GNU/Linux 2.6.32-042stab108.8 x86_64)

* Documentation: https://help.ubuntu.com/

You have new mail.

Last login: Thu Aug 3 04:31:07 2017 from 202.101.22.42

root@may:~# groups

root lin

root@may:~# groups root

root : root lin

root@may:~# usermod -G www-data root

root@may:~# groups

root lin

root@may:~# groups root

root : root www-data

root@may:~#

以上操作历史可见：groups 后接用户名，显示用户所在的组别，可能还未生效。直接执行 groups 返回当前用户的组别，当前的权限范围内的组别，也许一退出就发生变化。