主题：介绍Shadow软件包及其中的命令的简单介绍。

Shadow，该软件包中包含的程序与系统的密码处理有关，这些程序提供了一种处理密码的安全方式。

项目主页：https://github.com/shadow-maint/shadow

包含的命令列表

chage
Used to change the maximum number of days between obligatory password changes

chfn
Used to change a user’s full name and other information

chgpasswd
Used to update group passwords in batch mode

chpasswd
Used to update user passwords in batch mode

chsh
Used to change a user’s default login shell

expiry
Checks and enforces the current password expiration policy

faillog
Is used to examine the log of login failures, to set a maximum number of failures before an account is blocked, or to reset the failure count

gpasswd
Is used to add and delete members and administrators to groups

groupadd
使用给出的组名来创建新组。

groupdel
Deletes the group with the given name

groupmems
Allows a user to administer his/her own group membership list without the requirement of super user privileges.

groupmod
Is used to modify the given group’s name or GID

grpck
Verifies the integrity of the group files /etc/group and /etc/gshadow

grpconv
Creates or updates the shadow group file from the normal group file

grpunconv
Updates /etc/group from /etc/gshadow and then deletes the latter

lastlog
Reports the most recent login of all users or of a given user

login
Is used by the system to let users sign on

logoutd
Is a daemon used to enforce restrictions on log-on time and ports

newgidmap
Is used to set the gid mapping of a user namespace

newgrp
Is used to change the current GID during a login session

newuidmap
Is used to set the uid mapping of a user namespace

newusers
Is used to create or update an entire series of user accounts

nologin
Displays a message that an account is not available; it is designed to be used as the default shell for accounts that have been disabled

passwd
Is used to change the password for a user or group account

pwck
Verifies the integrity of the password files /etc/passwd and /etc/shadow

pwconv
Creates or updates the shadow password file from the normal password file

pwunconv
Updates /etc/passwd from /etc/shadow and then deletes the latter

sg
Executes a given command while the user’s GID is set to that of the given group

su
使用替代用户和组ID运行SHELL

useradd
使用给定的名字创建用户，或者更新默认的新用户信息。

userdel
Deletes the given user account

usermod
用于修改帐号的登录名，UID，SHELL，组，家目录等等。

vigr
修改/etc/group文件或者/etc/gshadow文件。

vipw
修改/etc/passwd文件或者/etc/shadow文件。