配置文件
在Linux中新建用户，如果不加任何参数，新建用户会自带一些属性。
这些自带属性就是通过/etc/default/useradd中设定的。
[22:11:24 root@C8-3-55 ~]#cat /etc/default/useradd
# useradd defaults file
GROUP=100 ## 若不创建主组，则默认主组为user组（100）
HOME=/home ## 若不特殊指定家目录，则默认新建家目录为用户名同名在/home下的目录
INACTIVE=-1 ##/etc/shadow 第7列 密码有效期
EXPIRE= ##/etc/shadow 第8列 账号有效期
SHELL=/bin/bash ## 默认shell
SKEL=/etc/skel ## 用户创建时自带家目录隐藏配置文件，模板文件夹
CREATE_MAIL_SPOOL=yes ## 启用邮件池
同样可以使用useradd -D命令查看。
实例
当你新创建用户后，使用户家目录自带某些文件。
答：在SKEL=/etc/skel目录下增删改相关文件即可。
用户初建相关其他默认设置全局配置文件
[22:26:45 root@C8-3-55 ~]#cat /etc/login.defs
#
# Please note that the parameters in this configuration file control the
# behavior of the tools from the shadow-utils component. None of these
# tools uses the PAM mechanism, and the utilities that use PAM (such as the
# passwd command) should therefore be configured elsewhere. Refer to
# /etc/pam.d/system-auth for more information.
#
# *REQUIRED*
# Directory where mailboxes reside, _or_ name of file, relative to the
# home directory. If you _do_ define both, MAIL_DIR takes precedence.
# QMAIL_DIR is for Qmail
#
#QMAIL_DIR Maildir
MAIL_DIR /var/spool/mail
#MAIL_FILE .mail
# Password aging controls:
#
# PASS_MAX_DAYS Maximum number of days a password may be used.
# PASS_MIN_DAYS Minimum number of days allowed between password changes.
# PASS_MIN_LEN Minimum acceptable password length.
# PASS_WARN_AGE Number of days warning given before a password expires.
#
PASS_MAX_DAYS 99999
PASS_MIN_DAYS 0
PASS_MIN_LEN 5
PASS_WARN_AGE 7
#
# Min/max values for automatic uid selection in useradd
#
UID_MIN 1000
UID_MAX 60000
# System accounts
SYS_UID_MIN 201
SYS_UID_MAX 999
#
# Min/max values for automatic gid selection in groupadd
#
GID_MIN 1000
GID_MAX 60000
# System accounts
SYS_GID_MIN 201
SYS_GID_MAX 999
#
# If defined, this command is run when removing a user.
# It should remove any at/cron/print jobs etc. owned by
# the user to be removed (passed as the first argument).
#
#USERDEL_CMD /usr/sbin/userdel_local
#
# If useradd should create home directories for users by default
# On RH systems, we do. This option is overridden with the -m flag on
# useradd command line.
#
CREATE_HOME yes
# The permission mask is initialized to this value. If not specified,
# the permission mask will be initialized to 022.
UMASK 077
# This enables userdel to remove user groups if no members exist.
#
USERGROUPS_ENAB yes
# Use SHA512 to encrypt password.
ENCRYPT_METHOD SHA512