添加新用户与以下文件有关联：

/etc/default/useradd

[root@localhost pam.d]# cat /etc/default/useradd
# useradd defaults file
GROUP=100
HOME=/home
INACTIVE=-1
EXPIRE=
SHELL=/bin/bash
SKEL=/etc/skel
CREATE_MAIL_SPOOL=yes

/etc/skel/下面所有文件会复制到新建用户家目录下

/etc/login.defs

[root@localhost pam.d]# 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 500
UID_MAX 60000

#
# Min/max values for automatic gid selection in groupadd
#
GID_MIN 500
GID_MAX 60000

#
# 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

useradd umask报错是因为用户在/etc/login.defs后面添加 了UMASK  0022

root用su 切换到普通用户提示输入密码并报密码错误 可以查看/var/log/secure /var/log/messages报错信息 用户认证与/etc/pam.d/password-auth /etc/pam.d/sshd /etc/pam.d/su /etc/pam.d/sudo相关