/etc/login.defs 文件为多个用户帐户参数提供默认配置信息。
useradd、usermod、userdel 和 groupadd 命令以及其他用户和组实用程序从该文件中获取默认值。
每行包含一个指令名称和相关值。

下面是来自 CentOS 7 系统的示例 /etc/logins.defs 文件。

# 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

以下是 /etc/login.defs 指令的部分列表：

• 用户邮箱的位置
• 密码时效控制
• 自动 UID 选择的最小值和最大值（1000 到 60000）
• 自动 GID 选择的最小值和最大值（1000 到 60000）
• 添加新用户时是否应创建主目录
• 默认 umask
• 用于加密密码的加密方法

如果 /etc/login.defs 中的 USERGROUPS_ENAB 指令设置为 YES ，则会为用户创建一个与用户名同名的组。
如果该指令设置为 NO ，则 useradd 命令将新用户的主要组设置为 /etc/default/useradd 文件中 GROUP 指令指定的值，或者默认为 100。