[root@localhost ~]# cat /etc/passwd
root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
adm:x:3:4:adm:/var/adm:/sbin/nologin
lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin
sync:x:5:0:sync:/sbin:/bin/sync
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
halt:x:7:0:halt:/sbin:/sbin/halt
mail:x:8:12:mail:/var/spool/mail:/sbin/nologin
operator:x:11:0:operator:/root:/sbin/nologin
games:x:12:100:games:/usr/games:/sbin/nologin
ftp:x:14:50:FTP User:/var/ftp:/sbin/nologin
nobody:x:99:99:Nobody:/:/sbin/nologin
systemd-network:x:192:192:systemd Network Management:/:/sbin/nologin
dbus:x:81:81:System message bus:/:/sbin/nologin
polkitd:x:999:998:User for polkitd:/:/sbin/nologin
libstoragemgmt:x:998:997:daemon account for libstoragemgmt:/var/run/lsm:/sbin/nologin
abrt:x:173:173::/etc/abrt:/sbin/nologin
rpc:x:32:32:Rpcbind Daemon:/var/lib/rpcbind:/sbin/nologin
apache:x:48:48:Apache:/usr/share/httpd:/sbin/nologin
sshd:x:74:74:Privilege-separated SSH:/var/empty/sshd:/sbin/nologin
postfix:x:89:89::/var/spool/postfix:/sbin/nologin
ntp:x:38:38::/etc/ntp:/sbin/nologin
chrony:x:997:995::/var/lib/chrony:/sbin/nologin
tcpdump:x:72:72::/:/sbin/nologin
quail:x:1000:1000:quail:/home/quail:/bin/bash

[root@localhost ~]# cat /etc/shadow
root:$6$ONr1v9vJOt8CnljA$xyqJy3PJUyDP7CIfUheuSa1MWjISSr4KRi.AHIEw8DCdVYQa36HjJ5TkFYnRinoiOu.kvmRKFs43HfMXz4UQi1::0:99999:7:::
bin:*:17834:0:99999:7:::
daemon:*:17834:0:99999:7:::
adm:*:17834:0:99999:7:::
lp:*:17834:0:99999:7:::
sync:*:17834:0:99999:7:::
shutdown:*:17834:0:99999:7:::
halt:*:17834:0:99999:7:::
mail:*:17834:0:99999:7:::
operator:*:17834:0:99999:7:::
games:*:17834:0:99999:7:::
ftp:*:17834:0:99999:7:::
nobody:*:17834:0:99999:7:::
systemd-network:!!:18085::::::
dbus:!!:18085::::::
polkitd:!!:18085::::::
libstoragemgmt:!!:18085::::::
abrt:!!:18085::::::
rpc:!!:18085:0:99999:7:::
apache:!!:18085::::::
sshd:!!:18085::::::
postfix:!!:18085::::::
ntp:!!:18085::::::
chrony:!!:18085::::::
tcpdump:!!:18085::::::
quail:$6$Kc3NU.IzbnLHJB/w$UQPSQJng5IAt4a0zn.be9QP/WWay3p8ou.hA.o/hr8mIqYT0UX7Gn1MTyzOsfioBL4ere3l4bYbEyI.HUspHb.::0:99999:7:::

　　字段解析: (用户名):(加密密码):(密码最后更新时间):(修改间隔):(密码时效性不):(快到期警告时间):(到期宽限时 间):(账号失效时间):(保留)

[root@localhost ~]# cat /etc/group
root:x:0:
bin:x:1:
daemon:x:2:
sys:x:3:
adm:x:4:
tty:x:5:
disk:x:6:
lp:x:7:
mem:x:8:
kmem:x:9:
wheel:x:10:
cdrom:x:11:
mail:x:12:postfix
man:x:15:
dialout:x:18:
floppy:x:19:
games:x:20:
tape:x:33:
video:x:39:
ftp:x:50:
lock:x:54:
audio:x:63:
nobody:x:99:
users:x:100:
utmp:x:22:
utempter:x:35:
input:x:999:
systemd-journal:x:190:
systemd-network:x:192:
dbus:x:81:
polkitd:x:998:
libstoragemgmt:x:997:
ssh_keys:x:996:
abrt:x:173:
rpc:x:32:
apache:x:48:
sshd:x:74:
slocate:x:21:
postdrop:x:90:
postfix:x:89:
ntp:x:38:
chrony:x:995:
tcpdump:x:72:
stapusr:x:156:
stapsys:x:157:
stapdev:x:158:
quail:x:1000:quail
字段解析: 用户组名：密码：GID号：组中附加的用户

[root@localhost ~]# 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 

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

[root@localhost home]# ll test.txt 
-rwxrw-r--. 1 root root 107 Jul 10 22:16 test.txt
#上面目录权限分解： 
#U：rwx----->user------>root 用户对于test.txt拥有的权限（属主） 
#G：rw------>group------>root 组对于text.txt拥有的权限 （属组） 
#O：r--------other-------> *  其他用户对于test.txt拥有的权限 （其他人）

[root@localhost home]# ll test
drwxrw-r--. 2 root root 107 Jul 10 22:16 test

· d 表示目录，同一位置如果为-则表示普通文件；
· rwxrw-r--表示三种角色的权限，每三位为一种角色，依次为U，G，O权限，如上则表示user的权限为rwx， group的权限为rw-，other的权限为r--；
· 2表示文件夹的链接数量，可理解为该目录下子目录的数量（目录的链接数量至少为2.因为内含：.和..两个特殊目录)
· 从左到右，第一个root表示该用户名(文件所属主，对应前面三位一体权限的“user”)，第二个root则为组名 （文件所属组，对应前面三位一体权限的“group”），其他人角色默认不显示（其他人权限，对应前面三位一体权限 的“other”）； 
· 107表示该文件夹占据的字节数； 
· Jul 10 22:16 表示文件创建或者修改的时间；
· test 为目录的名，或者文件名。