Linux system user rights pipe and directory, file management

User accounts and group accounts

Linux system is controlled access to resources based on user identity, Linux user account into the following three categories:

• Supervisor : that is the root user, all users it has the largest authority, manages the ordinary user, you can say you have a root access to the server, then this server basically is yours.
• Average user : general user, its authority is limited use of the system, you can use the root account for its management.
• User program : also known as system users, to protect the user of system operation, generally not man-made log into the system.

Common set of accounts are generally divided into two types:

• Basic set : Also called private group, generally created when the default system account for each account creates a basic group with the same name as their own.
• Additional groups : also called public groups, are generally created by the administrator account to manage user rights.

UID and GID:

• The UID : user identification number (User IDentity)
• The GID : group ID, (Group IDentity)

  User accounts associated with two files:

• User accounts file / etc / passwd: save user name, host directory, login shell and other basic information.
  
  The above is the basic format I cut out of the / etc / passwd file, Here I introduce the contents of each part represented by:

  root：用户名称
  x：密码占位符，密码保存在/etc/shadow文件内
  0：用户id，UID
  0：组id，GID
  root：注释信息
  /root：用户家目录
  /bin/bash：用户默认使用shell

  The root UID / GID is 0; the program user UID / GID is 1 to 499; normal user UID / GID from 500 to 60,000.

• User accounts file / etc / shadow: save the user's password, account expiration date information.
  
  The above is the basic format I cut out of the / etc / shadow file, Here I introduce the contents of each part represented by:

jiang:用户名称
$6$lA9A.3a/aE$5V.E3sipR6H1bcdNg.5uQOGeS0M6NDcoqinrfMI9f3g\
>rWYXZHZgGyCRYZ1cRifI1/5tIy5hcX3UtGsncq2jYr1:用哈希（Hash）算法加密过的密码
18117:用户创建的日期距离1970-01-01多少天（1970-01-01是官方的Unix启动时间）
0:表示密码的最短使用期限（天）
99999:表示密码的失效时间（天）
7:表示失效前多少天提醒用户

User Action: add users, change passwords, modify and remove users

• Adding user accounts: useradd command (Format: useradd ... [option] ... username)

  [选项]：
  -u-----------------UID
  -d-----------------用户的家目录
  -e-----------------失效时间（天）    格式：YYYY-MM-DD（如：1990-10-11）
  -g-----------------基本组
  -G-----------------附加组
  -M----------------不创建家目录
  -s-----------------shell登录（可以登录：/bin/bash；无法登录：/sbin/nologin）

Set / change the user password: passwd command (Format: passwd ... [option] ... username)

[选项]：
-d----------------------------删除密码
-l-----------------------------停止账号使用
-S（大写）---------------显示密码信息
-u---------------------------启用已被停止的账户
不加选项直接跟用户，是更改用户密码

Modify user account properties: the usermod command (Format: usermod ... [option] ... username)

[选项]：
-l（小写）-----------------------修改账号名称
-L（大写）----------------------锁定用户账号
-U（大写）---------------------解锁用户账号

There are other -u, -d, -e, -g, -G, -s option of the useradd command the same meaning

To delete a user account: userdel command (Format: userdel ... [- r] ... username)
when you add the -r option, even with the means to delete the user's home directory.

Added: switch user command: su [-] user name. Plus "-" indicates to maintain the current path access to other users, without "-" switch user to enter the user's home directory.

Group accounts related documents

Group account file with the user account file is similar to:

• / etc / group: save the group account basic information
  
• : / etc / gshadow password information stored group account

Add a group account: groupadd command (Format: group ... [- g GID] ... group account name)

add / delete group members: gpasswd command (Format: gpasswd ... [option] ... group account name)
this command can also be used to set the group account password, but with very little.

选项：
-a------------------向组内添加一个用户
-d------------------从组内删除一个用户
-M-----------------定义组成员列表，以逗号“，”分隔（同时向组内添加多个成员，但会覆盖之前添加的成员）

To delete a group account: groupdel command (Format: groupdel ... group account name)

Query account information:

groups命令：查询用户所属的组（格式：groups...[用户名]）

id命令：查询用户身份标识（格式：id...[用户名]）

finger命令：查询用户账号的详细信息（格式：finger...[用户名]）


w、who、users命令：查询已登录到主机的用户信息

文件目录的权限和归属：

访问权限：

• 读取r：允许查看文件内容、显示目录列表（对应数字4）
• 写入w：允许修改文件内容，允许在目录中新建、移动、删除文件或子目录（对应数字2）
• 可执行x：允许运行程序、切换目录（对应数字1）
  归属（所有权）：
• 属主：拥有该文件或目录的用户账号
• 属组：拥有该文件或目录的组账号

查看文件、目录的权限和归属：
一般用ls -l 命令去查看文件、目录的权限和归属，可以在目录下直接输入“ls -l”命令查看当前目录下的子目录与文件的权限和归属，也可以直接用“ls -l 目录名/文件名”直接查看你想看的目录或文件的权限与归属。

下面我截取一个文件的信息来仔细解释每个部分所代表的含义：
-rw-------. 1 root root 1801 8月 9 19:32 anaconda-ks.cfg
将这个文件的信息进行分解：

- ； rw- ； --- ； ---. ； 1 ； root ； root ； 1801 ； 8月 9 19:32 ； anaconda-ks.cfg

-：表示文件的类型（-表示普通文件、d表示目录文件、l表示链接文件、b设备文件）
rw-：表示文件属主对该文件拥有的权限（r表示读取权限、w表示写入权限、x表示执行权限、rwx三个权限的顺序固定，-在哪个位置就表示不具备该权限）
---：表示属组对该文件拥有的权限
---：表示所有人对该文件的拥有权限
1：表示硬链接的数量
root：表示文件的属主
root：表示文件的属组
1801：表示文件大小，单位字节
8月  9  19:32 ：表示文件创建时间
anaconda-ks.cfg：表示文件名

设置文件和目录的权限：
chmod命令（有两种格式，加上“-R”表示递归修改目录下所有子项的权限）

• Format 1: chmod ... [u, g , o, a] [+, -, =] [r, w, x] .... file / directory
  u, g, o, a, respectively owner, is group, other users, all users
  +, -, =, respectively added, removed, set permissions
  r, w, x represent read, write, execute permissions
  
  
• Format 2: chmod ... nnn .... file / directory ( this format is more convenient than the first, this format is recommended )
  n, n, n is three octets, n represents the first genus owner permissions, group permissions second represents genera, third means permission of all
  n corresponds to the number for each permission and only read authority is n = 4, has read and write access is n = 6, has all privileges then n = 7, n = 0 then do not have permission
  
  set file ownership and directories:
  chown command (add "-R" recursively modifying the home directory of all children)
  format: chown owner ... [: is a group] ... file or directory

Umask permissions mask:
the role of umask: the
control of the newly created file or directory permissions
default permissions umask permission to remove the permissions on the new file or directory
is set umask: command "umask ... nnn", and three n chmod command the same
umask View: command "umask", directly enter the "umask" you can view the current permission mask