Basic knowledge of Linux: User, password, file access permission and security

User main directory, Super User and other users added
For general users, the write operation on the hard disk may only have their own home directory, which is located under "/home/user name.
The/home directory stores all user files, including user settings, program configuration files, documents, data, netscape cache files, and user emails. Normal users can only create new sub-directories in their home directories to organize your files. In addition, other users do not have the permission to read or write the content in your home directory without having the permission granted to other users.
Except for the home directory, users can search, read, and execute files in other directories in the system. However, they generally cannot modify or move these files.
A Super User (also known as "root") is a special account with the power to modify any file in the system. In daily work, it is best not to use a Super User Account to enter the system, because any wrong operation may cause huge losses. A Super User Account is the only account provided after the system is created. Therefore, you need to create and use a general user account for daily work.
Super User root can create a new user account. For example, the following command creates a new user named joe.

# Adduser joe # passwd joe (enter the password of joe)

Root can modify the password of any user without knowing the user's current password (the password is encrypted using a one-way encryption algorithm, and save the encryption result in the file/etc/passwd, but the original password is not saved ). When you register, the entered password is calculated by a one-way encryption algorithm and the result is compared with the value saved in/etc/passwd.
Linux uses a policy that separates administrators from general users to ensure system robustness, at the same time, it also makes it difficult to write viruses in Linux (the program you write only has the write permission on your own directory, and is isolated from other parts of the operating system ).
Generally, you need to modify your password immediately when registering for the first time. The command is as follows:

$ Passwd (current) Unix Password [enter the old Password] New Unix password [enter a New Password] Retype New UNIX Password [enter a New password again]

For security reasons, the password you typed is not displayed on the screen.
Password Security
Vulnerable passwords are the primary cause of system insecurity. The following are examples of insecure passwords.
1. Use the word "password" as the password;
2. Use the name or Registration Name of yourself or others as the password;
3. Use the company name, Department name, or group name as the password;
4. Use birthday as a password;
5. Write the password on the calendar or beside the computer;
6. Use words or frequently used words in a dictionary;
A good password should contain at least six letters, including letters and numbers, and should be modified frequently.
The system administrator can configure a program to set a password security policy. For example, you can start the linuxconf utility as root:
# Linuxconf
In the menu, configure the password under user account authentication policies password & account authentication ies.

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