Linux file permissions control who can read, write, or execute files, and are managed through chmod and chown commands. 1. File permissions are divided into owner, group and other users. Each type of user has three permissions: read (r), write (w), and execute (x); 2. Use chmod to modify permissions through symbolic mode (such as chmod ux file) or digital mode (such as chmod 644 file); 3. Use chown to change the owner and group of the file (such as chown user:group file), which requires administrator permission; 4. In terms of security, chmod 777 should be avoided. It is recommended to allocate permissions reasonably and modify directory permissions recursively using the -R parameter.

In Linux systems, file permissions control who can read, write, or execute a file. If you need to modify these permissions, you will usually use chmod and chown commands. Understanding and using them correctly can help you manage files and directories more securely.

What are file permissions?

Each file in Linux has three types of user permission settings: owner , group , and other users . Each user has three permissions:

• r(read) : Allows reading file content or listing files in directories.
• w(write) : Allows to modify file content or create/delete files in a directory.
• x(execute) : Allows running files (such as scripts or programs) or entering directories.

You can view the permission settings of the current file through ls -l , for example:

 -rw-r--r-- 1 user group 0 Apr 5 10:00 file.txt

This means that the owner can read and write, and groups and others can only read.

How to modify permissions using chmod

The chmod command is used to change access rights to files or directories. There are two common methods: symbolic mode and digital mode.

Symbolic Mode

The syntax is: chmod [用戶][操作符][權限] 文件名

• Users can be:

  • u : Owner
  • g : Group
  • o : Others
  • a : Everyone (default)

• Operator:

  • : Add permissions
  • - : Remove permissions
  • = : Set permissions

Example:

 chmod ux script.sh # Add execution permissions to the owner chmod go-w file.txt # Remove write permissions from groups and others

Digital Mode

Each permission can be represented by numbers:

• r = 4, w = 2, x = 1

Combined it is:

• rw- = 6 (4 2)
• rx = 5 (4 1)
• rwx = 7 (4 2 1)

Common usage:

 chmod 644 file.txt # The owner can read and write, others read only chmod 755 directory/ # The owner can be fully controlled, others can read and execute

How to change file owners and groups

Sometimes you need to change the owner or group of the file, and then use the chown command.

Basic syntax:

 chown [Options] [User][:Group] File

Common usage:

 chown user file.txt # Change owner chown :group file.txt # Change the group to which chown user:group file.txt # Change both owner and group

Note: Ordinary users cannot change the owner of the file at will, and they usually require administrator rights (sudo) to operate.

Safety tips and precautions

• Don't give all users "write" or "execute" permissions easily, especially for system files or configuration files.
• Although it is convenient to use chmod 777 , it is very unrecommended because it allows everyone to modify and execute the file, which poses a security risk.
• When modifying directory permissions, adding the -R parameter can be recursively applied to all subfiles and subdirectories in the directory:

   chmod -R 755 mydir/

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  Basically that's it. By mastering chmod and chown , you can control the access permissions of the Linux file system well.

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