chmod is a command in Linux to modify file or directory permissions. To understand its usage, you need to master the permission representation method and user classification; permissions are divided into read (r), write (w), and execute (x), and users are divided into owner (u), group (g), and others (o); permissions can be set by character mode or numeric mode; 1. Character mode specifies user categories through u, g, o, a, combined with, -, = operation permissions; 2. The digital mode uses three-digit number to represent permissions, 4 is read, 2 is write, and 1 is execution, and after combination, it means 7 is used to represent rwx; 3. It is recommended to check the current permissions before modification to avoid misoperation, and use others to write permissions with caution. Sensitive files can be set with permissions recursively by the -R parameter.

In Linux systems, chmod is a command used to modify file or directory permissions. Understanding and using chmod correctly is very important for ensuring system security and collaborative development. It is actually not difficult to master its usage. The key is to understand how permissions are represented and the meaning of permissions of different users.

What are permissions: read, write, execute

Each file in Linux has three types of user permission settings: owner (user), group (group) and others . Each type of user can have the following three permissions:

• Read (r) : Allows viewing file content or listing files in a directory
• Write (w) : Allows to modify file content or delete/rename files in the directory
• Execution (x) : Allows running files (such as scripts or programs), or to enter a directory

For example, permission -rwxr-xr-- means:

• The owner can read, write, and execute
• Group members can read and execute
• Others can only read

These permissions can be modified by characters or numbers.

How to modify permissions using chmod: Character mode

Character mode is suitable for beginners to understand the process of permission changes. The basic format is:

 chmod [user category][operator][permission] file name

Common user categories include:

• u : Owner
• g : Group
• o : Others
• a : Everyone (equivalent to ugo)

Operator:

• : Add permissions
• - : Remove permissions
• = : Set to a specific permission

To give a few examples:

• Add execution permissions to the file owner:

   chmod ux filename

• Remove the write permissions of group members:

   chmod gw filename

• All people only reserve read permissions:

   chmod a=r filename

This method is intuitive and easy to understand and is suitable for partial adjustments to permissions.

Set permissions with numbers: simple and efficient

Permissions can also be represented by three digits, each representing the permission value of a type of user:

• 4 means reading (r)
• 2 means writing (w)
• 1 means execution (x)

Combining these three can get the corresponding permission values:

for example:

• Set file permissions to rwxr-xr-- :

   chmod 754 filename

This method is more suitable for use in scripts or when you need to set complete permissions at once.

Common precautions and suggestions

• It is best to use ls -l filename to view the current permission before modifying the permissions to avoid misoperation.
• Don't easily write permissions to "other people", especially sensitive configuration files or log files.
• If you want all files and subdirectories in the directory to apply the same permissions, you can add the -R parameter to recursively set:

   chmod -R 755 directoryname

• Some systems or services require specific permissions to work properly, such as web directory is generally set to 755 , while upload directory may be set to 775 or 777 (use with caution).

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Basically that's it. After mastering the basic usage of chmod , you can better manage file security in Linux systems.

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