A little Linux knowledge every day: file permissions and chmod

In Linux systems, file and directory permissions are crucial concepts. Properly understanding and setting these permissions can protect system security and data integrity. This article will explore file permissions in Linux in detail and explain how to use the chmod command to modify these permissions.

1. Overview of file permissions

In the Linux system, each file and directory is given specific permissions. These permissions determine which users can access, modify or perform operations. There are three main types of file permissions: read, write, and execute permissions.

Read permission (r): Allow the file to be read.

Write permission (w): Allow the file to be modified.

Execute permissions (x): Allow the file to be executed (for scripts or program files).

These permissions can be granted to three different user categories:

File Owner (u): The creator or owner of the file.

Group to which the file belongs (g): Other members of the user group where the file owner belongs.

Other users (o): Other system users who do not belong to the file owner or the group to which the file belongs.

2. View file permissions

Use the ls -l command to view detailed permission information of the file. For example:

$ ls -l example.txt
-rw-r--r-- 1 user group 1234 March 21 10:00 example.txt

The output here means:

The file type is an ordinary file (indicated by the first character "-").

The file owner (user) has read (r) and write (w) permissions.

The group to which the file belongs and other users (others) only have read (r) permissions.

3. Use the chmod command to modify file permissions

The chmod command is used to modify the permissions of files or directories. It has two basic uses: symbolic mode and octal mode.

1. Symbol mode

Symbol mode uses character operators to add, remove, or set permissions. Operators include:

: Add permissions.

-: Delete permissions.

=: Set permissions and delete all other permissions.

For example, to add execution permissions to the file owner, you can do this:

$ chmod u+x example.txt

2. Octal mode

Octal mode uses three octal numbers to represent the permissions of the file owner, the group to which the file belongs, and other users. Each number is a value between 0 and 7, corresponding to different permission combinations:

4: Read permission.

2: Write permission.

1: Execution permission.

For example, to set the file permissions so that the file owner has read, write, and execute permissions, and the group to which the file belongs and other users only have read permissions, you can do this:

$ chmod 755 example.txt

Here, 7 (4 2 1) means that the file owner has read, write, and execute permissions, and 5 (4 1) means that the group to which the file belongs and other users have read and execute permissions.

4. Precautions

Be careful when modifying file permissions to ensure that you do not accidentally deprive yourself of necessary permissions or grant too many permissions, which may affect the security and stability of the system.

For system files and directories, it is generally recommended to retain the default permission settings unless there is a clear reason to modify them.

Through today's introduction, I believe you have a deeper understanding of file permissions and chmod commands in Linux. In daily use, reasonable use of these knowledge and tools can help you better manage file and directory permissions in Linux systems.

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