Linux common commands chmod to modify file permissions 777 and 754

The following command is often used:

chmod 777 文件或目錄

Example: chmod 777 /etc/squid After running the command, the permissions of the squid folder (directory) are changed to 777 (readable, writable, and executable).

If it is Ubuntu system, you may need to add sudo to execute:

sudo chmod 777 /etc/squid

At the beginning of the story, there will always be a suspense.

In the series of jokes that only programmers can understand, there is a classic one:

Please describe our country's FL in the most concise language.

754.

So, what does 754 mean? What does 754 mean? What does 754 mean?

The following is a detailed introduction to the chmod command.

In the Linux system, the roles and permissions of each user are divided very carefully and strictly. Each file (directory) has access permission. This mechanism is used to determine whether a user can read, write, execute, and perform other operations on the file (directory) in a certain way.

There are three different types of users who operate files or directories: file owners, group users, and other users. The highest bit represents the permission value of the file owner, the middle bit represents the permission value of the group user, and the lowest bit represents the permission value of other users. Therefore, in chmod 777, the three numbers 7 correspond to the above three users, and the permission values are all 7.

There are three types of file or directory permissions: read-only, write-only, and executable.

Permissions	Permission Value	Binary	Specific role
r	4	00000100	read, read. The current user can read the contents of the file and browse the directory.
w	2	00000010	write, write. The current user can add or modify file contents, and the current user can delete or move directories or files within directories.
x	1	00000001	execute, execute. The current user can execute files and enter directories.

According to the table above, the permission combination is the sum of the corresponding permission values, as follows:

7 = 4 + 2 + 1 Read, write and execute permissions
5 = 4 + 1 Read and execute permissions

4 = 4 read-only permission

Therefore, everyone understands the meaning of the chmod 754 filename command.

This command means to grant the read, write and run permissions of the filename file to the file owner, grant the read and run permissions to the group users, and grant the read permission to other users.

For more official and detailed explanations, you can use the following command to view:

chmod --help or man chmod

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