Summary of Linux file basic attributes knowledge points

The Linux system is a typical multi-user system. Different users are in different positions and have different permissions. In order to protect the security of the system, the Linux system has different regulations on the permissions for different users to access the same file (including directory files).

In Linux, we can use the ll or ls –l command to display the attributes of a file and the user and group to which the file belongs, such as:

[root@www /]# ls -l
total 64
dr-xr-xr-x 2 root root 4096 Dec 14 2012 bin
dr-xr-xr-x 4 root root 4096 Apr 19 2012 boot
…

In the example, the first attribute of the bin file is represented by "d". "d" in Linux means that the file is a directory file.

In Linux, the first character represents whether the file is a directory, file, link file, etc.

• When [d], it is a directory
• When [-], it is a file;
• If [l], it means a link file;
• If [b], it means an interface device (random access device) in the device file that can be used for storage;
• If [c], it indicates a serial port device in the device file, such as a keyboard or mouse (a one-time read device).
• The following characters are grouped in groups of three and are all combinations of the three parameters of "rwx". Among them, [r] stands for readable (read), [w] stands for writable (write), and [x] stands for executable (execute). It is important to note that the positions of these three permissions will not change. If there are no permissions, a minus sign [-] will appear instead.

The properties of each file are determined by the 10 characters in the first part on the left (as shown below).

From left to right, they are represented by numbers 0-9.

Bit 0 determines the file type, and bits 1-3 determine the permissions the owner (the file's owner) has on the file.

Bits 4-6 determine the permissions that the group (users in the same group as the owner) has for the file, and bits 7-9 determine the permissions that other users have for the file.

Among them, the 1st, 4th, and 7th bits represent read permissions. If they are represented by the "r" character, read permissions are granted. If they are represented by the "-" character, read permissions are denied.

The 2nd, 5th, and 8th bits represent write permissions. If they are represented by the "w" character, there is write permission. If they are represented by the "-" character, there is no write permission. The 3rd, 6th, and 9th bits represent executable permissions. If they are represented by the "x" character, there is execute permission. If they are represented by the "-" character, there is no execute permission.

Linux file owner and group

[root@www /]# ls -l
total 64
drwxr-xr-x 2 root root 4096 Feb 15 14:46 cron
drwxr-xr-x 3 mysql mysql 4096 Apr 21 2014 mysql
…

For a file, it has a specific owner, that is, the user who has ownership of the file.

At the same time, in the Linux system, users are classified into groups, and a user belongs to one or more groups.

Users other than the file owner can be divided into users in the same group as the file owner and other users.

Therefore, the Linux system defines different file access permissions according to the file owner, users in the same group as the file owner, and other users.

In the above example, the mysql file is a directory file. Its owner and group are both mysql. The owner has read, write, and execute permissions. Other users in the same group as the owner have read and execute permissions. Other users also have read and execute permissions.

For the root user, file permissions generally have no effect on them.

Changing file attributes

1. chgrp: change the file group

grammar:

chgrp [-R] group name file name

Parameter options

-R: Recursively change the file group. That is, when changing the group of a file in a directory, if you add the -R parameter, the group of all files in the directory will be changed.

2. chown: change the file owner, and you can also change the file group at the same time

grammar:

chown [–R] owner name file name
chown [-R] owner name:group name file name

Enter the /root directory (~) and change the owner of install.log to the bin account:

[root@www ~] cd ~
[root@www ~]# chown bin install.log
[root@www ~]# ls -l
-rw-r--r-- 1 bin users 68495 Jun 25 08:53 install.log

Change the owner and group of install.log back to root:

[root@www ~]# chown root:root install.log
[root@www ~]# ls -l
-rw-r--r-- 1 root root 68495 Jun 25 08:53 install.log

3. chmod: change 9 file attributes

There are two ways to set Linux file attributes, one is numbers and the other is symbols.

There are nine basic permissions for Linux files, namely owner/group/others, each with its own read/write/execute permissions.

Let’s review the data just mentioned above: the file permission characters are: "-rwxrwxrwx", and these nine permissions are in groups of three! Among them, we can use numbers to represent each permission, and the score comparison table of each permission is as follows:

r:4
w:2
x:1

The scores of the three permissions (r/w/x) of each identity (owner/group/others) need to be accumulated. For example, when the permissions are: [-rwxrwx---], the scores are:

owner = rwx = 4+2+1 = 7
group = rwx = 4+2+1 = 7
others= --- = 0+0+0 = 0

So when we set the permission changes later, the permission number of the file will be 770! The syntax of the chmod command to change permissions is as follows:

chmod [-R] xyz file or directory

Options and parameters:

xyz: This is the numeric permission attribute just mentioned, which is the sum of the rwx attribute values.

-R: Perform recursive changes, that is, all files in the subdirectory will be changed

For example, if you want to enable all permissions of the .bashrc file, the command is as follows:

[root@www ~]# ls -al .bashrc
-rw-r--r-- 1 root root 395 Jul 4 11:45 .bashrc
[root@www ~]# chmod 777 .bashrc
[root@www ~]# ls -al .bashrc
-rwxrwxrwx 1 root root 395 Jul 4 11:45 .bashrc

What if you want to change the permissions to -rwxr-xr--? Then the authority score becomes [4+2+1][4+0+1][4+0+0]=754.

There is another way to change the permissions of files. From the previous introduction, we can find that there are basically nine permissions:

(1)user
(2)group
(3) others

Then we can use u, g, o to represent the permissions of three identities!

In addition, a stands for all, which means all identities. Read and write permissions can be written as r, w, x, which can be viewed using the following table:

If we need to set the file permissions to -rwxr-xr--, we can use chmod u=rwx,g=rx,o=r filename to set it:

# touch test1 // Create test1 file# ls -al test1 // View test1 default permissions -rw-r--r-- 1 root root 0 Nov 15 10:32 test1
# chmod u=rwx,g=rx,o=r test1 // Modify test1 permissions # ls -al test1
-rwxr-xr-- 1 root root 0 Nov 15 10:32 test1

What if you want to remove permissions without changing other existing permissions? For example, to remove the executable permission for everyone, you would:

# chmod ax test1
# ls -al test1
-rw-r--r-- 1 root root 0 Nov 15 10:32 test1

The above is the full content of this article. I hope it will be helpful for everyone’s study. I also hope that everyone will support 123WORDPRESS.COM.