Detailed explanation of soft links and hard links in Linux

Tip: Let's first explain the basic storage of files and directories in the Linux system, which will make it easier for us to understand and learn hard links and soft links in the Linux system.

1. Basic storage of files and directories

We have talked about partitions before. Each partition can be understood as being divided into two parts. A small part stores the i node information of the file, and a large part stores the Block file blocks.

The information stored in the i-node of a file includes: i node number, time, permissions, and save location. Block stores the data in the file.

Each folder will have i node and a corresponding Block file.

But there is a question, where is the file name of a file stored?

Let's take a file abc in the folder root as an example. The file path is /root/abc .

In the root directory, all files have i node names and corresponding Block blocks, and root directory also has its own i node information. Wherever there is i node information, there is a corresponding Block block. The data stored in Block block of root directory is the file names of all sub-files and sub-directories under root directory, and its corresponding i node number.

In other words, the file name of the file in root directory is not written in the i node of the file. Instead, it is written in Block block of the parent directory.
If I want to find the file name of the abc file, I must first find the i node of root folder, and then find the abc file name and the corresponding i node number in Block block of root directory. The data in the abc file is finally read through the i -node number of the abc file.

Where is the file name or i -node of root directory stored?

The root directory is under the root directory. The root directory will have its own i node information and its own Block . In summary, the file name and i node number information of root directory are placed in Block block of the root directory.
How to get the i node number of that root?

The root i -node number is fixed, and the root i node number is 2. (Because number 1 is occupied when the system kernel is loaded)

[root@192 ~]# ls -ild /
2 dr-xr-xr-x. 25 root root 4096 December 27 22:27 /
The first digit displayed is the i-node number.

Summarize

To sum up, to view the contents of the abc file, first use the root i node number to read the data in the root Block block, find root directory i node information, read root directory Block data, find the abc file i-node information, and read the abc file Block data.
The above is the basic storage of files and directories.

2. Introduction to In command

(1) Let's take a look at the basic information of the ln command:

The original meaning of ln in English: make links between file Location path: /bin/ln Execution permission: all users. Function description: Establish links between files.

(2) Basic format of the ln command

[root@localhost ~] # ln [options] Source file target file options:
  -s: Create a soft link file. If the "-s" option is not added, a hard link file will be created. -f: forced. If the target file already exists, delete the target file and then create the link file.

3. Create a hard link

(1) How to create a hard link

Here is an example:

[root@localhost ~] # touch cangls
[root@localhost ~ ]# ln /root/cangls /tmp/

# Create a hard link file. The target file does not have a file name, so it will be consistent with the original file. # That is, /root/cangls and /tmp/cangls are hard link files.

Description ln command application:

# Create a new file abc and check the number of hard links of the abc file [root@192 ~]# touch abc
[root@192 ~]# ls -il abc 
1043285 -rw-r--r--. 1 root root 0 12月29 18:18 abc
# You can see that the number of hard links of the abc file is 1.

# Create a hard link file for the abc file in the tmp directory, and then check the number of hard links between the two files.
[root@192 ~]# ln abc /tmp/abc_h
[root@192 ~]# ls -il abc /tmp/abc_h 
1043285 -rw-r--r--. 2 root root 0 12月29 18:18 abc
1043285 -rw-r--r--. 2 root root 0 12月29 18:18 /tmp/abc_h
# You can see that the reference count (number of hard links) of the abc file has increased by 1.
# You can also see that the i-node numbers of the two files above are the same.

(2) Hard link features

The source file and the hard link file have the same Inode and Block . Modifying either file changes the other. Delete either file and the other will be available. Hard links are not clearly marked, and it is difficult to confirm the location of hard link files (it is difficult to determine whether this is a hard link file. If you created it yourself, you may still remember it, but if it was created by someone else, I can't determine whether a file is a hard link file at all). It is not recommended to use. Hard links cannot link directories. Hard links cannot cross partitions (because two files need the same i node number of the same partition. So they cannot be found across partitions).

(3) Hard connection principle

I created an abc file in the root directory above, and then created an abc file hard link file abc_h in /tmp/ directory.

root directory, abc file, and tmp directory each have their own i node number and corresponding Block block. Find the corresponding Block through the i-node number of root directory. Block block records the name and i node number of the abc file, so you can find the abc file. i node number and file name of the hard-linked file abc_h are recorded in Block block of tmp directory. And the i node number of the abc file and abc_h hard-link file found above is the same, so the i node number of the abc file can also be found through the abc_h information in Block block of tmp .

Therefore, the abc file information recorded in root directory and the abc_h file information recorded in the tmp directory both point to the abc file in root directory.

See the picture below:

So the above figure proves that:
Modifying either file changes the other.
Delete either file and the other will be available.

4. Create a soft link

(1) How to create a soft link

Soft links can be directly regarded as shortcuts in the Windows system.

Create a soft link file as follows:

[root@localhost ~]# touch abc
[root@localhost ~]# ln -s /root/abc /tmp/

cde file in root directory, and then create a soft link /tmp/cde_s file to view these two files.

As shown in the following figure:

[root@localhost ~ ]# touch cde
[root@localhost ~ ] # ln -s /root/cde /tmp/cde_s
[root@localhost ~ ] # ls -il /root/cde /tmp/cde_s
1055972-rw-r--r--1 root root 0 Feb 1715：30 /root/cde
261634 1rwxrwxrwx 1root root 9 Feb 1715：30 /tmp/cde_s->/root/cde

illustrate:

First: You can see that the created soft link file has a clear prompt that this is a soft link file and where its source file is located.
/tmp/cde_s->/root/cde . This is why hard links are not clearly marked.

Second: The number of citation links to the file did not increase.

Third: The i-node numbers of the two files are completely different.

Fourth: The source file abc is 0 bytes and is empty. The file size of the soft link is 9 bytes.

(2) Soft link characteristics

Soft links and source files have different Inode and Block . If you modify either file, the other one will also be changed. If you delete the soft link, the source file will not be affected; if you delete the source file, the soft link cannot be used because the soft link has no actual data and only saves Inode of the source file. No matter how large the source file is, the size of the soft link file remains unchanged. The soft link has the maximum permission lrwxrwxrwx . However, since there is no actual data, the source file permissions need to be referenced for final access. Soft links can link directories and can cross partitions. The soft link feature is obvious, and it is recommended to use soft link.

(3) Soft connection principle

There is a cde file in root directory, and a soft link file cde_s of the cde file is created in the tmp directory. The block of root directory can be found through the i node number of root directory. block of root directory records the file name and i node number of the cde file. By finding the i node number of the cde file, you can find block of the cde file and read the data in it.

The tmp directory has its own i node number and corresponding block . block of the tmp directory contains the file name and i node number of the cde_s file (it was mentioned earlier that the two i node numbers of the hard link are the same, so they point to the same file), while i node numbers of the soft link file and the source file are different. This means that the cde_s file has its own i node number and cde_s own block .

Now the key is, what is stored in block block of the cde_s file?
If data is stored in it, the cde file and the cde_s file are not linked, but are completely copied. In this way, the situation where the soft link file changes when the source file is modified, and the source file is not changed after the soft link file is modified will not occur.

In fact, the i node number of the source file is stored in block of the cde_s file.

As shown below:

This also explains why the newly created soft link file has data after creation, and the data size does not change with the change of the source file.

Another point to note is that the soft link file has the largest permissions, but this permission is for the soft link file. block of the soft link file does not store data. If you want to operate the source file, you still have to look at the source file permissions. This requires attention.

(4) Description

In Windows, shortcuts are created because the software is installed too deep and difficult to find, so a shortcut is created on the desktop. What is the purpose of creating soft links in Linux? Anyway, they are all accessed through directories?

In Linux systems, the main function of soft links is to take care of the administrator's usage habits.

As shown below:

In the old Linux system, we are used to the location of the /etc/rc.local file. After the system is updated, the location of the /etc/rc.local file is changed for the upgrade needs. We have a lot of trouble finding this file. The solution to this problem is to create a soft link for these two files to solve the above problem, which takes into account both the upgrade needs and the usage habits.

Also note that a relative path is used to create a soft link to the def file, and an absolute path is used to create a soft link to the anaconda-ks.cfg file.

From the above figure, you can see that an error occurs when creating a soft link with a relative path.

Tip: When creating a soft link, be sure to use an absolute path; this is not required for a hard link.

This is the end of this article about the detailed explanation of soft links and hard links in Linux. For more relevant content about Linux soft links and hard links, please search for previous articles on 123WORDPRESS.COM or continue to browse the related articles below. I hope everyone will support 123WORDPRESS.COM in the future!