The difference between soft link and hard link and how the file system takes files

Catalog

One . The difference between soft link and hard link

Two . File system fetches files

One . The difference between soft link and hard link

1. How it was created

         How to create soft links ：ln -s target link_name （ Soft links can link files or folders , Often used to link folders ）

         How hard links are created ：ln target link_name （ Hard links can only link files , Cannot link folder ）

2. Hard links ：

         It's equivalent to an alias . New will not be created inode Number , The linked file and the source file share the same inode Number , Just add one to the number of references in the source file ;

         If you delete the source file , Linked files can still be used ;

         The file type of the linked file and the source file are the same ;

         Hard links cannot cross file systems , Soft links can ;

         You cannot create hard links to nonexistent files and directories ;

3. Soft link ：

         amount to Windows Shortcuts on the desktop . New will be created inode Number , Source and linked files inode Different numbers ;

         The number of links in the source file does not change , The number of links to the linked file is 1（ Will not be referenced by the upper layer ）;

         The file types of source files and linked files are different , The soft link file permission is 777, File attributes are identified , Access rights really depend on the permissions of the source file ;

         Delete source file , Linked files cannot be accessed ;

         You can create soft links to nonexistent files and directories ;

[[email protected] lianxi]# ll
 Total usage  0
-rw-r--r-- 1 root root 0 7 month   20 15:22 aabb
drwxr-xr-x 2 root root 6 7 month   20 15:22 xieshan
[[email protected] lianxi]# ll -a
 Total usage  4
drwxr-xr-x   3 root root   33 7 month   20 15:22 .
drwxr-xr-x. 32 root root 4096 7 month   20 15:22 ..
-rw-r--r--   1 root root    0 7 month   20 15:22 aabb
drwxr-xr-x   2 root root    6 7 month   20 15:22 xieshan
[[email protected] lianxi]# ln aabb aabb-link
[[email protected] lianxi]# ln -s xieshan xieshan-link
[[email protected] lianxi]# ll -a
 Total usage  4
drwxr-xr-x   3 root root   70 7 month   20 15:23 .
drwxr-xr-x. 32 root root 4096 7 month   20 15:22 ..
-rw-r--r--   2 root root    0 7 month   20 15:22 aabb
-rw-r--r--   2 root root    0 7 month   20 15:22 aabb-link
drwxr-xr-x   2 root root    6 7 month   20 15:22 xieshan
lrwxrwxrwx   1 root root    7 7 month   20 15:23 xieshan-link -> xieshan

Two . File system fetches files

Inode Equivalent to file pointer , Points to a block of the physical hard disk . In fact, the file system maintains a reference count （ That is, the number of links ）, As long as there are files pointing to this block , It won't disappear from the hard disk

After formatting, the disk is divided into two areas , A storage file metadata （inode), A piece of real data (block)

Folders are also files , Only what is saved in it is the directory entries corresponding to the files to be stored in this folder

Most of Linux file system （ Such as ext2、ext3） Regulations , A file consists of directory entries 、inode And data blocks ：

• Catalog items ： Include the file name and inode Node number .
• Inode： Also called file index node , Contains the basic information of the file and the pointer to the data block .
• Data blocks ： Contains the specific content of the file .

How the file system fetches files ？

        1. First , According to the file name , You will find the corresponding inode Number

        2. Will be based on inode Number , from inode-table File found in table block Number

        3.block There are specific data stored in , Read the contents of the file .

If it is a soft link file , Then there will be No 4 Step ,block What is stored in is not specific data , It is the name of the source file where the linked file is stored , Then according to the source file name , Repeat the three steps above , Find the real file data .

If it is a hard link file , The same as the source file will be found according to the hard link file name inode Number , Then proceed to the second and third steps , Read file data

About Inode、block、 More detailed contents of the catalog item , It is recommended to check this blog ：https://www.cnblogs.com/fxh0707/p/16164674.html​​​​​​​