硬连接和软连接

背景

项目的nginx日志太多了，需要迁移到其他磁盘，另外一个同学采用软链的方式迁移到其他磁盘上，这样不影响现有程序。对于软链和硬连接概念上还是有些模糊，所以总结一下，直接抄Quora上的一个通俗的回答.

Have you ever given a thought to what happens when you store a file on your hard disk?

I will show you.

Let’s create a file first.

touch myfile
cat > myfile
Hello, world!

cat file should display this text:

cat myfile
Hello, world!

Where do you think myfile is stored?

An obvious and correct answer is your hard drive.

How does Linux know myfile is on the hard drive?

When you entered “touch myfile” Linux stored the file on the hard drive and assigned it a number. That number is called inode.

An inode stores a file’s address, size and plenty of other information, but not its name. A name links to an inode which links to a file.

If, for some reason, the inode is linked with a file is removed, you cannot find the file, even if it is sitting right there on the hard drive.

Thankfully it rarely happens.

A more common scenario is for someone to erroneously write:

rm myfile

It will remove myfile.

Hard links can save you from such nightmarish scenarios.

Creating a hard link means choosing a new name for your file and linking that name of your file’s inode. It is relatively simple to do.

ln myfile myfileHardLink

Now even if someone removes myfile, you will still be able to access your data through myfileHardLink.

Impressive, isn’t it?

But where do symbolic links fit into all this?

I have said, hard links connect the name of a file with its inode and you can choose multiple names for a file.

Now, a symbolic link connects the new name of a file with its old name which in turn connects to the inode and finally to the data (file).

This image will summarize my answer more elegantly than I can:

linux soft and hard link

动手

root@instance-8alx1qc6-1:~/tmp# ll -ail
total 20
420794 drwxr-xr-x  2 root root 4096 Apr 24 23:41 ./
446 drwx------ 22 root root 4096 Apr 24 23:35 ../
395269 -rw-r--r--  3 root root   61 Apr 16 23:25 file
395269 -rw-r--r--  3 root root   61 Apr 16 23:25 file.h
395269 -rw-r--r--  3 root root   61 Apr 16 23:25 file.s
394365 lrwxrwxrwx  1 root root    6 Apr 24 23:41 file.ss -> file.s
394365 lrwxrwxrwx  2 root root    6 Apr 24 23:41 file.t -> file.s

其中file是原来的文件.

file.h, file.s是file的hard link, file.ss，file.t都是file.s的软链,其中file.t是通过file.ss生成硬链得来的。

可以看到通过硬连接得来的文件，inode id和原来的是一样的。