First we will create a discardable directory calls rm.dir into which we shall add two directories: dir1 and dir2.
~$ mkdir rm.dir && cd rm.dir ~$ mkdir dir1 dir2
Now paste some rubbish into a dir1/file1.txt (find your least favourite website and copy and paste everything):
~$ cat > dir1/file1.txt # paste some rubbish here ^D
Now create a hardlink to a file called dir2/file2.txt.
~$ ln dir1/file1.txt dir2/file2.txt
Let's view the tree view using tree.
~$ tree . ├── another_file.txt ├── dir1 │ └── file1.txt └── dir2 └── file2.txt 2 directories, 3 files
Let's view the file stats using stat.
~$ stat dir*/* 16777220 99414274 -rw-r--r-- 2 pkorir pkorir 0 14322 "Mar 9 08:38:00 2021" "Mar 9 08:37:33 2021" "Mar 9 08:37:59 2021" "Mar 9 08:37:16 2021" 4096 32 0 dir1/file1.txt 16777220 99414274 -rw-r--r-- 2 pkorir pkorir 0 14322 "Mar 9 08:38:00 2021" "Mar 9 08:37:33 2021" "Mar 9 08:37:59 2021" "Mar 9 08:37:16 2021" 4096 32 0 dir2/file2.txt
The second column shows the files inode number. Notice that they are identical: these are hardlinks - two directory entries to the same data blocks i.e. one thing with two names. Let's make a third directory and do an rsync of dir2/file2.txt.
~$ mkdir dir3 ~$ rsync -zarvh dir2/ dir3/ building file list ... done ./ file2.txt sent 6.45K bytes received 48 bytes 13.00K bytes/sec total size is 14.32K speedup is 2.20 ~$ tree . ├── another_file.txt ├── dir1 │ └── file1.txt ├── dir2 │ └── file2.txt └── dir3 └── file2.txt 3 directories, 4 files
What does stat tell us now?
~$ stat dir*/* 16777220 99414274 -rw-r--r-- 2 pkorir pkorir 0 14322 "Mar 9 08:38:00 2021" "Mar 9 08:37:33 2021" "Mar 9 08:37:59 2021" "Mar 9 08:37:16 2021" 4096 32 0 dir1/file1.txt 16777220 99414274 -rw-r--r-- 2 pkorir pkorir 0 14322 "Mar 9 08:38:00 2021" "Mar 9 08:37:33 2021" "Mar 9 08:37:59 2021" "Mar 9 08:37:16 2021" 4096 32 0 dir2/file2.txt 16777220 99414874 -rw-r--r-- 1 pkorir pkorir 0 14322 "Mar 9 08:39:03 2021" "Mar 9 08:37:33 2021" "Mar 9 08:39:01 2021" "Mar 9 08:37:33 2021" 4096 32 0 dir3/file2.txt
We have copied the hardlink to an actual file. How much disk space are we using?
~$ du -h 16K ./dir2 16K ./dir3 0B ./dir1 36K .
What?
~$ du -h dir*/* 16K dir1/file1.txt 16K dir3/file2.txt
Hardlinks don't take any disk space!? Cool!