Understanding hard links

Question

I created a file in home directory.

touch demo
echo "This is demo" > demo

Now I created a hard link in /tmp

ln /home/leo/demo demoLink

Now when checking the inode using

ls -ltri demoLink

It shows the same inode number as the original file. I can digest that. But the file type of the Hard Link is Regular! How is it a regular file? How is it different from the original file? I read a file is an anonymous blob of data. So these two file are pointing to the same data? Is the file size of both hard link and Original file same?

Answer 1

The hard link is not actually a "link". The first, second, third, Nth hard link to the same file inode are all equal to each other. There is no difference between an "original entry" and a "hard link".

Say, if one creates the file1 and then add the hard link file2 to the same file inode, there is no method to find out which link to the given inode is original.

I.e.

cryo@CryoNest:~ $ touch file1

cryo@CryoNest:~ $ ls -il file*
9505656 -rw-r--r-- 1 cryo cryo 0 кві  8 20:24 file1

cryo@CryoNest:~ $ ln file1 file2

cryo@CryoNest:~ $ ls -il file*
9505656 -rw-r--r-- 2 cryo cryo 0 кві  8 20:24 file1
9505656 -rw-r--r-- 2 cryo cryo 0 кві  8 20:24 file2

cryo@CryoNest:~ $ rm file1

cryo@CryoNest:~ $ ls -il file*
9505656 -rw-r--r-- 1 cryo cryo 0 кві  8 20:24 file2

cryo@CryoNest:~ $ ln file2 file1

cryo@CryoNest:~ $ ls -il file*
9505656 -rw-r--r-- 2 cryo cryo 0 кві  8 20:24 file1
9505656 -rw-r--r-- 2 cryo cryo 0 кві  8 20:24 file2

As you see, the inode number (9505656) and all attributes stored in the inode are equal for all hard links. Including size, date, mode, etc.

Pay attention to the 3rd field, this is a number of links to the inode. This field can be used to understand "mystical" situations when one "deletes" a file (actually, one of the hard links' directory entry) but space usage doesn't change :)

If one wants to find all hard links of the given file, the "find" tool has the option "-inum"

cryo@CryoNest:~ $ ln file1 tmp/file3

cryo@CryoNest:~ $ find . -maxdepth 2 -inum 9505656 -ls
  9505656  0 -rw-r--r--   2 cryo  cryo     0 кві  8 20:24 ./tmp/file3
  9505656  0 -rw-r--r--   2 cryo  cryo     0 кві  8 20:24 ./file1