Can someone explain the detail of the 'inode count' when using command 'll'
 

i can understand why hard links can change that number,but still confused with the number of a directory.

in fact, create a sub dir will change the number of his parent, regular files will not.

hard link:

file1
|
|——> inode -> data block
|
file2

the 2 files above have a common inode with the count value 2

directoy:

assume ./a/b, b is the only sub dir of a

inode(a) ——> data block(a) ——> contains {inode:[10, 11, 12], filename:[., .., b]} // this editor will ignore the extra blank, so here i use json instead

above show the struct of storage of a dir
if a's inode is 9, b's is 12, they are not the same
means only 2 filenames are linked to inode 9(a and .)

someone can help me understand why?

Do you mean the "Total <number>" part of the command output? if yes, then check this from the GNU page on the ls command :

Directory b contains a ".." link, which is to the inode of directory a. The link count for a directory is always ( 2 + number_of_immediate_subdirs ) due to these ".." links.

hmm, i see, except the root, . and .. are same.

The formula (2 + number_of_subdirs) is correct for the root directory too. It's just that in the root directory, one of those "2" links is different. In a non-root directory, there is one link from the parent directory plus the directory's own "." link. In the root directory, there is no parent directory, but both "." and ".." point to the root directory, so there are still 2 links.

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