Why is number of hard links for a directory 2 + the number of subdirectories?

Question

When using ls -l, I noticed that directories start with 2 hard links, and gain one for each subdirectory. I understand that the current directory link . counts as one link and the parent directory link .. of each subdirectory counts as a hard link, but:

1. Why don't subfiles count towards hard links when subdirectories do?

2. Why does the parent directory link .. count as a hard link for both this directory and the parent directory?

Answer 1

Assume the following directory tree

/a
  /b
    /c
    /d

Then directory b at path /a/b has 4 hard links. The links are the following:

• b (from the parent directory)
• . (from the directory itself)
• c/..
• d/..

Files don't count against the link count of a directory, because files do not have a nested .. directory, thus do not point back to the directory.

We can calculate the number of hardlinks for each directory shown above:

• a: 3
  • a (from /)
  • .
  • b/..
• c: 2
  • c (from b)
  • .
• d: 2
  • d (from b)
  • .