EXPLANATION
Users cannot hard link directories:
$ mkdir This_dir
$ ln This_dir That_dir
ln: ‘This_dir’: hard link not allowed for directory
But the system makes two hard links within a directory when it
is created. One of these is named "." and it is a link to the newly
created directory itself, so the new directory will have a link count of
2, which is the minimum link count for a directory.
The other is named ".." and it is a link to the parent directory, and will add to the
parent's link count.
$ ls -ild Link.test ## list "i"node, "l"ong list, for the "d"irectory itself.
ls: cannot access Link.test: No such file or directory
$ mkdir Link.test ## make the directory.
$ ls -ild Link.test
2238981 drwxr-xr-x 2 dan users 4096 Dec 26 13:53 Link.test
$ #Link count......^ The newly created directory has 2 hard links.
$ ls -ild "Link.test" "Link.test/." #Quoting not required, but more readable
2238981 drwxr-xr-x 2 dan users 4096 Dec 26 13:53 Link.test
2238981 drwxr-xr-x 2 dan users 4096 Dec 26 13:53 Link.test/.
When making a new directory, the system will also create a link to the parent directory named ".."
So every time a directory is created, the link count of the parent directory increases by 1.
$ pwd
/home/dan/Test.link.dir
$ ## List inode numbers in a long listing, directory only, for "Link.test" and "Link.test/."
$ ls -ild "Link.test" "Link.test/."
2238981 drwxr-xr-x 2 dan users 4096 Dec 26 13:53 Link.test
2238981 drwxr-xr-x 2 dan users 4096 Dec 26 13:53 Link.test/.
$ #................^ Link count is 2 for each entry and inode numbers are identical, 2238981.
$ mkdir Link.test/subdir ## make a subdirectory under "Link.test".
$ ## Now check the link count, including the newly created subdirectory's parent, " .. "
$ ls -ild "Link.test/" "Link.test/." "Link.test/subdir/.."
2238981 drwxr-xr-x 3 dan users 4096 Dec 26 15:55 Link.test/
2238981 drwxr-xr-x 3 dan users 4096 Dec 26 15:55 Link.test/.
2238981 drwxr-xr-x 3 dan users 4096 Dec 26 15:55 Link.test/subdir/..
$ #................^ Link count has increased from 2 to 3 after creating "subdir".
$ #+ The 3 names above, "Link.test", "Link.test/." and "Link.test/subdir/.." reference
$ #+ the same inode, 2238981.
c.f.
https://www.tldp.org/LDP/gs/node5.html
"...
a directory is actually just a file containing information about
link-to-inode associations. Also, every directory contains at least two
hard links: ``.'' (a link pointing to itself), and ``..'' (a link
pointing to the parent directory)."
The root directory of a
filesystem does not have a parent directory, so "/" and "/." and
"/.." all reference the same inode. That is, the parent directory of
root, "/..", is "/"
E.g.
$ ls -ild "/" "/." "/.."
2 drwxr-xr-x 32 root root 4096 Oct 26 13:54 /
2 drwxr-xr-x 32 root root 4096 Oct 26 13:54 /.
2 drwxr-xr-x 32 root root 4096 Oct 26 13:54 /..
$ #..........^^ Note that this link count also includes parent references (..) from subdirectories.
See also:
http://teaching.idallen.com/cst8207/13w/notes/notes/450_file_system.html
SOURCE
https://www.tldp.org/LDP/gs/node5.html
