Link value is 2 by default for folders

Question

Can anyone help me to understand the logic behind the link value being "2" for the first time when we create any folders/ directory in linux.

I searched a lot , but couldn't get satisfactory logic

Answer 1

Every directory has link count at least two due to

1. the reference to the directory from its parent directory
2. the self-reference within the directory (.)

Example: Suppose within some base directory there is a directory foo:

$ ls -i /basedir
1056868 foo
$ ls -ia /basedir/foo
1056868 .  1056849 ..

The ls option -i shows the inode numbers of the directory entries, the option -a shows entries that begin with a dot (ls does not show those by default). Note that /basedir/foo and /basedir/foo/. have the same inode number. Since there are (at least) two different references to the according inode, the link count for that inode is two (or more).

Answer 2

The fundamental design of the Unix filesystem goes back to the early days. It is described in the paper The UNIX Time-Sharing System by Dennis M. Ritchie and Ken Thompson.

The designers wanted to be able to refer to the current directory, and to have a way to go from a directory to its parent directory. Rather than introduce special shell syntax, they decided to use a feature that already existed anyway: directories can contain entries for other directories, so they decided that an entry with the special name . would always point to the directory itself, and an entry with the special name .. would always point to a directory's parent. For example, if the root directory contains a subdirectory called foo, then foo's .. entry points to the root directory and foo's . entry points to foo itself.

Thus a directory's link count (the number of directory entries pointing to it) would always be 2 for a directory with no subdirectory: the expected entry in the parent directory, plus the . directory. Each subdirectory adds 1 to the link count due to the .. entry.

The special entries . and .. were originally created by the mkdir command by mucking with the on-disk representation of the filesystem directly. Later systems moved this into the kernel. Today, many filesystems don't include . and .. entries in the on-disk representation anymore. The filesystem driver doesn't need ., and doesn't need .. either if it always remembers the location of a directory's parent (which increases memory consumption a bit, negligible by today's standards but not by the 1970's standards). In filesystems that include on-disk . and .. entries, the filesystem driver ensures that these entries are always present. In filesystems that don't include these entries in the on-disk representation, the filesystem driver pretends that these entries are present.