Since everything in Unix is a file, when we call "cd ." are we actually cding into the directory . ? Is it a protected symbolic reference to the parent directory of each directory?
Yes, everything in Unix, is a file. Like any directory, a file of any type, any device(speaker, keyboard,. etc.) and even a file-system itself, all act like a file for OS. In Unix every file has a inode attached with it, which contains the file metadata like info about permissions, size, time-stamps and most importantly file data block pointers which point to data block containing actual file data.
Hence each Directory(being a file) also has a inode. The content of the directory is the sequence of records. Each record has at least two fields which are filename and the inode number.
file1name file1_inode_number
Exact structure of record depends on the filesystem implementation. So basically directory file contain a (record)entry corresponding to each file and immediate sub-directory inside it. In addition to that, Directory file also contain 2 more entries which are
. : mapped with self inode
and
.. : mapped with parent's inode
so all over directory structure looks like
. inode_number_of_self
.. inode_number_of_parent_dir
file1name inode_number_of_file1
file2name inode_number_of_file2
.
.
so on
So whenever you cd ./ or cd ../ OS is referring current or parent directory(respectively) relative to your current directory.