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Similarly files are referred by file name, not by inode number. So what is the purpose of a directory? You can groups the files according to your usage. For example all configuration files are stored under /etc directory. So the purpose of a directory is to make a connection between file names and their associated inode number. Inside every directory you will find out two directories . (current directory) and .. (pointer to previous directory i.e. the directory immediately above the one I am in now). The .. appears in every directory except for the root directory.

Directory

A directory contained inside another directory is called a subdirectory. At the end the directories form a tree structure. Use tree command to see directory tree structure:
$ tree /etc | less
Again a directory has an inode just like a file. It is a specially formatted file containing records which associate each name with an inode number. Please note the following limitation of directories under ext2/3 file system:

• There is an upper limit of 32768 subdirectories in a single directory.
• There is a “soft” upper limit of about 10-15k files in a single directory

However according to official documentation of ext2/3 file system points that “Using a hashed directory index (which is under development) allows 100k-1M+ files in a single directory without performance problems’. Here are my two favorite alias commands related to directory :
$ alias ..='cd ..'
alias d='ls -l | grep -E "^d"'

Well I’m sure all of you know the basic commands related to directories and files managment. Click above (or here) to see summery of all basic commands related to directories and files managment. See interesting  about soft links and directories. This is 6th part of “Understanding  system, continue reading rest of the Understanding Linux file system series (this is part IV):

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