We use DNS (domain name system) to translate between domain names and IP addresses. For example, one can use the dig command/host command for DNS lookup on a Linux and Unix-like systems. Similarly, Linux files are referred by file names, not by inode number. So what is the purpose of a directory? You can group 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 sub-directories named:

1. . (single period) – The current directory
2. .. (double period) – The 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. The ‘..‘ always points to the same inode as ‘.‘

Use the ls command to list files and directories including . and .. directories on Linux or Unix:
ls -la

Directory

A directory contained inside another directory is called a sub-directory. At the end the directories form a tree structure. Use the 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
• Ext4 and other modern Linux file systems allows an unlimited number of subdirectories

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 bash shell alias commands related to directory :
alias ..='cd ..'
alias d='ls -l | grep -E "^d"'

Unix and Linux directory management commands

A list of standard Linux/Unix commands to work with directories and files:

Command	Description	Example(s)
mkdir command	Creates a new directory	mkdir dir1
rmdir command	Deletes the specified directory if it is already empty	rmdir dir1
cd command	Change the current directory	cd /etc/
cd .. command	Go back to previous directory	cd ..
pwd command	Display name of current/working directory	pwd
cd command	cd without any parameters changes to the user’s home directory	cd
mv command	Copies source to target then deletes the original source	mv dir1 dir2
cp command	Copies source to target	cp -r dir1 /path/to/dir2
rm command	Removes the specified files from the file system. Directories are not removed by rm unless the option -r is used	rm file1 rm -r dir1
ln command	Creates an internal link from source to target	ln -s /etc/hosts /tmp/link
chown command	Transfers ownership of a file to the user with the specified username	chown userName file
chgrp command	Transfers the group ownership of a given file to the group with the specified group name	chgrp dir1
chmod command	Changes the access permissions	chmod 0444 dir1
locate command	The locate command can find in which directory a specified file is located	locate file1
find command	Search for a file in a given directory	find $HOME -name "hello.c"
file command	Detect the contents of the specified files	file /etc/resolv.conf
cat command	Displays the contents of a file	cat data.txt
less command	See the contents of the specified file	less resume.txt
more command	It is a filter for paging through text one screenful at a time	more /etc/hosts
grep command	Finds a specific search string in the specified files	grep "nameserver" /etc/resolv.conf
egrep command	Same as grep but extended regular expression supported	egrep -i 'err|cri|warn|' /var/log/messages
diff command	Compares the contents of any two files	diff old.c new.c

I am sure all of you know the basic commands related to directories and files management. Click the link on the left column to learn more about each command and same usage.