Linux file system and directory structure

Overview of the FHS

Everything in Linux can be reduced to a file. Partitions are associated with files such as /dev/hda1. Hardware components are associated with files such as /dev/modem. Detected devices are documented as files in the /proc directory. The Filesystem Hierarchy Standard (FHS) is the official way to organize files in Unix and Linux directories.

Linux file system and directory structure

Several major directories are associated with all modern Unix/Linux operating systems. These directories organize user files, drivers, kernels, logs, programs, utilities, and more into different categories. The standardization of the FHS makes it easier for users of other Unix-based operating systems to understand the basics of Linux. Every FHS starts with the root directory, also known by its label, the single forward slash (/). All of the other directories shown in Table are subdirectories of the root directory. Unless they are mounted separately, you can also find their files on the same partition as the root directory.

/	The root directory, the top-level directory in the FHS. All other directories are subdirectories of root, which is always mounted on some partition. All directories that are not mounted on a separate partition are included in the root directory?s partition.
/bin	Essential command line utilities. Should not be mounted separately; otherwise, it could be difficult to get to these utilities when using a rescue disk.
/boot	Includes Linux startup files, including the Linux kernel. Can be small; 16MB is usually adequate for a typical modular kernel. If you use multiple kernels, such as for testing a kernel upgrade, increase the size of this partition accordingly.
/etc	Most basic configuration files.
/dev	Hardware and software device drivers for everything from floppy drives to terminals. Do not mount this directory on a separate partition.
/home	Home directories for almost every user.
/lib	Program libraries for the kernel and various command line utilities. Do not mount this directory on a separate partition.
/mnt	The mount point for removable media, including floppy drives, CD-ROMs, and Zip disks.
/opt	Applications such as WordPerfect or StarOffice.
/proc	Currently running kernel-related processes, including device assignments such as IRQ ports, I/O addresses, and DMA channels.
/root	The home directory of the root user.
/sbin	System administration commands. Don't mount this directory separately.
/tmp	Temporary files. By default, Red Hat Linux deletes all files in this directory periodically.
/usr	Small programs accessible to all users. Includes many system administration commands and utilities.
/var	Variable data, including log files and printer spools.

Types of Files Used by Linux

When working with Linux, you need to be aware of the fact that there are a number of different file types used by the file system. This is another area where the Linux file system differs significantly from the Windows file system. With a Windows file system you basically have two entry types in the file system:

• Directories
• Files

Granted, you can have normal files, hidden files, shortcut files, word processing files, executable files, and so on. However, these are all simple variations of the basic file when working with Windows.
With Linux, however, there are a variety of different file types used by the file system. These include the file types shown in Table

File Type	Description
Regular files	These files are similar to those used by the file systems of other operating systems—for example, executable files, OpenOffice.org files, images, text configuration files, etc.
Links	These files are pointers that point to other files in the file system.
FIFOs	FIFO stands for First In First Out. These are special files used to move data from one running process on the system to another. A FIFO file is basically a queue where the first chunk of data added to the queue is the first chunk of data removed from the queue. Data can only move in one direction through a FIFO.
Sockets	Sockets are similar to FIFOs in that they are used to transfer information between sockets. With a socket, however, data can move bi-directionally.

Important configuration files form /etc directory for RHCE exam

File	Function
/etc/fstab	Lists the partitions and file systems that will be automatically mounted when the system boots.
/etc/group	Contains local group definitions.
/etc/grub.conf	Contains configuration parameters for the GRUB bootloader (assuming it's being used on the system).
/etc/hosts	Contains a list of hostname-to-IP address mappings the system can use to resolve hostnames.
/etc/inittab	Contains configuration parameters for the init process.
/etc/init.d/	A subdirectory that contains startup scripts for services installed on the system. On a Fedora or Red Hat system, these are located in /etc/rc.d/init.d.
/etc/modules.conf	Contains configuration parameters for your kernel modules.
/etc/passwd	Contains your system user accounts.
/etc/shadow	Contains encrypted passwords for your user accounts.
/etc/X11/	Contains configuration files for X Windows.