Linux Directory Structure and Important Files Paths Explained

For any person, who does not have a sound knowledge of Linux Operating System and Linux File System, dealing with the files and their location, their use may be horrible, and a newbie may really mess up.

This article is aimed to provide the information about Linux File System, some of the important files, their usability and location.

Linux Directory Structure Diagram

A standard Linux distribution follows the directory structure as provided below with Diagram and explanation.

Linux Directory Structure

Each of the above directory (which is a file, at the first place) contains important information, required for booting to device drivers, configuration files, etc. Describing briefly the purpose of each directory, we are starting hierarchically.

1. /bin : All the executable binary programs (file) required during booting, repairing, files required to run into single-user-mode, and other important, basic commands viz., cat, du, df, tar, rpm, wc, history, etc.
2. /boot : Holds important files during boot-up process, including Linux Kernel.
3. /dev : Contains device files for all the hardware devices on the machine e.g., cdrom, cpu, etc
4. /etc : Contains Application’s configuration files, startup, shutdown, start, stop script for every individual program.
5. /home : Home directory of the users. Every time a new user is created, a directory in the name of user is created within home directory which contains other directories like Desktop, Downloads, Documents, etc.
6. /lib : The Lib directory contains kernel modules and shared library images required to boot the system and run commands in root file system.
7. /lost+found : This Directory is installed during installation of Linux, useful for recovering files which may be broken due to unexpected shut-down.
8. /media : Temporary mount directory is created for removable devices viz., media/cdrom.
9. /mnt : Temporary mount directory for mounting file system.
10. /opt : Optional is abbreviated as opt. Contains third party application software. Viz., Java, etc.
11. /proc : A virtual and pseudo file-system which contains information about running process with a particular Process-id aka pid.
12. /root : This is the home directory of root user and should never be confused with ‘/‘
13. /run : This directory is the only clean solution for early-runtime-dir problem.
14. /sbin : Contains binary executable programs, required by System Administrator, for Maintenance. Viz., iptables, fdisk, ifconfig, swapon, reboot, etc.
15. /srv : Service is abbreviated as ‘srv‘. This directory contains server specific and service related files.
16. /sys : Modern Linux distributions include a /sys directory as a virtual filesystem, which stores and allows modification of the devices connected to the system.
17. /tmp :System’s Temporary Directory, Accessible by users and root. Stores temporary files for user and system, till next boot.
18. /usr : Contains executable binaries, documentation, source code, libraries for second level program.
19. /var : Stands for variable. The contents of this file is expected to grow. This directory contains log, lock, spool, mail and temp files.

Exploring Important file, their location and their Usability

Linux is a complex system which requires a more complex and efficient way to start, stop, maintain and reboot a system unlike Windows. There is a well defined configuration files, binaries, man pages, info files, etc. for every process in Linux.

1. /boot/vmlinuz : The Linux Kernel file.
2. /dev/hda : Device file for the first IDE HDD (Hard Disk Drive)
3. /dev/hdc : Device file for the IDE Cdrom, commonly
4. /dev/null : A pseudo device, that don’t exist. Sometime garbage output is redirected to /dev/null, so that it gets lost, forever.
5. /etc/bashrc : Contains system defaults and aliases used by bash shell.
6. /etc/crontab : A shell script to run specified commands on a predefined time Interval.
7. /etc/exports : Information of the file system available on network.
8. /etc/fstab : Information of Disk Drive and their mount point.
9. /etc/group : Information of Security Group.
10. /etc/grub.conf : grub bootloader configuration file.
11. /etc/init.d : Service startup Script.
12. /etc/lilo.conf : lilo bootloader configuration file.
13. /etc/hosts : Information of Ip addresses and corresponding host names.
14. /etc/hosts.allow : List of hosts allowed to access services on the local machine.
15. /etc/host.deny : List of hosts denied to access services on the local machine.
16. /etc/inittab : INIT process and their interaction at various run level.
17. /etc/issue : Allows to edit the pre-login message.
18. /etc/modules.conf : Configuration files for system modules.
19. /etc/motd : motd stands for Message Of The Day, The Message users gets upon login.
20. /etc/mtab : Currently mounted blocks information.
21. /etc/passwd : Contains password of system users in a shadow file, a security implementation.
22. /etc/printcap : Printer Information
23. /etc/profile : Bash shell defaults
24. /etc/profile.d : Application script, executed after login.
25. /etc/rc.d : Information about run level specific script.
26. /etc/rc.d/init.d : Run Level Initialisation Script.
27. /etc/resolv.conf : Domain Name Servers (DNS) being used by System.
28. /etc/securetty : Terminal List, where root login is possible.
29. /etc/skel : Script that populates new user home directory.
30. /etc/termcap : An ASCII file that defines the behaviour of Terminal, console and printers.
31. /etc/X11 : Configuration files of X-window System.
32. /usr/bin : Normal user executable commands.
33. /usr/bin/X11 : Binaries of X windows System.
34. /usr/include : Contains include files used by ‘c‘ program.
35. /usr/share : Shared directories of man files, info files, etc.
36. /usr/lib : Library files which are required during program compilation.
37. /usr/sbin : Commands for Super User, for System Administration.
38. /proc/cpuinfo : CPU Information
39. /proc/filesystems : File-system Information being used currently.
40. /proc/interrupts : Information about the current interrupts being utilised currently.
41. /proc/ioports : Contains all the Input/Output addresses used by devices on the server.
42. /proc/meminfo : Memory Usages Information.
43. /proc/modules : Currently using kernel module.
44. /proc/mount : Mounted File-system Information.
45. /proc/stat : Detailed Statistics of the current System.
46. /proc/swaps : Swap File Information.
47. /version : Linux Version Information.
48. /var/log/lastlog : log of last boot process.
49. /var/log/messages : log of messages produced by syslog daemon at boot.
50. /var/log/wtmp : list login time and duration of each user on the system currently.

A Basic Guide to Linux Boot Process

As promised in our earlier post, in this post we are going to review boot process in Linux Operating System. How Operating system passes through different stage of booting states. This article is written for those readers who has just steps in Linux world. Understanding how Linux boots up is very important in terms of effectively troubleshooting in case of system failure. When a system switched on and after few moment we get a login prompt. Have we try to find out what all stage of booting sequence has crossed and what happened behind the scene during system boots up.

Power on

1. BIOS (Basic Input Output System) is a software program comes pre-built in a motherboard chipset.
2. BIOS loads and scans for devices such as Hard Disk, CD-ROM, RAM, etc.
3. BIOS searches for MBR (Master Boot Record: 1st sector) of the primary hard drive, it scans for 1st stage loader (In our case boot loader is (GRUB LILO) and hands over the responsibility to MBR.
4. Boot PROM/FLASH/BIOS is proficient of loading the MBR into RAM and executing it.

MBR (Master Boot Record)

• 512 bytes of space –> MBR
• MBR contains the information of loader of most operating system e.g UNIX, Linux and WINDOWS
• MBR holds the small binary information of 1st stage of loader
• MBR consist physical sector of the first disk drive (i.e 512 bytes) and it’s not part of any partition.
• Placed on the prime disk drive, in the prime sector of the first cylinder of track is 0 and head is 0 (this whole path is generally booked for boot programs)
• MBR involve a mini executable programs and a table specify the primary partitions.

Boot Code (GRUB) 446 bytes
partition 1: 16 bytes
partition 2: 16 bytes
partition 3: 16 bytes
partition 4: 16 bytes
magic Number: 2 bytes

1. MBR also document which primary partition is ACTIVE.
2. The BIOS surrender rights to the first stage boot loader, which then scans partition table and finds second stage boot loader on the partition configured as bootable.

Boot Loader

1. The boot loader termed from 1st stage loader and loads itself into RAM. All this go on in milliseconds.
2. The default stage 2 boot loader is a GRUB (Grand Unified Boot Loader) or LILO (Linux Loader)
3. Once GRUB is loaded into RAM, then it’s search for the location of Kernel.
4. GRUB will scrutinize the map file to find the kernel image, that is located under (/boot) and load it.
5. GRUB loads the kernel (vmlinuz-version) from /boot partition

Trivia 1

GRUB organize RAMDISK for initrd —> (RAMDISK is reserved space from RAM). In addition, it drives initrd into RAM to ready the kernel for loading itself into memory and depended modules so that it can leave the system to “init” process

In, Linux most of the drivers are pre-built as modules, these would be initial ram drive (initrd.img) where it can keep all the information of additional modules. So, when the kernel boots, it creates ramdrive, loads the initrd.img and its depended modules.

GRUB reads /boot/grub/grub.conf & shows us a clean interface for selecting Operating SystemOnce Kernel loads its depended modules and then it hand over to “init” process. The kernel image has a small, unpacked program that un-compresses kernel and runs it.

Trivia 2

LILO needed to indicate MBR in order to locate operating systems on the hard drive. Any modifications done to /etc/lilo.conf, that must be updated in MBR, but in GRUB‘s case no need to update, it reads directly from the file /boot/grub/grub.conf.

After making changes in /etc/lilo.conf, we’ll have to update the MBR manually

# /sbin/lilo -v

Trivia 3

The GRUB second stage loader resides within the MBR and within /boot partition. Once GRUB is loaded into memory it becomes 2nd stage loader.

Trivia 4

The /initrd directory should not be removed it is a temporary place holder for kernel to have quick access to the modules that it needs to start the system modules include device drivers.

Kernel initialization highlights include:

1. initialize CPU components, eg, MMU
2. initialize the scheduler (PID 0)
3. mount the root filesystem in rw mode
4. fork off the init process (PID 1)

In essence, kernel initialization does two things:

1. Start the core system of shared resource managers (RAM, processor and mass storage).
2. Starts a single process, /sbin/init.

Init process (sbin/init) is the very fist process which loads all the various daemons and mounts all the partitions which are listed under /etc/fstab.

About /etc/fstab

1. The /sbin/init reads /etc/inittab file
2. Set default runlevel ( the telinit command allows administrators to tell the init process to change its current runlevel)
3. Calls /etc/rc.d/rc.sysinit and /etc/rc.d/rc x (where ‘x‘ is a runlevel)
4. In /etc/rc.d/rc5.d directory files starting with letter K –> kill scripts and files starting with letter S –> Startup scripts.
5. Start up the tty processes and xdm ( X display manager)
6. Starts User’s login screen