Linux File System Structure

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

 
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.

That’s all for now. Keep connected to Tecmint for any News and post related to Linux and Foss world. Stay healthy and Don’t forget to give your value-able comments in comment section.

Booting with an NFS Root Filesystem

It is possible to boot most EMAC OE systems using NFS (Network File System) as the root filesystem. This method can be especially useful during development where the root filesystem is changing frequently. This can save time as well as wear on the on-board flash device. This page describes the steps required to boot over NFS. 

Prerequisites

Some prerequisites must be met prior to being able to boot a root filesystem over NFS.

TFTP Server

A TFTP server is required to host the kernel image. For information on installing a TFTP server, see Installing TFTP server.

NFS Server

To boot an EMAC OE system over NFS, an NFS server must be available on the local network. This is often the same machine that is being used for software development. EMAC recommends using the nfs-kernel-server package available on most desktop Linux distributions if setting up a new NFS server. Once the server has been installed, export a directory to use as the root filesystem. This is often done using the /etc/exports file. This document assumes that the root filesystem for the board will be located at /srv/nfs/rootfs on the NFS server.
For more information on setting up an NFS server, see Setting up an NFS File Server.

Root Filesystem

A complete root filesystem for the EMAC OE system to boot from must be stored on the NFS server. The NFS server must be configured to allow clients to access this filesystem. The root filesystem does not have to be the directory shared by the NFS filesystem; it can be in a subdirectory, which means many root filesystems can be shared by one NFS server.

Configuring the Client to Boot from NFS

The method used to configure a client to boot with an NFS mounted root filesystem depends on the type of bootloader the machine uses. Configuring U-Boot for booting over NFS is covered here.

Configuring U-Boot

A series of U-Boot commands are used to tell U-Boot to boot an NFS filesystem:

U-Boot> set autoload no
U-Boot> dhcp
DHCP client bound to address 10.0.0.100

Here, the first command, set autoload no, tells U-Boot not to try to load an image after they've gotten an IP address from the DHCP server. The second command, dhcp, tells U-Boot to use DHCP to acquire an IP address. In this example, the DHCP server assigned 10.0.0.100 as the IP address for the machine.

U-Boot> set serverip 10.0.0.20

This line sets the IP address of the TFTP server to 10.0.0.20. This is the server which hosts the Linux kernel which will be used by the machine.

U-Boot> setenv bootargs console=${console} root=/dev/nfs rootfstype=nfs ip=dhcp nfsroot=10.0.0.20:/srv/nfs/test_fileystem

This line sets up the environment needed to boot from the NFS server. These options will be passed to the Linux kernel when booting it. The console=${console} part tells Linux to use the console setting from the U-Boot environment variable; this will usually be something along the lines of console=ttyS0,115200n8. The root=/dev/nfs directive tells Linux to instantiate with the virtual device, /dev/nfs, as the root filesystem. The rootfstype=nfs directive tells Linux that the root filesystem is of the NFS variety. The ip=dhcp directive tells Linux to acquire an IP address by requesting one from the DHCP server. The nfsroot=10.0.0.20:/srv/nfs/test_filesystem directive tells Linux to look for an NFS server at 10.0.0.20 and to mount the /srv/nfs/test_filesystem directory as the root filesystem for the machine.

U-Boot> tftp ${loadaddr} uImage
...
Loading: #################################################################
         #################################################################
         #################################################################
         #################################################################
         #################################################################
         #################################################################
         #################################################################
         #################################################################
         #################################################################
         #####################

This line tells U-Boot to load the kernel from the TFTP server, whose IP address was set earlier with the set serverip 10.0.0.20 command. The filename U-Boot requests from the TFTP server is uImage, and it stores it in the memory location pointed to by loadaddr. The text which follows displays U-Boot's progress in loading the kernel into RAM. When the U-Boot prompt returns, the environment is fully setup and ready to boot from an NFS root filesystem.

U-Boot> bootm ${loadaddr}

The bootm command tells U-Boot to boot from RAM. This means it boots the kernel which was just loaded into RAM, passing the commandline arguments specified by bootargs to the Linux kernel as it does so.
The machine should now boot, and show output on its console as it does so.

Conclusion

Using NFS for the root filesystem allows boards to boot quickly into test versions of a filesystem, provides a tool for remotely backing up the filesystem installed on an embedded machine, can provide a central filesystem image for many boards which can allow updating one filesystem to update all boards which use it simultaneously, and more. Setting up a machine to boot to a root filesystem via NFS requires little work; most of the work is in setting up the server. The ability to boot a root filesystem over NFS provides a powerful tool for many applications.


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