About RFS
In this lecture let’s discuss about the Root File System(RFS) and its directory
structure.
Later videos in this course will show you how you can create your own root
file system by using busybox and buildroot and we can also test it on the
hardware.
Now the root file system, as the name indicates, it’s a file system which Linux
mounts to the "/" (root)
File system is nothing but a collection of files organized in standard folder
structure.
Yes!!! There is a standard for the Linux file system. That is called "File system
Hierarchy Standard"
You may have to check this document if you are interested.
https://wiki.Linuxfoundation.org/lsb/fhs-30
https://en.wikipedia.org/wiki/File system_Hierarchy_Standard#cite_note-2
In a typical file system you will find the below folder structure, even though all
these folders are not required for Linux to boot and mount the file system
successfully.
Now, let’s understand what the purpose of each folder is and what exactly it
contains.
The official documentation of the FHS says this about the directories present
in the "/"
http://refspecs.Linuxfoundation.org/FHS_3.0/fhs/ch03s02.html
Directory Description
bin Essential command binaries
boot Static files of the boot loader
dev Device files
etc Host-specific system configuration
lib Essential shared libraries and kernel modules
media Mount point for removable media
mnt Mount point for mounting a file system temporarily
opt Add-on application software packages
run Data relevant to running processes
sbin Essential system binaries
srv Data for services provided by this system
tmp Temporary files
usr Secondary hierarchy
var Variable data
bin/ :
This directory contains binaries of Linux commands which are used by both
the system admins and users.
You don’t need privileges from your System admin to execute these
commands neither you need root access. Remember that this folder will not
contain binaries for all the Linux commands. There is a restriction on what
types of commands have to be placed in this directory, because these
binaries can be executed by the common user.
Below are some the commands you will find it in the bin/ directory.
cat Utility to concatenate files to standard output
chgrp Utility to change file group ownership
chmod Utility to change file access permissions
chown Utility to change file owner and group
cp Utility to copy files and directories
date Utility to print or set the system data and time
dd Utility to convert and copy a file
df Utility to report file system disk space usage
dmesg Utility to print or control the kernel message buffer
echo Utility to display a line of text
false Utility to do nothing, unsuccessfully
hostname Utility to show or set the system's host name
kill Utility to send signals to processes
ln Utility to make links between files
login Utility to begin a session on the system
ls Utility to list directory contents
mkdir Utility to make directories
mknod Utility to make block or character special files
more Utility to page through text
mount Utility to mount a file system
mv Utility to move/rename files
ps Utility to report process status
pwd Utility to print name of current working directory
you can see that commands related to "repairing", "recovering", "restoring",
"network configuration" ,”modules install remove” are not found in this
directory.
boot/:
This directory contains the boot related files, which are required to boot the
Linux. This directory may be read by the boot loader to read the boot images
like Linux kernel image, dtb, vmLinux, initramfs, etc.
So this directory may be accessed by boot loader even before the kernel
boots and mounts the file system.
dev/:
This is the place where you can find the "device files”.
You may be heard or read this statement "in unix/Linux devices are treated
like file access" .
Yes, if you want to access any i/o, networking devices, memory devices,
serial devices, parallel devices, input output devices such as keyboard,
mouse, display, everything will be treated like a file.
So, this directory will have the file entry for every device
For example: the i2c devices may have a file entries like this
/dev/i2c-0 ,
/dev/i2c-1,
The user Space application can use this device files to access those devices .
The ram may have a device file entry like /dev/ram0
The 2 partitions of the SD card may have entries like this:
/dev/mmcblk0p1
/dev/mmcblkop2.
The serial devices may have entries like this:
/dev/ttyS0,
/dev/ttyS1,
/dev/ttyO0
It’s the responsibility of the respective drivers to populate this directory with
the device files
etc/ :
This is the place where all the start-up scripts, networking scripts, scripts to start
and stop networking protocols like NFS, networking configuration
files, different run level scripts will be stored.
1) Contains run level scripts, which will be used during different run levels
2) Contains start-up and shutdown scripts
3) Contains various scripts related to services like start/stop networking,
start/stop NFS, etc
4) Contains various configuration files, like passwd, hostinfo, etc.
5) Contains various network configuration files
lib/ :
The major contents of this directory are
1) The dynamically loadable kernel modules. ( later you will see, when we
compile the kernel modules and when we run “modules install” command ,
all the kernel modules will go and sit in this directory under the sub directory
"modules" .)
2) To store the Essential shared libraries (.so.*) for dynamic linking.
for example, 'C' shared library(libc), math library, python libray, etc,
media/ :
This is the mount point for the removable media like your USB flash drive, SD
cards, camera, cell phone memory, etc .
For example, when i connect my SD card to the PC, there will be 2 device
files will be created for each partition 1) /dev/sdb1 and 2) /dev/sdb2
and theses 2 device files are automatically mounted under the /media
directory. So that i can access those 2 partitions just like folders.
Some examples:-
/media/cdrom for CD-ROM
/media/<your usb flash drive name>
mnt/ :
This is the place where you can mount the temporary file system.
The system admins can use a Linux commands to temporarily mount and un-
mount the file system, if they want to transfer any file.
opt/ :
"opt" stands for "optional"
This directory will be used when you install any software packages for your
Linux distribution.
For example if i run the command apt-get install <some packages name>
then the package will be installed in this directory.
sbin/ :
The commands which come in the category of system administration will be
stored in this directory, which is used by your sys admins for the purpose of
networking configurations, repairing, restoring and recovering .
may be sbin stands for "System Admin's bin" ??
It also has root only command and need privileges to execute those
commands.
These are the commands which you will find in sbin/
Command Description
fastboot Reboot the system without checking the disks (optional)
fasthalt Stop the system without checking the disks (optional)
fdisk Partition table manipulator (optional)
fsck File system check and repair utility (optional)
File system check and repair utility for a specific file system
fsck.*
(optional)
getty The getty program (optional)
halt Command to stop the system (optional)
ifconfig Configure a network interface (optional)
init Initial process (optional)
mkfs Command to build a file system (optional)
mkfs.* Command to build a specific file system (optional)
mkswap Command to set up a swap area (optional)
reboot Command to reboot the system (optional)
route IP routing table utility (optional)
swapon Enable paging and swapping (optional)
swapoff Disable paging and swapping (optional)
update Daemon to periodically flush file system buffers (optional)
home/:
The /home directory contains a home folder for each user.
Each user only has write access to their own home folder and must obtain
elevated permissions (become the root user) to modify other files on the
system.
This directory will be used to store personal data of the user .
In a single user mode you may have folder like this
/home/ramesh
In a multiple user mode, you may have folder dedicated to each user.
/home/ramesh, /home/suresh, /home/ram etc.
srv/ :
SRV stands for "Service"
The /srv directory contains “data for services provided by the system.” If you
are using the Apache HTTP server to serve a website, you’d likely store your
website’s files in a directory inside the /srv directory.
tmp/ :
Applications store temporary files in the /tmp directory.
usr/ :
According to FHS, it’s a "secondary hierarchy", the usr/ directory may contain
the below sub directories
Directory Description
bin Most user commands
include Header files included by C programs
lib Libraries
local Local hierarchy (empty after main installation)
sbin Non-vital system binaries
share Architecture-independent data
/usr/bin contains binary of the commands for user programs
For example, if you have firefox on your system then just check it must be
available under "/usr/bin" not under /bin. Because it is a binary related to
user installed programs, similarly, "zip" command also you will find it under
/usr/bin.
/usr/sbin contains, again privileged commands which may be used by the
system admins, but these commands are for system administration purposes.
/usr/include will hold the header files which will be included from the C
programs you write.
/usb/lib will again hold the shared libraries, linker/loader files , which enable
your /usb/bin and /usr/sbin commands to execute.
For more information you may check this official link:
http://refspecs.linuxfoundation.org/FHS_3.0/fhs/ch03.html
