What to prepare for Linux Administrator Job Interview?

What to prepare for Linux Administrator Job Interview?

You should be familiar with below concepts:

1) Port number of different servers {cat /etc/services}
2) Linux Installation(through FTP,HTTP,NFS)
3) Boot process
4) Diff b/w ext3 and ext2
5) RAID LEVELS and Selection of raid
6) backup methods
7) Package management such as Yum server
8) Kernel Tuning
9) IPTABLES
10) TCP WRAPPERS
11) DIFFERENT RUN LEVELS 
12) USER AND GROUP MANAGEMENT
13) QUOTA SETTING(user and group)
14) DIFF B/W CRON AND AT
15) BASIC SHELL SCRIPTING
16) Troubleshooting different issues.
17) Tell me why we should hire you?
18) DAILY ACTIVITES IN YOUR CURRENT COMPANY
19) RECENTLY SOLVED CRITICAL ISSUE
20) LVM (Very Imp)
21) Vertias Volume manager
22) cluster basic like HAD , GAB , LLT , HEARTBEAT , CONFIG FILES , RESOURSE , 
SERVICE GROUPS etc
23 ) kernel panic troubleshooting
24) Process management 
25)Configuration part of NFS , NIS , Samba , DHCP , DNS,Apache, Sendmail etc.
26)Remote administration experience.

Some Linux Interview Questions and Answers

You need to see the last fifteen lines of the files dog, cat and horse. What 
command should you use?
tail -15 dog cat horse The tail utility displays the end of a file. The -15 tells tail to display the last 
fifteen lines of each specified file. 

Who owns the data dictionary? 
The SYS user owns the data dictionary. The SYS and SYSTEM users are 
created when the database is created. 
You routinely compress old log files. You now need to examine a log from 
two months ago. In order to view its contents without first having to 
decompress it, use the _________ utility. 
zcat The zcat utility allows you to examine the contents of a compressed file much 
the same way that cat displays a file. 

You suspect that you have two commands with the same name as the 
command is not producing the expected results. What command can you 
use to determine the location of the command being run? 
which The which command searches your path until it finds a command that 
matches the command you are looking for and displays its full path. 
You locate a command in the /bin directory but do not know what it 
does.

What command can you use to determine its purpose?
whatis The whatis command displays a summary line from the man page for the 
specified command. 

You wish to create a link to the /data directory in bob's home directory
so you issue the command ln /data /home/bob/datalink but the
command fails. What option should you use in this command line to be 
successful.
Use the -F option In order to create a link to a directory you must use the -F option. 

When you issue the command ls -l, the first character of the resulting 
display represents the file's ___________. 
type The first character of the permission block designates the type of file that is 
being displayed. 

What utility can you use to show a dynamic listing of running processes? 
top The top utility shows a listing of all running processes that is dynamically 
updated.

Where is  standard output usually directed? 
to the screen or display By default, your shell directs standard output to your screen or display. 

You wish to restore the file memo.ben which was backed up in the tarfile 
MyBackup.tar. What command should you type? 
tar xf MyBackup.tar memo.ben 
This command uses the x switch to extract a file. Here the file memo.ben will 
be restored from the tarfile MyBackup.tar. 

You need to view the contents of the tarfile called MyBackup.tar. What 
command would you use?
tar tf MyBackup.tar The t switch tells tar to display the contents and the f modifier specifies which 
file to examine. 

You want to create a compressed backup of the users' home directories. 
What utility should you use? 
tar You can use the z modifier with tar to compress your archive at the same time 
as creating it. 
What daemon is responsible for tracking events on your system? 
syslogd The syslogd daemon is responsible for tracking system information and saving 
it to specified log files.

 
You have a file called phonenos that is almost 4,000 lines long. What text 
filter can you use to split it into four pieces each 1,000 lines long? 
split The split text filter will divide files into equally sized pieces. The default length 
of each piece is 1,000 lines. 

You would like to temporarily change your command line editor to be vi. 
What command should you type to change it? 
set -o vi The set command is used to assign environment variables. In this case, you 
are instructing your shell to assign vi as your command line editor. However, 
once you log off and log back in you will return to the previously defined 
command line editor. 

What account is created when you install Linux? 
root Whenever you install Linux, only one user account is created. This is the 
superuser account also known as root. 

What command should you use to check the number of files and disk 
space used and each user's defined quotas? 
repquota The repquota command is used to get a report on the status of the quotas you 
have set including the amount of allocated space and amount of used space.

In order to run fsck on the root partition, the root partition must be 
mounted as 
readonly You cannot run fsck on a partition that is mounted as read-write.

In order to improve your system's security you decide to implement 
shadow passwords. What command should you use? 
pwconv The pwconv command creates the file /etc/shadow and changes all passwords 
to 'x' in the /etc/passwd file. 

Bob Armstrong, who has a username of boba, calls to tell you he forgot 
his password. What command should you use to reset his command? 
passwd boba The passwd command is used to change your password. If you do not specify 
a username, your password will be changed.

 
The top utility can be used to change the priority of a running process? 
Another utility that can also be used to change priority is ___________? 
nice Both the top and nice utilities provide the capability to change the priority of a 
running process. 

What command should you type to see all the files with an extension of 
'mem' listed in reverse alphabetical order in the /home/ben/memos 
directory. 
ls -r /home/ben/memos/*.mem The -c option used with ls results in the files being listed in chronological 
order. You can use wildcards with the ls command to specify a pattern of filenames. 

What file defines the levels of messages written to system log files? 
kernel.h To determine the various levels of messages that are defined on your system, 
examine the kernel.h file. 

What command is used to remove the password assigned to a group? 
gpasswd -r The gpasswd command is used to change the password assigned to a group. 
Use the -r option to remove the password from the group. 

What command would you type to use the cpio to create a backup called 
backup.cpio of all the users' home directories? 
find /home | cpio -o > backup.cpio 
The find command is used to create a list of the files and directories contained 
in home. This list is then piped to the cpio utility as a list of files to include and 
the output is saved to a file called backup.cpio. 

What can you type at a command line to determine which shell you are 
using? 
echo $SHELL The name and path to the shell you are using is saved to the SHELL 
environment variable. You can then use the echo command to print out the 
value of any variable by preceding the variable's name with $. Therefore, 
typing echo $SHELL will display the name of your shell. 

What type of local file server can you use to provide the distribution 
installation materials to the new machine during a network installation?
A) Inetd            B) FSSTND        C) DNS
D) NNTP           E) NFS 
E - You can use an NFS server to provide the distribution installation materials 

to the machine on which you are performing the installation. Answers a, b, c, 
and d are all valid items but none of them are file servers. Inetd is the 
superdaemon which controls all intermittently used network services. The 
FSSTND is the Linux File System Standard. DNS provides domain name 
resolution, and NNTP is the transfer protocol for usenet news. 

If you type the command cat dog & > cat what would you see on your 
display? Choose one: 
a. Any error messages only.
b. The contents of the file dog.
c. The contents of the file dog and any error messages.
d. Nothing as all output is saved to the file cat. 
d When you use & > for redirection, it redirects both the standard output and 
standard error. The output would be saved to the file cat.

You are covering for another system administrator and one of the users
asks you to restore a file for him. You locate the correct tarfile by
checking the backup log but do not know how the directory structure
was stored. What command can you use to determine this? 
Choose one:
a. tar fx tarfile dirname
b. tar tvf tarfile filename
c. tar ctf tarfile
d. tar tvf tarfile 
d The t switch will list the files contained in the tarfile. Using the v modifier will 
display the stored directory structure. 

You have the /var directory on its own partition. You have run out of 
space. What should you do? Choose one:
a. Reconfigure your system to not write to the log files.
b. Use fips to enlarge the partition.
c. Delete all the log files.
d. Delete the partition and recreate it with a larger size. 
d The only way to enlarge a partition is to delete it and recreate it. You will then 
have to restore the necessary files from backup. 

You have a new application on a CD-ROM that you wish to install. What 
should your first step be? 
Choose one:
a. Read the installation instructions on the CD-ROM.
b. Use the mount command to mount your CD-ROM as read-write.
c. Use the umount command to access your CD-ROM.
d. Use the mount command to mount your CD-ROM as read-only. 
d Before you can read any of the files contained on the CD-ROM, you must first 
mount the CD-ROM. 

When you create a new partition, you need to designate its size by 
defining the starting and ending _____________. 
cylinders When creating a new partition you must first specify its starting cylinder. You 
can then either specify its size or the ending cylinder.

What key combination can you press to suspend a running job and place 
it in the background?
ctrl-z Using ctrl-z will suspend a job and put it in the background. 

The easiest, most basic form of backing up a file is to _____ it to another 
location. copy The easiest most basic form of backing up a file is to make a copy of that file 
to another location such as a floppy disk. 

What type of server is used to remotely assign IP addresses to machines 
during the installation process? 
A) SMB
B) NFS
C) DHCP
D) FT
E) HTTP
C - You can use a DHCP server to assign IP addresses to individual machines 
during the installation process. Answers a, b, d, and e list legitimate Linux 
servers, but these servers do not provide IP addresses. The SMB, or Samba, 
tool is used for file and print sharing across multi-OS networks. An NFS server 
is for file sharing across Linux net-works. FTP is a file storage server that 
allows people to browse and retrieve information by logging in to it, and HTTP 
is for the Web. 

Which password package should you install to ensure that the central 
password file couldn't be stolen easily? 
A) PAM
B) tcp_wrappers
C) shadow
D) securepass
E) ssh
C - The shadow password package moves the central password file to a more 
secure location. Answers a, b, and e all point to valid packages, but none of 
these places the password file in a more secure location. Answer d points to 
an invalid package. 

When using useradd to create a new user account, which of the following 
tasks is not done automatically.
Choose one:
a. Assign a UID.
b. Assign a default shell.
c. Create the user's home directory.
d. Define the user's home directory.
c The useradd command will use the system default for the user's home 
directory. The home directory is not created, however, unless you use the -m 
option.

You want to enter a series of commands from the command-line. What 
would be the quickest way to do this? 
Choose One
a. Press enter after entering each command and its arguments
b. Put them in a script and execute the script
c. Separate each command with a semi-colon (;) and press enter 
after the last command
d. Separate each command with a / and press enter after the last 
command
c The semi-colon may be used to tell the shell that you are entering multiple 
commands that should be executed serially. If these were commands that you 
would frequently want to run, then a script might be more efficient. However, 
to run these commands only once, enter the commands directly at the 
command line. 

You attempt to use shadow passwords but are unsuccessful. What 
characteristic of the /etc/passwd file may cause this? 
Choose one:
a. The login command is missing.
b. The username is too long.
c. The password field is blank.
d. The password field is prefaced by an asterisk. 
c The password field must not be blank before converting to shadow passwords. 

When you install a new application, documentation on that application is 
also usually installed. Where would you look for the documentation after 
installing an application called MyApp?
Choose one:
a. /usr/MyApp
b. /lib/doc/MyApp
c. /usr/doc/MyApp
d. In the same directory where the application is installed.
c The default location for application documentation is in a directory named for 
the application in the /usr/doc directory. 

What file would you edit in your home directory to change which window 
manager you want to use? 
A) Xinit
B) .xinitrc
C) XF86Setup
D) xstart
E) xf86init
B - The ~/.xinitrc file allows you to set which window man-ager you 
want to use when logging in to X from that account. 
Answers a, d, and e are all invalid files. Answer c is the main X server 
configuration file.

What command allows you to set a processor-intensive job to use less 
CPU time? 
A) ps
B) nice
C) chps
D) less
E) more
B - The nice command is used to change a job's priority level, so that 
it runs slower or faster. Answers a, d, and e are valid commands but are not 
used to change process information. Answer c is an invalid command. 

While logged on as a regular user, your boss calls up and wants you to 
create a new user account immediately. How can you do this without first 
having to close your work, log off and logon as root? 
Choose one:
a. Issue the command rootlog.
b. Issue the command su and type exit when finished.
c. Issue the command su and type logoff when finished.
d. Issue the command logon root and type exit when finished. 
b You can use the su command to imitate any user including root. You will be 
prompted for the password for the root account. Once you have provided it 
you are logged in as root and can do any administrative duties.

There are seven fields in the /etc/passwd file. Which of the following 
lists all the fields in the correct order? 
Choose one:
a. username, UID, GID, home directory, command, comment
b. username, UID, GID, comment, home directory, command
c. UID, username, GID, home directory, comment, command
d. username, UID, group name, GID, home directory, comment
b The seven fields required for each line in the /etc/passwd file are username, 
UID, GID, comment, home directory, command. Each of these fields must be 
separated by a colon even if they are empty. 

Which of the following commands will show a list of the files in your 
home directory including hidden files and the contents of all 
subdirectories?
Choose one:
a. ls -c home
b. ls -aR /home/username
c. ls -aF /home/username
d. ls -l /home/username
b The ls command is used to display a listing of files. The -a option will cause 
hidden files to be displayed as well. The -R option causes ls to recurse down 
the directory tree. All of this starts at your home directory. 

In order to prevent a user from logging in, you can add a(n) ________at 
the beginning of the password field. 
asterick If you add an asterick at the beginning of the password field in the 
/etc/passwd file, that user will not be able to log in. 

You have a directory called /home/ben/memos and want to move it 
to /home/bob/memos so you issue the command mv 
/home/ben/memos /home/bob. What is the results of this action?
Choose one:
a. The files contained in /home/ben/memos are moved to the 
directory /home/bob/memos/memos.
b. The files contained in /home/ben/memos are moved to the 
directory /home/bob/memos.
c. The files contained in /home/ben/memos are moved to the 
directory /home/bob/.
d. The command fails since a directory called memos already exists 
in the target directory. 
a When using the mv command to move a directory, if a directory of the same 
name exists then a subdirectory is created for the files to be moved.

Which of the following tasks is not necessary when creating a new user 
by editing the /etc/passwd file? 
Choose one:
a. Create a link from the user's home directory to the shell the user 
will use.
b. Create the user's home directory
c. Use the passwd command to assign a password to the account.
d. Add the user to the specified group.
a There is no need to link the user's home directory to the shell command. 
Rather, the specified shell must be present on your system. 

You issue the following command useradd -m bobm But the user cannot 
logon. What is the problem?
Choose one:
a. You need to assign a password to bobm's account using the 
passwd command.
b. You need to create bobm's home directory and set the appropriate 
permissions.
c. You need to edit the /etc/passwd file and assign a shell for bobm's 
account.
d. The username must be at least five characters long.
a The useradd command does not assign a password to newly created 
accounts. You will still need to use the passwd command to assign a 
password. 

You wish to print the file vacations with 60 lines to a page. Which of the 
following commands will accomplish this? Choose one:
a. pr -l60 vacations | lpr
b. pr -f vacations | lpr
c. pr -m vacations | lpr
d. pr -l vacations | lpr
a The default page length when using pr is 66 lines. The -l option is used to 
specify a different length. 

Which file defines all users on your system? 
Choose one:
a. /etc/passwd
b. /etc/users
c. /etc/password
d. /etc/user.conf
a The /etc/passwd file contains all the information on users who may log into 
your system. If a user account is not contained in this file, then the user 
cannot log in. 

Which two commands can you use to delete directories? 
A) rm
B) rm -rf
C) rmdir
D) rd
E) rd –rf

 B, C - You can use rmdir or rm -rf to delete a directory. Answer a is 
incorrect, because the rm command without any specific flags will not delete 
a directory, it will only delete files. Answers d and e point to a non-existent 
command.

 
Which partitioning tool is available in all distributions? 
A) Disk Druid
B) fdisk
C) Partition Magic
D) FAT32
E) System Commander
B - The fdisk partitioning tool is available in all Linux distributions. 
Answers a, c, and e all handle partitioning, but do not come with all 
distributions. Disk Druid is made by Red Hat and used in its distribution along 
with some derivatives. Partition Magic and System Commander are tools 
made by third-party companies. Answer d is not a tool, but a file system type. 
Specifically, FAT32 is the file system type used in Windows 98. 

Which partitions might you create on the mail server's hard drive(s) 
other than the root, swap, and boot partitions? 
[Choose all correct answers]
A) /var/spool
B) /tmp
C) /proc
D) /bin
E) /home
A, B, E -Separating /var/spool onto its own partition helps to 
ensure that if something goes wrong with the mail server or spool, the output 
cannot overrun the file system. Putting /tmp on its own partition prevents 
either software or user items in the /tmp directory from overrunning the file 
system. Placing /home off on its own is mostly useful for system re-installs or 
upgrades, allowing you to not have to wipe the /home hierarchy along with 
other areas. Answers c and d are not possible, as the /proc portion of the file 
system is virtual-held in RAM-not placed on the hard drives, and the /bin 
hierarchy is necessary for basic system functionality and, therefore, not one 
that you can place on a different partition. 

When planning your backup strategy you need to consider how often you 
will perform a backup, how much time the backup takes and what media 
you will use. What other factor must you consider when planning your 
backup strategy? _________ 
what to backup Choosing which files to backup is the first step in planning your backup 
strategy.

What utility can you use to automate rotation of logs? 
logrotate The logrotate command can be used to automate the rotation of various logs. 

In order to display the last five commands you have entered using the 
history command, you would type ___________ .
history 5 The history command displays the commands you have previously entered. 
By passing it an argument of 5, only the last five commands will be displayed. 

What command can you use to review boot messages? 
dmesg The dmesg command displays the system messages contained in the kernel 
ring buffer. By using this command immediately after booting your computer, 
you will see the boot messages. 

What is the minimum number of partitions you need to install Linux? 
2 Linux can be installed on two partitions, one as / which will contain all files 
and a swap partition. 

What is the name and path of the main system log? 
/var/log/messages  By default, the main system log is /var/log/messages. 

Of the following technologies, which is considered a client-side script? 
A) JavaScript
B) Java
C) ASP
D) C++
A - JavaScript is the only client-side script listed. Java and C++ are 
complete programming languages. Active Server Pages are parsed on the 
server with the results being sent to the client in HTML

when you create a user in linux , the default 
settings comes from -------------
/etc/skel

What is the difference between hard links and soft links
Hard links for files only and soft links are for both directoris and files.

what is the difference between name base and ip base 
virtual hosting in ip base you are running more than one web site on the same server 
machine,and in name base you have to put more than one DNS record for your IP 
address in the DNS.

where are the kernel parameters stored in linux 
 in /boot.

you want to deploy linux on identical machines , which 
method you will use 
Unattendend Installation Method With the help of PXE Server & NFS server.

Name of the file which describe the kickstart installation 
Anaconda ks.cfg

How you give another Ip address on the same NIC 
(network card)
copy  a file in /etc/sysconfig/network-script/ifcfg-eth0 to ifcfg-eth0:1

you defined samba shares and you want these shares to 
be available on network , you don't want to restart samba 
server which command you will use ??
Service SMB  restart

you want to monitor systems on your network ,what 
utility you will use 
TOP and ps 

can you define the linux boot process
1.BIOS: The Basic Input/Output System is the lowest level interface between 
the computer and peripherals. 
The BIOS performs integrity checks on memory and seeks instructions on 
the Master Boor Record (MBR) on the floppy drive or hard drive. 
2.The MBR points to the boot loader (GRUB or LILO: Linux boot loader). 
3.Boot loader (GRUB or LILO) will then ask for the OS label which will identify 
which kernel to run and where it is located (hard drive and partition 
specified). The installation process requires to creation/identification of 
partitions and where to install the OS. GRUB/LILO are also configured 
during this process. The boot loader then loads the Linux operating 
system. 
4.The first thing the kernel does is to execute init program. Init is the 
root/parent of all processes executing on Linux. 
5.The first processes that init starts is a script /etc/rc.d/rc.sysinit 
6.Based on the appropriate run-level, scripts are executed to start various 
processes to run the system and make it functional
Boot process takes place in 4 scenes with 4 main characters.
Scene 1
when the computer is switched on,it automatically invokes BIOS[a ROM chip 
embedded in the motherboard].The BIOS will start the processor and perform a 

POST[power on self test] to check whether the connected device are ready to use 
and are working properly.
Once the POST is completes BIOS will jump to a specified location in the RAM and 
check for the booting device. The boot sector is always the first sector of the hard 
disk and BIOS will load the MBR into the memory.
Scene 2
Here the boot loader takes the control of the booting process. LILO or GRUB are 
the boot loaders commonly available. It will help the user to select various boot 
options. Depending on the boot option selected the kernel is loaded.
scene 3
After kernel is loaded the kernel will take the control of the booting process and it 
will initialize all the hardwares including I/O processors etc. kernel then creates a 
root device and mounts the partitions.
Scene 4
INIT is loaded.

what is initrd 
initrd  image is the initial ram disk image While the system getting booted the kernel image will 
get loaded into the main memory.

can you name of the actual kernel in linux 
vmlinuxz

how you get the kernel version 
uname -r

how you change run levels in linux 
/etc/inittab

you want /dev/hda10 mounted whenever your system boots what you will do 
put the entry in fstab with proper drivers and mount point(/dev/hda10    mountpoint    ext2/3      defaults    0 0).

you want bind to be run at boot time ,which command you will use 
chkconfig.

You want to know if nfs package is installed on your 
system 
rpm -qa nfs and you can check export file in /etc.

 What is the difference between rpm , yum and RHEN 
rpm required dependency software and yum can install automatically with dependancy

what is a proc directory
/proc directory are not real files--they are hooks to look at information available to the kernel.

what is the difference between current directory and home directory 
home directory is one over which user have complete controland it is its default working 
directory when its logs in. while the current directory is the users current directorywhich may or 
may not be his home directory.

what is exit status in linux 
You can use $? to find out the exit status of command($ echo $?)

As the system administrator you have created a directory containing some scripts 
that you have written. You want to have all your users to be able to run this 
scripts. Which file should you edit to ensure that the scripts will run without your 
users having to type the complete path to the script?
Choose one:
a. ~/.profile
b. /etc/profile
c. /etc/bash
d. ~/.bash
b. /etc/profile

What are the files to be used in the network installation of linux 
os?/etc/sysconfig/network,/etc/resolv.conf,/etc/hosts

 What is the command to see the installed rpm's in the linux system.you can view the installed rpm in your linux box by :
rpm -qa | grep rpm name

  

What is Kernel? Explain the task it performs.
Kernel is used in UNIX like systems and is considered to be the heart of the operating system. It is 
responsible for communication between hardware and software components. It is primarily used for 
managing the systems resources as well. 

Kernel Activities:
§The Kernel task manager allows tasks to run concurrently. 
§Managing the computer resources: Kernel allows the other programs to run and use the 
resources. Resources include i/o devices, CPU, memory. 
§Kernel is responsible for Process management. It allows multiple processes to run 
simultaneously allowing user to multitask. 
§Kernel has an access to the systems memory and allows the processes to access the memory 
when required. 
§Processes may also need to access the devices attached to the system. Kernel assists the 
processes in doing so. 
§For the processes to access and make use of these services, system calls are used.

What does nslookup do? Explain its two modes.
Nslookup is used to find details related to a Domain name server. Details like IP addresses of a 
machine, MX records, servers etc. It sends a domain name query packet to the corresponding DNS.
Nslookup has two modes. Interactive and non interactive. Interactive mode allows the user to interact by 
querying information about different hosts and domains.
Non interactive mode is used to fetch information about the specified host or domain. 
Interactive mode:
Nslookup [options] [server] 

What is Linux and why is it so popular?
Linux is an operating system that uses UNIX like Operating system. However, unlike UNIX, Linux is an 
open source and free software. Linux was originally created by Linus Torvalds and commonly used in 
servers.
Popularity of Linux is because of the following reasons
§It is free and open source. We can download Linux for free and customize it as per our needs. 
§It is very robust and adaptable. 
§Immense amount of libraries and utilities

What is 'inode'?
Each file in UNIX has a unique number called as an inode. Using this number the file information like 
user, group, ownership and access mode information can be found. A files inode number can be found 
using the following command:
ls –i 
If the inode number is known, the following command can be used to get details of the file:
ls –l 

What is use of sed command?
Sed command in UNIX is commonly used for processing of files. Sed stands for Stream Editor which 
parses text files and used for making textual transformations to a file. The command specified to Sed, is 
applied on the file line by line.
Example:
To replace all matching occurrences of some text to another.
sed -e 's/olddata/newdata/g' inputFileName > outputFileName
Here, g is global which replaces ALL occurrences. 

What is the difference between Telnet and SSH? 
SSH using PORT 22 and telnet using Port 23 ssh is a secured shell, where telnet is not 
a secured one.when you ssh to trasnfer data between a system, the data will be send in 
the encrypted form, where the hacker cannot encode or decode it. While you telnet,the 
data send between the system is alphabetical format(ASCII), where every one can 
understand. More over as per network security, telnet and ftp are prohibited. Always, 
trust SSL based data transfer. 

How to recover a deleted file in linux ? 
Determine the partition in which the file you wish to recover was located by using the 
command pwd (present work directory) and unmount the current directory by using 
the command umount. Use the "debugfs" command. The "debugfs" command is used 
to handle most major errors and bugs that can occur in Linux by using #debugfs 
/usr/directory name.Use the "lsdel" once you have begun the debug of the file system. 
Recover those files you wish to undelete and take backup of that file.