Linux Interview Questions

1) What is GRUB

Ans) GNU GRUB is a Multiboot boot loader. It was derived from GRUB, the GRand Unified Bootloader,

which was originally designed and implemented by Erich Stefan Boleyn.

Briefly, a boot loader is the first software program that runs when a computer starts. It is

responsible for loading and transferring control to the operating system kernel software

(such as the Hurd or Linux). The kernel, in turn, initializes the rest of the operating

system (e.g. GNU)

2) Explain Linux Boot Process

Ans) http://www.thegeekstuff.com/2011/02/linux-boot-process/

3) Which files are called for user profile by default when a user gets login

Ans) $HOME/.bash_profile, $HOME/.bash_bashrc

4) Which file needs to update if srequired to change default runlevel 5 to 3

Ans) File is /etc/inittab and required to change below lines:

id:5:initdefault: to id:3:initdefault:

5) What command used for showing user info like Login Name, Canonical Name, Home Directory,Shell etc..

Ans) FINGER command can be used i.g; finger username

6) What is inode number

Ans) An inode is a data structure on a traditional Unix-style file system such as UFS or ext3. An

inode stores basic information about a regular file, directory, or other file system object

iNode number also called as index number, it consists following attributes:

File type (executable, block special etc)

Permissions (read, write etc)

Owner

Group

File Size

File access, change and modification time (remember UNIX or Linux never stores file creation

time, this is favorite question asked in UNIX/Linux sys admin job interview)

File deletion time

Number of links (soft/hard)

Extended attribute such as append only or no one can delete file including root user

(immutability)

Access Control List (ACLs)

Following command will be used to show inodes of file and folders:

ls -i

Following command will show complete info about any file or folders with inode number

stat file/folder

Files/Folders can also be deleted using inode numbers with following command:

find out the inode number using 'ls -il' command then run below command

find . -inum inode_number -exec rm -i {} \;

7) How can we increase disk read performance in single command

Ans) blockdev command

This is sample output - yours may be different.

# Before test

$ blockdev --getra /dev/sdb

256

$ time dd if=/tmp/disk.iso of=/dev/null bs=256k

2549+1 records in

2549+1 records out

668360704 bytes (668 MB) copied, 6,84256 seconds, 97,7 MB/s

real 0m6.845s

user 0m0.004s

sys 0m0.865s

# After test

$ blockdev --setra 1024 /dev/sdb

$ time dd if=/tmp/disk.iso of=/dev/null bs=256k

2435+1 records in

2435+1 records out

638390272 bytes (638 MB) copied, 0,364251 seconds, 1,8 GB/s

real 0m0.370s

user 0m0.001s

sys 0m0.370s

8) .... command to change user password expiration time

Ans) CHAGE

9) Command used to lock user password

Ans) usermod -L username

10) How many default number of Shells available and what are their names?

Ans) SH, BASH, CSH, TCSH, NOLOGIN, KSH

11) Which file defines the attributes like UID, PASSWORD expiry, HOME Dir create or not while

adding user

Ans) /etc/login.defs

12) ...... command used for changing authentication of linux system to LDAP/NIS/SMB/KERBOS

Ans) authconfig

13) ...... command used for changing the attributes of any file

Ans) chattr

14) What is the path of network (ethX) configuration files

Ans) /etc/sysconfig/network-scripts/ethX

15) How can we change speed and make full duplex settings for eth0

Ans) We can do this with below given 2 methods:

ethtool -s eth0 speed 100 duplex full

ethtool -s eth0 speed 10 duplex half

mii-tool -F 100baseTx-HD

mii-tool -F 10baseT-HD

16) File which stores the DNS configuration at client side

Ans) /etc/resolve.conf

17) Main configuration file and command used for exporting NFS directories and it's deamons

Ans) /etc/exports and exportfs -av , deamons are quotad, portmapper, mountd, nfsd and nlockmgr/status

18) What is command to check ports running/used over local machine

Ans) netstat -antp

19) What is the command to check open ports at remote machine

Ans) nmap

20) What is the difference between soft and hard links

Ans) Soft Links => 1) Soft link files will have different inode numbers then source file

2) If original file deleted then soft link file be of no use

3) Soft links are not updated

4) Can create links between directories

5) Can cross file system boundaries

Hard Links => 1) Hard links will have the same inode number as source file

2) Hard links can not link directories

3) Can not cross file system boundaries

4) Hard links always refers to the source, even if moved or removed

21) How to setup never expired user password

Ans) chage -E never username

22) Restricting insertion into file if full permission are assigned to all

Ans) chattr +i filename

23) Display or Kill all processes which are accessing any folder/file

Ans) Display User who are using file/folder : fuser -u file/folder

Kill All Processes which are using file/folder: fuser -k file/folder

24) Kill any user's all processes

Ans) killall -u username

25) How can we have daily system analysis and reports over mail

Ans) Use logwatch

26) How can we rotate logs using logrotate without performing any operation like move and gzip'ng over original file and then creating new file (which is very lengthy process)

Ans) We can use "logrotate"'s "copytruncate" option which will simply copy original file and

truncate original file :)

27) Command to collect detailed information about the hardware and setup of your system

Ans) dmidecode , sysreport

28) Command to check PCI devices vendor or version

Ans) lspci

29) What is the difference between cron and anacron

Ans ) Cron :

1) Minimum granularity is minute (i.e Jobs can

be scheduled to be executed every minute)

2) Cron job can be scheduled by any normal

user ( if not restricted by super

user )

3) Cron expects system to be running 24 x 7. If a job is scheduled, and

system is down during that time, job is not executed

4) Ideal for servers

5) Use cron when a job has to be executed at a

particular hour and minute

Anacron :

1)Minimum granularity is only in days

2)Anacron can be used only by super user ( but there are workarounds to make
it usable by normal user )

3)Anacron doesn’t expect system to be running

24 x 7. If a job is scheduled,

and system is down during that time, it

start the jobs when the system comes back up.

4) Ideal for desktops and laptops

5) Use anacron when a job has to be executed

irrespective of hour and minute

30) Default Port numbers used by ssh,ftp,http,https,telnet,smtp,pop3,pop3s,imap,imaps

Ans) SSH 22, ftp 20/21, http 80, https 443, SMTP/SMPTS 25/465, POP3/POP3S 110/995, IMAP/IMAPS 143/993

31) How to setup ACLs in following case:

1) Create a file FILE1 and this should be read,write,executable for all user but Read only for user USER1

2) Copy FILE1 ACLs to FILE2 ACL

3) Delete a USER1's rule for FILE1 which were setup in step 1)

Ans) 1) touch FILE1 ; chmod 777 FILE1 ; setfacl -m u:USER1:r FILE1

2) getfacl FILE1 | setfacl --set-file=- FILE2

3) setfacl -x u:USER1 FILE1

32) How to make USB bootable?

Ans) Write efidisk.img from RHEL 6 DVD images/ subdirectory to USB

dd if=efidisk.img of=/dev/usb (usb device name)

33) How can we check disk/device status/failure/errors using smartctl utility?

Ans) Try following to check:

Enable/Disable SMART on device/disk : smartctl -s on /dev/sda

Check device SMART health : smartctl -H /dev/sda

Check device SMART capabilities : smartctl -c /dev/sda

Enable/Disable automatic offline testing on device : smartctl -o on/off /dev/sda

Show device SMART vendor-specific Attributes and values : smartctl -A /dev/sda

Show device log [TYPE : error, selftest, selective, directory,background,

scttemp[sts,hist]] : smartctl -l TYPE /dev/sda

Run test on device [TEST: offline short long conveyance select,M-N pending,N

afterselect,[on|off] scttempint,N[,p] : smartctl -t /dev/sda

34) What is the difference between ext2 vs ext3 vs ext4?

Ans) http://www.thegeekstuff.com/2011/05/ext2-ext3-ext4/

35) Disable ping to avoid network/ICMP flood

Ans) Set following in /etc/sysctl.conf : net.ipv4.icmp_echo_ignore_all = 1

Then "sysctl -p"

echo "1" > /proc/sys/net/ipv4/icmp_echo_ignore_all

36) What is SYN Flood, ICMP Flood

Ans) SYN Flood : A SYN flood occurs when a host sends a flood of TCP/SYN packets, often with a fake/forged sender address. Each of these packets is handled like a connection request, causing the server to spawn a half-open connection, by sending back a TCP/SYN-ACK packet(Acknowledge), and waiting for a packet in response from the sender address(response to the ACK Packet). However, because the sender address is forged, the response never comes. These half-open connections saturate the number of available connections the server is able to make, keeping it from responding to legitimate requests until after the attack ends

ICMP Flood : There are three types of ICMP Flood :

1) Smurf Attack : http://en.wikipedia.org/wiki/Smurf_attack

2) Ping Flood : http://en.wikipedia.org/wiki/Ping_flood

3) Ping of Death : http://en.wikipedia.org/wiki/Ping_of_death

37) What is the difference between Unix vs Linux Kernels?

Ans) Please find below given link :

http://www.thegeekstuff.com/2012/01/linux-unix-kernel/

38) How to setup Password less remote login/ssh?

Ans) Use "ssh-keygen -t dsa or rsa" at local system for creating public and private keys

Then copy /root/.ssh/id_dsa.pub to remote_server by name /root/.ssh/authorized_keys

Change permissions of /root/.ssh/authorized_keys file at remote_server "chmod 0600 ~/.ssh/authorized_keys"

Now try to login from local system to remote_server "ssh root@remote_server"

39) Command to see default kernel image file

Ans) "grubby --default-kernel"

40) How to create lvm mirror

Ans) lvcreate -L 50G -m1 -n LVMmirror vg0

41) Command to check last runlevel

Ans) who -r

42) What do you mean by File System?

Ans) File System is a method to store and organize files and directories on disk. A file system can have different formats called file system types. These formats determine how the information is stored as files and directories.

43) What is the requirement of udev daemon?

Ans) Create and remove device nodes or files in /dev/ directory

44) What are block and character devices?

Ans) Both the devices are present in /dev directory

Block device files talks to devices block by block [1 block at a time (1 block = 512 bytes to 32KB)].

Examples: - USB disk, CDROM, Hard Disk (sda, sdb, sdc etc....)

Character device files talk to devices character by character.

Examples: - Virtual terminals, terminals, serial modems, random numbers (tty{0,1,2,3......})

45) How to Convert ext2 to ext3 File System?

Ans) tune2fs -j /dev/{device-name}

46) File required to modify for setting up kernel parameters permanent

Ans) /etc/sysctl.conf

47) Commands used to install, list and remove modules from kernel

Ans) Installing/adding a module:

insmod mod_name

modprobe mod_name

List installed modules : lsmod

Removing a module : modprobe -r mod_name

48) How to create swap using a file and delete swap

Ans) Adding swap :

dd if=/dev/zero of=/opt/myswap bs=1024 count=4

mkswap /opt/myswap

swapon -a

For adding this myswap at boot time, add following in /etc/fstab file:

/opt/myswap swap swap defaults 0 0

Deleting Swap :

Run "swapoff /opt/myswap" command

Remove the entry from /etc/fstab file

Remove /opt/myswap file (using rm command)

49) What vmstat show

Ans) vmstat (virtual memory statistics) is a computer system monitoring tool that collects and displays summary information about operating system memory, processes, interrupts, paging and block I/O

50) What is tmpfs File System

Ans) Reference : http://en.wikipedia.org/wiki/Tmpfs

tmpfs is a common name for a temporary file storage facility on many Unix-like operating systems. It is intended to appear as a mounted file system, but stored in volatile memory instead of a persistent storage device. A similar construction is a RAM disk, which appears as a virtual disk drive and hosts a disk file system.

Everything stored in tmpfs is temporary in the sense that no files will be created on the hard drive; however, swap space is used as backing store in case of low memory situations. On reboot, everything in tmpfs will be lost.

The memory used by tmpfs grows and shrinks to accommodate the files it contains and can be swapped out to swap space.

51) What is the difference between screen and script commands?

Ans) Screen is an screen manager with VT100/ANSI terminal emulation and used to take GNU screen session remotely or locally and while Script make typescript of terminal session

Screen : needs to be detached, should not be exited to access remotely/locally

Script : creates a file and store all the terminal output to this file

52) How can we check which process is assigned to which processor?

Ans) Run "ps -elFL" and find out the PSR column which is showing the processor number to the process

53) How can we check vendor, version, release date, size, package information etc... of any installed rpm?

Ans) rpm -qi package-name , for example:

rpm -qi ypbind-1.19-12.el5

54) What is perl (lowercase) and Perl (uppercase)

Ans) Perl is a programming language

perl is binary executable used to run Perl.

55) How to change PE size of a Volume Group

Ans) vgchange -s vg_name

56) How to set block size of an filesystem

Ans) mkfs -t ext3 -b 8192 /dev/sdd

To be continued....

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