Friday, August 28, 2015

some useful linux commands and their full forms

ACL                            Access control list.
awk                             Aho, Weinberger, and Kernighan.
BIOS                          basic input output system.
cd                               change Directory.
chmod                        change mode.
chown                        change owner.
CIFS                          common Internet File System
df                                disk free.
DHCP                        Dynamic Host Configuration Protocol.
DNS                           Domain Name System.
du                              disk usage.
ext2                            second extended file system.
ext3                            third extended file system.
ext4                            fourth extended file system.
fdisk                           fixed disk.
FTP                            File Transfer Protocol.
grep                            Global  regular expression print.
GRUB                        Grand unified Bootloader.
http                            HyperText Transfer Protocol.
ifconfig                       Interface Configuration.
IMAP                         Internet Message Access Protocol.
KVM                          Kernel Virtual Machine.
LVM                          logical volume manager.
MBR                          Master Boot Record.
mkfs                           make filesystem.
NFS                            Network file system
pop                             Post Office Protocol.
ps                                process status.
pwd                            present working directory.
rm                              remove.
RPM                          Redhat package Manger.
scp                              secure copy.
sed                              Stream Editor.
SGID                          set group id.
SMB                           Server Message Block
ssh                              Secure Shell.
su                               substitute user or switch user.
sudo                           substitute user do or super user do .
SUID                          set user id .
tar                              tape archive.
TFTP                         Trivial File Transfer Protocol.
umask                        User's File Creation Mask.
Vim                            Visual Improved
VSFTP                       very secure  File Transfer Protocol.
YUM                          Yellowdog updater modified.

Wednesday, August 26, 2015

Commands to check the Linux Version, Release name & Kernel version.

uname -a (print all information)

uname -r (print the kernel name)

cat /proc/version

cat /etc/issue

cat /etc/redhat-release

lsb_release -a

Friday, August 21, 2015

what is virtual memory, paging , swap space ?

A process normally runs on physical memory where the memory is divided into sets of pages. A page is a 4kb area of memory and is the basic unit of memory with which both kernel and CPU deal.

There might be a situation when all the pages in physical memory goes full. In such cases all the inactive pages inside physical memory is shifted to the secondary storage or the swap space using the paging technique. By doing this physical memory gets free pages which can again be utilized by new processes. This entire process is termed as swapping.

NOTE: Swapping is a good idea as it gives you an additional space to store data files and programs when your physical memory is out of space but accessing a hard disk is hundred times slower than accessing memory.

Virtual memory is a memory management technique that is implemented using both hardware and software which gives an application program the impression that it has contiguous working memory (an address space).

In simple terms Virtual memory is a logical combination of RAM memory and swap space which is used by running process

NOTE: It is NOT just an additional space used in hard disk to make it act as physical memory

This is one of the memory management technique schemes by which a computer can store and retrieve data from secondary storage for use in main memory.

Swap space
This is a space on the hard disk which is used by the operating system to store data pages that are currently not needed. This swap sapce can be a partition as well as swap file. Generally swap space is double of the RAM .

Amount of RAM in the system recommended amount of swap space
4GB of RAM or less  a minimum of 2GB of swap space
4GB to 16GB of RAM  a minimum of 4GB of swap space
16GB to 64GB of RAM  a minimum of 8GB of swap space
64GB to 256GB of RAM  a minimum of 16GB of swap space

256GB to 512GB of RAM  a minimum of 32GB of swap space

Wednesday, August 5, 2015

linux interview questions and answers part 2

1. What is ldd?
List dynamic dependencies – print shared library dependencies
Eg: – # vim new.c
# gcc new.c –o new
# ldd new

2. What is the command to uninstall processes in Linux?
rpm –e sendmail
-e – remove

3. What is the command for finding the highest memory occupied file in Linux?
du –ah / | sort –n –r | head –n 1
du – estimate file space usage
-a – write counts for all files, not just directories
-h – print sizes in human readable format (eg. 1K 234M 2G)
sort – sort lines of text files
-n – compare according to string numerical value
-r – reverse the result of comparisons
head – output the first part of files
-n – number of lines

4. What are the Linux boot files?
1./boot/grub/grub.conf: contains boot disk parameters
2./etc/fstab: contains File systems which need to mount at boot time
3./etc/initab: Contains default run level
4./etc/init.d/rc.d/rcN.d: This is a dir it contains

5. Difference between swap partition and swap file?
Swap partition is maintained as a separate partition. Same swap partition can be used for two OS within single machine.
Suppose if the system crashes, there is a chance to recover or it may not  corrupt the partition.
Less fragmented.
Where as swap file takes very less space. We can increase the space very easily, compared to swap partition. Swap file system fragmented.
If  the system crashes then there is a huge chance to lost the swap file system.

6. A file which is not deleted by normal user and also root (using rm), for that type of file how we delete it?
Using chattr command, we need to change the attributes and then we remove using rm command.
Eg: – chattr -iIu example
rm -rf example

7. Difference between nfs soft and hard mounting points?
Hard mount option: – If the client fails to access the server, then the connection hangs and once the system is up then it will again access the server.
Soft: – If the client failed to connect the server, it immediately gives the error report and closes the connection.

8. If we transfer 100 files by ftp to remote server, how to know the files are successfully transfer or some file are not transferred?
ftp>mput 1 2 3 …. 100
ftp> ls –l

9. I know ssh, telnet, dns,apache all are worked on TCP/UDP but i want to know any one service which are working on UDP only?
snmptrap 162/udp
snmptrap – simple network management protocol trap
snmptrapd is an SNMP application that receives and logs

10. I want to built a fire wall using iptables. My condition is ” inbound to with a port of 80 from should accept”
iptables –A INPUT –p tcp –dport 80 –s –d –j ACCEPT

11. I want to see how many interfaces (Ethernet cards) are working using single command?

12. What is the status code 403,404 represented in apache server?
403 represent forbidden error, means if a file misses some selinux security context.
404 represent that there is a cgi script missing or web pages missing.

13. How to monitor ports in a linux machine, with single command?
nmap localhost

14. In my linux machine, i lost /etc/passwd file and /etc/shadow file, then how can i recover it?
Normally in linux we must have backup by default for /etc/passwd and /etc/shadow files
/etc/passwd —> /etc/passwd-
/etc/shadow —> /etc/shadow-
from there we can copy or restore. If both are not available. Then follow below steps:
1. reboot
2. Single user mode [ single init=/bin/bash ]
3. pwconv
4. check /etc/passwd and /etc/shadow files are there
5. sync
6. init 3 or reboot with init 3

15. what r the different command to check ram,process and hdd of linux machine
To check ram in your system:
To check process
To check hdd
#sfdisk -l

16.If i run ls command it will show me the junk output what is problem and how to resolve it
Set your terminal setting by stty and before that, export ls command to PATH variable

AIX (Advanced Interactive eXecutive) is an open operating system from IBM which is based on a version of UNIX. AIX/ESA was designed for IBM’s System/390 or large server hardware platform. AIX/6000 is an operating system that runs on IBM’s workstation platform, the RISC System/6000.

18. Why ls -F dev/log file output indicate as = sign at end of the file name?
Standard output

19. What is nis server?
NIS is a service that provides any user on a network with the same working environment irrespective of the system on that network, which has been used for login purpose.
For example if NIS server is set up in a single system and configured to hold user accounts and their passwords and access information. Then any user on that network can login to his/her account from any system (with nis client running) on that configured network. This gives a look and feel that the user is logged into his/her own system. But actually it’s the account on the NIS server that is mounted on the local sytem user login.

20. What command can you use to review boot messages?