Suresh Kumar Pakalapati's Linux Administration: Linux Interview Questions:Google

Wednesday, August 11, 2010

Linux Interview Questions:Google

Q2) Write a command to find all of the files which have been accessed within the last 30 days.

A1) The Command is , find / -atime +30

A2) # find / -type f -mtime +30

A3) find / -type f -mtime -30

A4) find / -type f -atime -30 = accessed

find / -type f -ctime -30 = changed

find / -type f -ctime -30 = modified

Q3) How to schedule cron backup to run on 4th saturday of month??

A1) crontab -e

** ** ** ** /5

Q4) how to see unallocated hard disk space on linux

A1) simply type

cat /proc/partitions

A2) df -h /dev/devicename

device name could be hda,sda

Q5) find out what file systems supported by kernel?

A1) # fdisk /dev/hdx

option: t

it will show the supported FS in the kernel

A2) cat /proc/filesystems

will show all the file system types that the kernel can

handle currently. Be aware that kernel will load the

necessary modules automatically if it have, for a new file

system type present in a new device you plug into it and

then the output of the above command will vary.

A3) you can give following commands

#fdisk

then press t

this command will show all the file system with code

supported by the current linux kernel.

Q6) how do u extract files from iso cd images in linux?

A1) From ISO MAGIC

A2) mount -o loop

A3) mount -t iso9660 -o ro,loop=/dev/loop0 /home/ste/cd.iso /mnt/iso

Q7) how do u find remote machine operating system and version?

A1) XProbe

A2) by ssh to that machine then give command

#uname -r

A3) for OS,

cat /etc/issue and version, uname –r

Q8) how do you port scaning with netcat command?

A1) nc -z

Q9) how do find all failed login attempts via ssh?

A1) check network connectivity check correct ipaddress

ping ipaddress

A2) 'who' or 'w' is the command used to find the users who logged in

The system and their attempts, with the help of some options

A3) Failed ssh logs are either written in /var/log/messages, or

/var/log/secure (configurable in /etc/syslog.conf). I am

assuming that the failed login attempts are recorded in

/var/log/secure:

grep ' authentication failure' /var/log/secure | sed -e

's/^$.*$$rhost.*$$/\2/p' | tr -s " " | cut -f2 -d"=" |

cut -f1 -d" " | sort -n | uniq -c

Will show you the count, and the IP/hostname of machines that tried

to access the system via ssh

A4) The command is ,

cat /var/log/messages | grep "Failed password"

it will show all the user which is not able to login.

Q10) How do display error messages instantly when command fails?

A1) Suppose you are trying to cat a non existing file:

cat /etc/shado 2>/dev/null || echo “Failed to open file”

command 2 "echo" is executed only if command 1 "cat" returns

a non-zero exit status

# cat /etc/shado 2>/dev/null || echo "Failed to open file"

Failed to open file

Q11) How do find hard disk revolutions speed?

A1) A typical desktop hard disk rotate at 7,200 revolutions per

minute (RPM). A typical server hard disk spin at 10,000 or

15,000 rpm to achieve sequential media transfer speeds. You

can use hard disk model number to obtain disk RPM. For

example, a typical Seagat disk Model # ST373455SS can

provide following information:

* ST - Brand identity

* 3 - Form Factor (3 = 3.5")

* 73 - Disk size / Capacity in GB i.e. 73GB

* 4 - Reserved for future use

* 5 - RPM ( 5 = 15k and 0 = 10K)

* 5 - Generation

* SS - Indicates interface i.e Serial Attached SCSI

How do I find out hard disk model and serial number?

Use any one of the following command from shell prompt to

find out hard disk model number:

$ cat /proc/scsi/scsi

OR use scsi_id command to querys a SCSI device via the SCSI

INQUIRY vital product data (VPD) page 0x80 or 0x83 and uses

the resulting data to generate a value that is unique across

all SCSI devices that properly support page 0x80 or page 0x83.

$ /sbin/scsi_id -g -p 0x80 -s /block/sdd

$ sudo grep -i sdd /var/log/boot.log

OR use sginfo / scsiinfo command from sg3_utiles package, enter:

# sginfo -a /dev/sdd | more

Q12) when zombie process fully cleared?

A1) When the server gets restarted!

A2) Do a ps and identify the zombie processes

# ps -el | grep 'Z'

Occasionally, these processes are in such a state that the

only way to get rid of them is to reboot to clear them.

In most cases, you can get rid of a zombie by normal means,

"kill -15 zombie PPID", "kill -9 zombie PPID".

Q13) how do you configure linux system as a router?

A1) Give the following command

echo 1 > /proc/sys/net/ipv4/ip_forward

A2) above anwser also write,

but must me know this,

vi /etc/sysctl.conf

# Controls IP packet forwarding

net.ipv4.ip_forward = 1

save &

sysctl –p

Q14) Which priority has the process?how do u view?

A1) nice command is used to find the priority

A2) Each process has a niceness value associated with it, which

is what the kernel uses to determine which processes require

more processor time than others. The higher the nice value,

the lower the priority of the process. In other words, the

“nicer” the program, the less CPU it will try to take from

other processes; programs that are less nice tend to demand

more CPU time than other programs that are nicer.

The priority is noted by a range of -20 (the highest) to 20

(the lowest). Using ps, you can see the current nice value

of all programs:

# ps axl

F UID PID PPID PRI NI VSZ RSS WCHAN STAT TTY

TIME COMMAND

4 0 1 0 16 0 2648 112 - S ?

0:01 init [3]

1 0 2 1 34 19 0 0 ksofti SN ?

0:02 [ksoftirqd/0]

5 0 3 1 10 -5 0 0 worker S< ?

0:00 [events/0]

You can see that init has a nice value of 0, while other

kernel tasks associated with PID 2 and 3 have a nice value

of 19 and -5 respectively.

Typically, a program inherits its nice value from its

parent; this prevents low priority processes from spawning

high priority children

Q15) Any one example of uninteruptable sleeping process?

A1) init (PID 1)

Q16) how many limitations of under directories in ext2/3 linux

file system?

A1) Ext2 Limits

============

Max file size: 2-64 TiB

Max number of files: 10 raised 18

Max filename length: 255 characters

Max volume size: 16-32 TiB

Allowed characters in filenames: Any byte except NULL and '/'

Ext3 Limits

===========

Max file size: 2 TiB

Max number of files: Variable, allocated at creation time[1]

Max filename length: 255 bytes

Max volume size: 2 TiB – 16 TiB

Allowed characters in filenames: All bytes except NULL

Q17) what is a superblock ?

A1) A superblock is a record of the characteristics of a

filesystem, including its size, the block size, the empty

and the filled blocks and their respective counts, the size

and location of the inode tables, the disk block map and

usage information, and the size of the block groups.

Q18) what is the command to check network interfaces in our system?

how to set etho to 10 full duplex speed?

A1) ifconfig, ifconfig -a, ip dev ls,

ethtool -s eth0 speed 10 duplex full

to watch the info on eth0 ( mii-diag --watch eth0 )

Q19) what is the UID and GID of root user? Can a normal user can

change the ownership of a file? what is the command to

change ownership of a file?

A1) the root UID/GID is 0 (zero). which is why he can able to

intervene in all normal users files even though he don't

had permission. A normal user will don't have the permission

to change ownership of file. The command to change ownership

is < chown user.user file >

Q20) what is soft mount and hard mount? i have to make permanent

nfs mount permanent what shall i do?

A1) to make permanent nfs mount, write it into /etc/fstab

Server nfsmount mountpoint filesystem defaults 0 0

192.168.0.1 /var/ftp/pub /mnt nfs defaults 0 0

A2) This is a UNIX terminology as to what the client does when

it can't talk to an NFS Server. If you just mount a file

system without specifying hard or soft, the default is a

hard mount. Hard mounts are preferable because of the

stateless nature of NFS. If a client sends an I/O request to

the server (such as an ls -la), and the server gets

rebooted, the client will wait until the server comes back

on line. This preserves data transfers in the event of a

server failure. There are disadvantages to this, as a simple

mount request could hang. A soft link will return with an

error and fail. This kills the wait time, but can cause

problems with data transfers.

To make permanent nfs mount, the above answer is right

Q21) what is major and minor?

A1) Major number

============

A number indicating which device driver should be used to

access a particular device. All devices controlled by the

same device driver have a common major device number.

Minor number

============

A number serving as a flag to a device driver.The minor

device numbers are used to distinguish between different

devices and their controllers.

Eg:

# ls -l /dev/sda1

brw-rw---- 1 root disk 8, 1 2008-09-04 08:28 /dev/sda1

rigel:~# ls -l /dev/sda2

brw-rw---- 1 root disk 8, 2 2008-09-04 08:28 /dev/sda2

You can see the major and minor device number (8,1) and

(8,2) in the ls listing for /dev/sdax

# ls -l /dev/scd0

brw-rw---- 1 root cdrom 11, 0 2008-09-04 08:28 /dev/scd0

You can see the major and minor device number (11,0)in the

ls listing for /dev/scd0

Q22) what is the command to make a process to run in the

background from foreground?

A1) To make the command run in the background suppose the

Command is "command_name" then the syntax for the same will

be such as follows;

[root@neo root]# command_name &

Q23) 1.what is Kernel parameters?

2.how many cpu have use in your machine?

A1) Kernal parameters are nothing but all the parameters in

/etc/system hear we can edit according to our requrement.

to check the how many cpus in the machine jus type this command

#psrinfo

to check the cpu speec #psrinfo –v

A2) 1.kernel parameters

parameters provide mechanisms to adjust the functiong of linux kernel.

the sysctl command used to view kernel parameters.

2.simply type in root

# cat /proc/cpuinfo

Q24) 1.I want to change runlevel but the Users shall not be disturbed?how?

2.Disk have 5gb disk utilization even though files unable to reate, why?

3.what are the internal and external command in linux?

4.sar command o/p?

5.how list the open files?

6.what is kernel compiling?

7.How do u See complete configuration in ur system?

8.how will u make a daily updates with cron daily?

9.which port is associated with ttys0?

10.specific some problems linux admin(if u are linuxadmin)faced

And how did u overcome it?

A1) 1. For changing the runlevel you should have to go in

/etc/inittab. There you can change the runlevel what you

want.It will not affecting current user. After restarting

system your system will boot in which run level you have set.

3. Internel command means whichever command are builtin the

system BIOS. And External command means Which are

nonbuiltin.It outside the shell. It require shell.

4. VMstat & sar are showing the free memory of the system.

5. ls command is use list a file & cat & vi is to open a file.

A2) 1) Same answer as above

2) Question not understood, may be ACLs

3) It is not BIOS,

A built-in (internal/resident) command is one that is

contained within the bash tool set and execute faster than

external commands.

An non-built-in (external/non-resident) command is a command

outside the shell and requires a $PATH (environmentvariable) to findit.

4)Displays the activity for the CPU

# sar

Linux 2.6.18-6-686 (server.domain.local) 09/16/2008

11:31:56 AM LINUX RESTART

5) # lsof

6)The Linux kernel is a complex program which provides the

underlying services to the rest of a Linux distribution. But

it is easy to add new features or improvements to it,

requiring a kernel recompiling. There are three reasons for

a kernel compile. Firstly, you may have some hardware that

is so new that there's no kernel module for it in on your

distribution CD. Secondly, you may have come across some

kind of bug which is fixed in a revision of the operating

system. Lastly, you may have some new software which

requires a newer version of the operating system.

7)A vague question

8) Suppose we have a daily update script called dailyupdate.sh

Copy the script to /etc/cron.daily/

# cp dailyupdate.script /etc/cron.daily/

Make the file executable

# chmod +x /etc/cron.daily/dailyupdat.sh

9) COM1

10) Depends...

A3) 4) Detailed sar output

Linux 2.6.18-6-686 (server.domain.local) 09/16/2008

11:31:56 AM LINUX RESTART

11:35:01 AM CPU %user %nice %system

%iowait %steal %idle

11:45:01 AM all 0.71 0.00 0.19

0.18 0.00 98.92

11:55:02 AM all 1.53 0.00 0.19

0.26 0.00 98.02

Average: all 1.12 0.00 0.19

0.22 0.00 98.47

A4) 1. use init , at runtime.

2. disk quota has been set and reached for the current user.

3. same as above

4. same as above

5. same as above

6. lay man definition, adding support for required hardware

and modules in the Linux kernel.

7. sysreport, question too generalized,

8. same as above

9. same as above

10. :)

Q25) what is user mode & kernel mode?

A1) KERNEL-MODE

The kernel-mode programs run in the background, making sure

everything runs smoothly - things like printer drivers,

display drivers, drivers that interface with the monitor,

keyboard, mouse, etc. These programs all run in such a way

that you don't notice them.

When the computer boots up, Windows calls the KERNEL, the

main kernel-mode program that allows all the other programs

to run, even the user-mode programs.

USER-MODE

These are the programs that you run when you want specific

programs - e.g., MS Paint, MS Word, and Calculator. These

are heavily restricted, as to not crash the system. Windows

uses memory-protection services offered by the processor to

prevent malicious programs from interfering with the rest of

the system and corrupting it.

A2) A process can run in two modes:

1.User Mode.

2.Kernel Mode.

1.User Mode:

=>A mode of the CPU when running a program.

=>In this mode ,the user process has no access to the

memory locations used by the kernel.When a program is

running in User Mode, it cannot directly access the kernel

data structures or the kernel programs.

2.Kernal Mode:

=>A mode of the CPU when running a program.

=>In this mode, it is the kernel that is running on behalf

of the user process and directly access the kernel data

structures or the kernel programs.Once the system call

returns,the CPU switches back to user mode.

When you execute a C program,the CPU runs in user mode till

the system call is invoked.In this mode,the user process has

access to a limited section of the computer's memory and can

execute a restricted set of machine

instructions.however,when the process invokes a system

call,the CPU switches from user mode to a

more privileged mode the kernel. In this mode ,it is the

kernel that runs on behalf of the user process,but it has

access to any memory location and can execute any machine

Instruction. After the system call has returned,the CPU

switches back to user mode.

A3) KERNEL-MODE

Kernel mode, also referred to as system mode, is one of the

two distinct modes of operation of the CPU in Linux. When

the CPU is in kernel mode, it is assumed to be executing

trusted software, and thus it can execute any instructions

and reference any memory addresses (i.e., locations in

memory). The kernel (which is the core of the operating

system and has complete control over everything that occurs

in the system) is trusted software, but all other programs

are considered untrusted software.

USER-MODE

User mode is the normal mode of operating for programs, web

browsers etc. They don't interact directly with the kernel,

instead, they just give instructions on what needs to be

done, and the kernel takes care of the rest. Kernel mode, on

the other hand, is where programs communicate directly with

the kernel. A good example of this would be device drivers.

A device driver must tell the kernel exactly how to interact

with a piece of hardware, so it must be run in kernel mode.

Because of this close interaction with the kernel, the

kernel is also a lot more vulnerable to programs running in

this mode, so it becomes highly crucial that drivers are

properly debugged before being released to the public

Q26) what is the diff b/w ext2 and ext3?

A1) ext3 is the advanced version of ext2

ext2+journaling=ext3

this journaling feature is very much useful while

retrieving data and writing data into a File System

Latest versions of all Linux flavors are coming with ext3

compatible file systems.

A2) ext 2 & ext3 are the two file systems in linux.ext 2 will

take more time while logon the system. this is bcoz ext2

will check all harddisk peripherals at the time of switch on

your sytem. But ext3 is less time consuming,bcoz instead of

checking all harddisk peripherals it will check only the

bootloader peripherals.

Q27) what is initrd image?

A1) initrd image is the initial ram disk image While the system

getting booted the kernel image will get loaded into the main

memory after POST to improve I/O performance.this initrd image

will contain the same version number of kernel and if we face any error

in booting regardingthis initrd we can reinstall it with 'mkinitrd' command

A2) initial RAM disk (initrd) is a temporary root file system

that is mounted during system boot to support the two-state

boot process. The initrd contains various executables and

drivers that permit the real root file system to be

mounted, after which the initrd RAM disk is unmounted and

its memory freed. In many embedded Linux systems, the

initrd is the final root file system

A3) initrd image is a image file which has initial modules which

are loaded in to the kernel while booting.

Q28) what is jumbi process? Oracle

A1) its a dead process it's parent process has been killed abruptly.

A2) zoombie process or defunct process. - Process that finish execution

Or we can say the process died but still it has entry in the

process table.

A process finished execution but parent of that process is

Not ready to accept the exit status from it . At that time

process go to zoombie state.

Q29) My machine is running half duplex mode how to change half

duplex to full duplex? ORACLE

A1) [root@ns2 ~]# mii-tool -V

mii-tool.c 1.9 2000/04/28 00:56:08 (David Hinds)

eth0: negotiated 100baseTx-FD flow-control, link ok

[root@ns2 ~]# ethtool -s eth0 duplex half autoneg off

[root@ns2 ~]# mii-tool -V

mii-tool.c 1.9 2000/04/28 00:56:08 (David Hinds)

eth0: 100 Mbit, half duplex, no link

[root@ns2 ~]# mii-tool -V

mii-tool.c 1.9 2000/04/28 00:56:08 (David Hinds)

eth0: 100 Mbit, half duplex, link ok

Q30) some one is asking my machine is slow what is your steps? ORACLE

A1) The increase in the size of SWAP partition may show you the

little effect in increasing the system speed and better run

in Single user mode rather than GUI mode

A2) use top command, then check which process use the most

resource and find the problem.

A3) Re-read all process

#killall –HUP

Q31) what is mean by system calls INFOTECH

A1) A system call is the mechanism used by an application

program to request service from the operating system.

On Unix-based and POSIX-based systems, popular system

calls are open, read, write, close, wait, exec, fork, exit,

and kill. Many of today's operating systems have hundreds of

system calls. For example, Linux has 319 different system

calls. FreeBSD has about the same (almost 330).

Tools such as strace and truss report the system calls made

by a running process.

Q32) What is trusted/untrusted operating system

A1) As per my perspective , I used to say any/all UNIX flavors

rather than MS WINDOWS

Because of

1. File/Directory permission (Action:Read/Write/Execute,

permissions can be given on behalf of the particular

User/group/Others).

2. All the unix configuration as files rather than

Registry (windows)

a. which cause the application to fail when a newer

or incompatible dynamic library is installed.

b. Some times you may need to restart the entire

system to update the changes with registry.

3. Linux Strong password protection. It won't get booted

(Drop you @ the GUI) without the successful

authentication (even though you don't have any more

users rather than admin/root).In Linux setting

password to the admin/root users is must during the

installation itself.

Where as in windows, let allow you to leave the

password as blank for admin/other user, System also

booting without authenticating.

4. Linux is open source, free under GNU. It doesn't mean

that its only free to install or use, but in fact you

can have these source codes and they are allowing you

to change it and redistribute it.

Windows is Lincesed os. Windows is developed by

Microsoft.

5. Linux is more secure, windows is less secure than

linux in case of virus, worms.

Because in Linux all the files/directory won't get

executable permission, Perhaps you need to set it

manually. Though by default all the incoming trojen,

virus, malwares and worms won't affect the system.

6. Distribution

Linux : Linux has many distributors like Radhat,

mandrake, Corel etc

Windows : whereas windows has only Micrsoft.

7. Run Level

Linux works on both GUI & Console mode (Checkout the

various run levels runlevel 0 - runlevel 6 Generally

known as init level)).

Windows : Only GUI.

8. By default LINUX have multiuser, multitasking,

multiprocessing, multiprogramming features. Perhaps

this will not be available on windows(Except some

versions), In that case you need to PAY & BUY

the extra features in windows.

9. You can play around the Linux through LIVE CDS on a

Windows machine.

Only is a live CD that runs over windows, in RAM,

and doesn't change your system at all..the other is

an install disk. I've used the live CD on two XP

machines and they both ran flawlessly. When you

want to quit, just go to logout under system tools,

and shut down.

Linux will turn itself off, eject the disk, and turn

of your PC.

Remove the disk, boot up, and you're running your

Windows again.

This is the way we are checking the newly bought

NIC/others hardwares support with all the Linux

distributors. Once it got detected, its very easy to

get the device driver names and other details about

that new hardwares, then compile the new kernel

image (bzImage) on your own build version systems.

Q33) Working in Linux/Unix platform, if any command dose not

work, say # ls reports as command not found.

what to do? GOOGLE

A1) Need to check the path first body ....

A2) reinstall that shell

A3) Check whether PATH variable as got the entry /bin

It should be something like this

=========================

[root@bc ~]# echo $PATH

/usr/kerberos/sbin:/usr/kerberos/bin:/bin:/sbin:/usr/bin

=========================

Q34) in unix how to change old name to new name

A1) mv // for rename the file in the same