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
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?
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
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
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 /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.
# 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? 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
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? list the open files?
6.what is kernel compiling?
7.How do u See complete configuration in ur system? 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
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
Copy the script to /etc/cron.daily/
# cp dailyupdate.script /etc/cron.daily/
Make the file executable
# chmod +x /etc/cron.daily/
9) COM1
10) Depends...
A3) 4) Detailed sar output
Linux 2.6.18-6-686 (server.domain.local) 09/16/2008
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?
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.
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.
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 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
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
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
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
Q34) in unix how to change old name to new name
A1) mv // for rename the file in the same
mv // for rename and
move the file in other diretory.
A2) 1. open the file /etc/sysconfig/network and change the
parameter HOSTNAME= to new name.
2. restart the network services.
3. open the file vi /etc/hosts, and change the old name to
new name. localhost.localdomain
A3) mv
[root@localhost]# touch file1 //created a file named file1
root@localhost]# ls //displays the file created
[root@localhost]# mv file1 file2 //command to rename file1
[root@localhost]# ls //displays the renamed file
A4) see there is no rename command in unix ,
so what u can use is "move" (mv) command
mv "(absolute path) old file name" "new file name
A5) rename
Q35) what is the standard output in a file as well as disply it
on the terminal options like
A)pr B) tail C) tee D)nl
A1) tail as it display ten current line
tail -f
option with this
tail -f -n 100
A2) I think, framing the question should be like
"What is the command used to write to the standard output
as well as to a file simultaneously ?
Answer) tee
It is a command that deals with pipes in unix. It
redirects the output of a command to an other command and
at the same time it redirects to a file also.
Q36) what is command like " #(cd unixos; pwd) " tell me OUTPUT
A1) first it will change the directory to unixos and then gives
the output as //unixos
A2) it is two commands runnung one by one.first the cd unixos
command will be executed,then the present working directory
will the output will be showing the presentworking
dirctory that is unixos dir.
A3) WRONG when I hit this command no output or error is printed
Q37) in unix how to change old name to new name