Linux interview questions basics and experienced

Basic Questions

1. What is initrd image and what is its function in the linux booting process?

The initial RAM disk (initrd) is an initial root file system that is mounted prior to when the real root file system is available.The initrd is bound to the kernel and loaded as part of the kernel boot procedure. The kernel then mounts this initrd as part of the two-stage boot process to load the modules to make the real file systems available and get at the real root file system. Thus initrd image plays a vital role in linux booting process.

2. Explain the terms suid, sgid and sticky bit?

In addition to the basic file permissions in Linux, there are few special permissions that are available for executable files and directories.
SUID : If setuid bit is set, when the file is executed by a user, the process will have the same rights as the owner of the file being executed.
SGID : Same as above, but inherits group privileges of the file on execution, not user privileges. Similar way when you create a file within the directory, it will inherit the group ownership of the directories.
Sticky bit : Sticky bit was used on executables in linux so that they would remain in the memory more time after the initial execution, hoping they would be needed in the near future. But mainly it is on folders, to imply that a file or folder created inside a stickybit enabled folder could only be deleted by the owner. A very good implementation of sticky bit is /tmp , where every user has write permission but only users who own a file can delete them.

3. List out few of the differences between Softlink and Hardlink?

a) Hardlink cannot be created for directories. Hard link can only be created for a file.
b) Symbolic links or symlinks can link to a directory.
c) Removing the original file that your hard link points to does not remove the hardlink itself; the hardlink still provides the content of the underlying file.
d) If you remove the hard link or the symlink itself, the original file will stay intact.
e) Removing the original file does not remove the attached symbolic link or symlink, but without the original file, the symlink is useless

4. How do you sent a mail attachment via bash console?

"mutt" is an opensource tool for sending emails with attachments from the linux bash command line. We can install "mutt" from the binary rpm or via package manager.
For Ubuntu / Debian based destros.

# apt-get install mutt

For Redhat / Fedor based destros,

# yum install mutt

Usage :

# mutt -s "Subject of Mail" -a "path of attachment file" "email address of recipient" < "message text containing body of the message"

Eg : mutt -s "Backup Data" -a /home/backup.tar.gz admin@mywebsite.com < /tmp/message.txt

5. What is the difference between umask and ulimit ?

umask stands for ‘User file creation mask’, which determines the settings of a mask that controls which file permissions are set for files and directories when they are created. While ulimit is a linux built in command which provides control over the resources available to the shell and/or to processes started by it.
You can limit user to specific range by editing /etc/security/limits.conf at the same time system wide settings can be updated in /etc/sysctl.conf

6. What are the run levels in linux and how to change them?

A run level is a state of init and the whole system that defines what system services are operating and they are identified by numbers.There are 7 different run levels present (run level 0-6) in Linux system for the different purpose. The descriptions are given below.

0: Halt System (To shutdown the system)
1: Single user mode
2: Basic multi user mode without NFS
3: Full multi user mode (text based)
4: unused
5: Multi user mode with Graphical User Interface
6: Reboot System

To change the run level, edit the file “/etc/inittab” and change initdefault entry ( id:5:initdefault:). If we want to change the run level on the fly, it can be done using ‘init’ command.
For example, when we type ‘init 3' in the command line , this will move the system from current runlevel to runlevl 3. Current level can be listed by typing the command 'who -r'

7. What is the functionality of a Puppet Server ?

Puppet is an open-source and enterprise application for configuration management toll in UNIX like operating system. Puppet is an IT automation software used to push the configuration to its clients (puppet agents) using code. Puppet code can do a variety of tasks from installing new software, to check file permissions, or updating user accounts and lots of other tasks.

8. What is SeLinux?

SELinux is an acronym for Security-enhanced Linux. It is an access control implementation and security feature for the Linux kernel. It is designed to protect the server against misconfigurations and/or compromised daemons. It put limits and instructs server daemons or programs what files they can access and what actions they can take by defining a security policy.

9. What is crontab and explain the fields in a crontab ?

The cron is a deamon that executes commands at specific dates and times in linux. You can use this to schedule activities, either as one-time events or as recurring tasks. Crontab is the program used to install, deinstall or list the tables used to drive the cron daemon in a server. Each user can have their own crontab, and though these are files in /var/spool/cron/crontabs, they are not intended to be edited directly. Here are few of the command line options for crontab.
crontab -e Edit your crontab file.
crontab -l Show your crontab file.
crontab -r Remove your crontab file.
Traditional cron format consists of six fields separated by white spaces:
<Minute> <Hour> <Day_of_the_Month> <Month_of_the_Year> <Day_of_the_Week> <command/program to execute>
The format is explained in detail below.

* * * * * *
| | | | | |
| | | | | +-- Year (range: 1900-3000)
| | | | +---- Day of the Week (range: 1-7, 1 standing for Monday)
| | | +------ Month of the Year (range: 1-12)
| | +-------- Day of the Month (range: 1-31)
| +---------- Hour (range: 0-23)
+------------ Minute (range: 0-59)

10. What are inodes in Linux ? How to find the inode associated with a file ?

An inode is a data structure on a filesystem on Linux and other Unix-like operating systems that stores all the information about a file except its name and its actual data. When a file is created, it is assigned both a name and an inode number, which is an integer that is unique within the filesystem. Both the file names and their corresponding inode numbers are stored as entries in the directory that appears to the user to contain the files. The concept of inodes is particularly important to the recovery of damaged filesystems. When parts of the inode are lost, they appear in the lost+found directory within the partition in which they once existed.
The inode entries store metadata about each file, directory or object, but only points to these structures rather than storing the data. Each entry is 128 bytes in size. The metadata contained about each structure can include the following:
Inode number
Access Control List (ACL)
Extended attribute
Direct/indirect disk blocks
Number of blocks
File access, change and modification time
File deletion time
File generation number
File size
File type
Group
Number of links
Owner
Permissions
Status flags
Inode structure of a directory consists of a name to inode mapping of files and directories in that directory.In a directory, You can find the inode number corresponding to the files using the command ls -i
786727 -rw------- 1 root root 4226530 May 29 13:17 sudo.log
786437 -rw-------. 1 root root 32640 Jun 23 20:11 tallylog
786440 -rw-rw-r--. 1 root utmp 276096 Jul 20 06:45 wtmp
786741 -rw------- 1 root root 9653 Jul 17 09:38 yum.log

Similar way, the number of inodes allocated, used and free in a Filesystem can be listed using df -i command

# df -i /root
Filesystem Inodes IUsed IFree IUse% Mounted on
/dev/mapper/RootVol-lvmroot
524288 80200 444088 16% /

The other way we can get the inode details of a file by using the stat command.
Usage : # stat <file name>
Example :

-sh-4.1$ stat note.txt
File: `note.txt'
Size: 4 Blocks: 8 IO Block: 4096 regular file
Device: fd05h/64773d Inode: 8655235 Links: 1
Access: (0644/-rw-r--r--) Uid: (69548/nixuser) Gid: (25000/ UNKNOWN)
Access: 2014-06-29 15:27:56.299214865 +0000
Modify: 2014-06-29 15:28:28.027093254 +0000
Change: 2014-06-29 15:28:28.027093254 +0000

11. Why should I use DAS either NAS or SAN

When we talk about storage, there are some solutions which exist but before choosing one solution, we need to know their role:

• DAS is a block device from a disk which is physically attached to the host machine (such as /dev/sda or /dev/sda1) . You must place a filesystem upon it before it can be used. There are limitations like the number of servers that can access it. Storage device, or say DAS storage has to be near to the server storage and the resources are dedicated but generally, you are not able to dedicate the hard disks to multiple computers. DAS solution is inexpensive and simple to configure. Technologies to do this include IDE, SCSI, SATA, etc.
• NAS  authenticates clients and provides shared to other computers and users over a network so it requires a dedicated ip address to be accessible. NAS devices generally run an embedded operating system on simplified hardware and lack peripherals like a monitor or keyboard. Network file systems can be considered safe enough to be used in a concurrent way, the protocol implementation will take care of problems due to concurrent access to the same resource (file), normally by locking the file to a single user/requester. You can set up automatic or manual backups and file copies between the NAS and all other connected devices by using a software program. It is an easy way to provide RAID redundancy to mass amount of users, it allows users permissions, folder privileges, restricted access to documents, etc
• SAN has the particularity to be a block level storage solution that NAS doesn’t provide. It is optimized for high volume of block level data transfer. SAN is performed best when used with fiber channel medium (optical fibers, and a fiber channel switch). It provides synchronous replication and it is an architecture to attach remote storage to make it appear as though it is locally attached. There are highly scalable, both from a capacity and performance perspective. If offers a centralized storage management. It is a solution for terabytes of storage and multiple simultaneous access to files e.g. streaming audio/video and it allows virtual environments, cloud computing, etc.

12. If you are allowed to choose 5 commands, what are your choices?

1) rsync command

The rsync command can be used to synchronize two directories or directory trees whether they are on the same computer or on different computers but it can do so much more than that. rsync creates or updates the target directory to be identical to the source directory.

rsync -aH sourcedir targetdir

The -a option is for archive mode which preserves permissions, ownerships and symbolic (soft) links. The -H is used to preserve hard links. Note that either the source or target directories can be on a remote host.

2) sed command

You might want to select specific lines of a file. sed, short for stream editor, is one way to do this. you want to combine multiple files that all had headers or to do a bulk find and replace a file.
insert a blank line above every line which matches "regex"

$ sed '/regex/{x;p;x;}'

change "scarlet" or "ruby" or "puce" to "red"

$ sed 's/scarlet/red/g;s/ruby/red/g;s/puce/red/g'

3) awk command

awk is a programming language which allows easy manipulation of structured data and the generation of formatted reports. It is mostly used for pattern scanning and processing. It searches one or more files to see if they contain lines that match with the specified patterns and then perform associated actions. It is like sed
Print Specific Field

$ awk -F':' '{ print $1 }' /etc/group
$ date | awk '{print $2 " " $6}'

4) lsof command

lsof is a command line utility which is used to list the information about the files that are opened by various processes. In unix, everything is a file: pipes, sockets, directories, devices, etc. So by using lsof, you can get the information about any opened files.
List processes which opened a specific file

# lsof /var/log/syslog

lists all open files belonging to processes owned by the user

# lsof -u username

Kill all process that belongs to a particular user

# kill -9 `lsof -t -u username

List all network connections

# lsof -i

List all network files in use by a specific process

# lsof -i -a -c ssh

List processes which are listening on a particular port

# lsof -i :25

5) grep command

grep is a command used to search text or searches the given file for lines containing a match to the given strings or words. By default, grep displays the matching lines.
print network connection used by firefox

# netstat -pltnu | grep firefox

print the line which contains "root" on /etc/passwd file

# cat /etc/passwd | grep root

Apart from the above basic questions, be prepared for answers for the below questions
1. How to set linux file/directory permissions ?
2. How to set ownership for files/directories ?
3. How to create user/group and how to modify it ?
4. How to find kernel / OS version and its supported bit (32/64) version ?
5. How to set / find interface ip address ?
6. How to find linux mount points and disk usage ?
7. What command to find memory and swap usage ?
8. Have a look on ps, top, grep, find, awk and dmesg commands ?

 Linux Scenario Questions

13. What is the difference between name based virtual hosting and IP based virtual hosting? Explain the scenario where name based virtual hosting seems useful?

Virtual hosts are used to host multiple domains on a single apache instance. You can have one virtual host for each IP your server has, or the same IP but different ports, or the same IP, the same port but different host names. The latter are called "name based vhosts".
n IP-based virtual hosting, we can run more than one web site on the same server machine, but each web site has its own IP address while In Name-based virtual hosting, we host multiple websites on the same IP address. But for this to succeed, you have to put more than one DNS record for your IP address in the DNS database.
In the production shared webhosting environment, getting a dedicated IP address for every domains hosted in the server is not feasible in terms of cost. Most of the customers won't be able to afford the cost of having a dedicated IP address. Here is the place where the concepts of Name based virtual hosting find its place.

14. What is network bonding in Linux and where the important configuration files involved? What is the advantage of Network Bonding?

Network Bonding is a Linux kernel feature that allows to aggregate multiple network interfaces into a single virtual link. This is a great way to achieve redundant links, fault tolerance or load balancing networks in production system. If one of the physical NIC is down or unplugged, it will automatically move traffic to the other NIC card. Similar way the bonding will increase the interface throughput to handle the traffic it it is configured in active-active mode.
There are 7 modes starting from 0 to 6 which decides how the bonding configuration behaves.
mode=0 (balance-rr) - Round-robin policy
It the default mode. It transmits packets in sequential order from the first available slave through the last.
This mode provides load balancing and fault tolerance.
mode=1 (active-backup)
Active-backup policy: In this mode, only one slave in the bond is active. The other one will become active, only when the active slave fails. The bond’s MAC address is externally visible on only one port (network adapter) to avoid confusing the switch. This mode provides fault tolerance.
mode=2 (balance-xor)
Transmit the traffic based on [(source MAC address XOR'd with destination MAC address) modulo slave count]. This selects the same slave for each destination MAC address. This mode provides load balancing and fault tolerance.
mode=3 (broadcast)
Broadcast policy: transmits everything on all slave interfaces. This mode provides fault tolerance.
mode=4 (802.3ad)
Creates aggregation groups that share the same speed and duplex settings. Utilizes all slaves in the active aggregator according to the 802.3ad specification.
mode=5 (balance-tlb) - Adaptive transmit load balancing
channel bonding that does not require any special switch support. The outgoing traffic is distributed according to the current load (computed relative to the speed) on each slave. Incoming traffic is received by the current slave. If the receiving slave fails, another slave takes over the MAC address of the failed receiving slave.
mode=6 (balance-alb) - Adaptive load balancing
It includes balance-tlb plus receive load balancing (rlb) for IPV4 traffic, and does not require any special switch support. The receive load balancing is achieved by ARP negotiation.
Important Configuration Files involved:

/etc/sysconfig/network-scripts/ifcfg-bond0
/etc/modprobe.d/bonding.conf
/etc/sysconfig/network-scripts/ifcfg-eth[0-4]
/proc/net/bonding/bond0

15. Explain briefly the procedure for re-installing Grub in Linux ?

1) Download Ubuntu Installation / Live cd
2) Boot from Ubuntu Installation / Live cd - usb, burned cd etc.
3) During boot select "Try Ubuntu" , Don't select install !
4) Mount your Linux root partition
sudo mount /dev/sda6 /mnt ( Assuming /dev/sda6 is the Linux root partition)
5) Install / reinstall grub
$ sudo grub-install --root-directory=/mnt/ /dev/sda ( where /dev/sda is your primary disk)
Installation finished. No error reported.
6) Reboot your system, remove bootable CD and we should have the boot menu ready when the system starts.
Note : There would be slight difference when using with other distros.

16. Explain the fields in /etc/passwd and /etc/shadow?

The /etc/shadow file stores actual password in encrypted format with some additional properties related to user password.It mainly holds athe account aging parameters. All fields are separated by a colon (:) symbol. It contains one entry per line for each user listed in /etc/passwd file Generally, shadow file entry looks as below.

steve:$1$XOdE07rn$WA6qFm4W5UIqNfaqE5Uub.:13775:0:99999:7:::

Here is the explanation of each field.
User name : Your login name
Password: Your encrypted password.
Last password change : Days since Jan 1, 1970 that password was last changed
Minimum: The minimum number of days required between password changes.
Maximum: The maximum number of days the password is valid.
Warn : The number of days before password is to expire that user is warned that his/her password must be changed
Inactive : The number of days after password expires that account is disabled
Expire : days since Jan 1, 1970 that account is disabled. It indicates an absolute date specifying when the login may no longer be used
The /etc/passwd file stores essential information, which is required during login /etc/passwd is a text file, that contains a list of user account related parameters like user ID, group ID, home directory, shell, etc.
Here is the sample entry from /etc/passwd file

steve:x:6902:6902::/home/steve:/bin/bash

Username: User's login name.
Password: An x character indicates that encrypted password is stored in /etc/shadow file.
User ID (UID): Each user must be assigned a user ID (UID). UID 0 (zero) is reserved for root.
Group ID (GID): The primary group ID
User Info: The comment field. It allow you to add extra information about the user.
Home directory: The absolute path to the directory the user will be in when they log in.
Command/shell: The absolute path of a command or shell (/bin/bash).

17. How do you boot your system into the following modes, when you are in some trouble?

a) Rescue mode
b) Single user mode
c) Emergency mode
Rescue mode provides the ability to boot a small Linux environment from an external bootable device like a CD-ROM, or USB drive instead of the system's hard drive.Rescue mode is provided to help you with your system from repairing the file system or fixing certain issues which prevent your normal operations.
In order to get into the rescue mode, change the BIOS settings of the machine to boot from the external media. Once the system started booting using bootable disk, add the keyword rescue as a kernel parameter or else you can give the parameter "linux rescue" in the graphical boot interface.
In single-user mode, the system boots to runlevel 1, but it will have many more additional functionalities compared to switching to runlevel 1 from other levels.
The local file systems can be mounted in this mode, but the network is not activated.
Use the following steps to boot into single-user mode:
1) At the GRUB splash screen during the booting process, press any key to enter the GRUB interactive menu.
2) Select the proper version of kernel that you wish to boot and type "a" to append the line.
3) Go to the end of the line and type "single" as a separate word.
4) Press Enter to exit edit mode and type "b" to boot into single usermode now.
In emergency mode, you are booting into the most minimal environment possible. The root file system is mounted read-only and almost nothing is set up. The main advantage of emergency mode over single-user mode is that the init files are not loaded. If the init is corrupted, you can still mount file systems to recover data that could be lost during a re-installation. To boot into emergency mode, use the same method as described for single-user mode, with one exception, replace the keyword single with the keyword "emergency".

18. In the ps results, few of the processes are having process state as "D". What does it mean? Briefly explain different process states?

To have a dynamic view of a process in Linux, always use the top command. This command provides a real-time view of the Linux system in terms of processes. The eighth column in the output of this command represents the current state of processes. A process state gives a broader indication of whether the process is currently running, stopped, sleeping etc.
A process in Linux can have any of the following four states…
Running – A process is said to be in a running state when either it is actually running/ executing or waiting in the scheduler’s queue to get executed (which means that it is ready to run). That is the reason that this state is sometimes also known as ‘runnable’ and represented by (R).
Waiting or Sleeping – A process is said to be in this state if it is waiting for an event to occur or waiting for some resource-specific operation to complete. So, depending upon these scenarios, a waiting state can be subcategorised into an interruptible (S) or uninterruptible (D) state respectively.
Stopped – A process is said to be in the stopped state when it receives a signal to stop. This usually happens when the process is being debugged. This state is represented by (T).
Zombie – A process is said to be in the zombie state when it has finished execution but is waiting for its parent to retrieve its exit status. This state is represented by (Z).
Apart from these four states, the process is said to be dead after it crosses over the zombie state; ie when the parent retrieves its exit status. ‘Dead’ is not exactly a state, since a dead process ceases to exist.

19. What is drop cache in Linux and how do you clear it ?

Cache in Linux memory is where the Kernel stores the information it may need later, as memory is incredible faster than disk.
It is great that the Linux Kernel takes care about that.Linux Operating system is very efficient in managing your computer memory, and will automatically free the RAM and drop the cache if some application needs memory.
Kernels 2.6.16 and newer provide a mechanism to have the kernel drop the page cache and/or inode and dentry caches on command, which can help free up a lot of memory. Now we can throw away that script that allocated a ton of memory just to get rid of the cache.
To free pagecache:

# echo 1 > /proc/sys/vm/drop_caches

To free dentries and inodes:

# echo 2 > /proc/sys/vm/drop_caches

To free pagecache, dentries and inodes:

echo 3 > /proc/sys/vm/drop_caches

This is a non-destructive operation in normal scenarios and will only free things that are completely unused. Dirty objects will continue to be in use until written out to disk and are not freeable. However it is always preferred to run "sync" first to flush useful things out to disk.

20. Password based authentication is disabled in your infrastructure. So how do you login to the servers ?

To improve the system security even further, most of the organizations turned to use keybased authentications instead of Password based authentication. We can enforce the key-based authentication by disabling the standard password authentication, which involves a public key private key pair. The public key is added in the server configuration file while private key is kept confidential on the client side.
Below listed is the procedure, to set up keybased authentication.
1) Generating Key Pairs
a) Generate an RSA key pair by typing the following at a shell prompt:
$ ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/home/steve/.ssh/id_rsa):
b) Press Enter to confirm the default location (that is, ~/.ssh/id_rsa) for the newly created key.
c) Enter a passphrase, and confirm it by entering it again when prompted to do so.
d) Copy the content of ~/.ssh/id_rsa.pub into the ~/.ssh/authorized_keys on the machine to which you want to connect,
appending it to its end if the file already exists.
e) Change the permissions of the ~/.ssh/authorized_keys file using the following command:
$ chmod 600 ~/.ssh/authorized_keys
2) Now on your client side, open the remote connection agent like putty and browse your public key and try SSH to the server, you should be able to login without a password now.
# ssh server1.myserver.com
The authenticity of host 'server1.myserver.com (192.168.44.2)' can't be established.
RSA key fingerprint is e3:c3:89:37:4b:94:37:d7:0c:d5:6f:9a:38:62:ce:1b.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'server1.myserver.com' (RSA) to the list of known hosts.
Last login: Tue July 13 12:40:34 2014 from server2.myserver.com
3) Public key authentication can prevent brute force SSH attacks, but only if all password-based authentication methods are disabled. Once public key authentication has been confirmed to be working, disable regular password authentication by editing /etc/ssh/sshd_config and set the following option to "no".
PasswordAuthentication no

21. Explain the different Scenarios involved in TCP 3 way handshake?

The TCP three way handshake is the process for establishing a TCP connection.We can explain 3 way handshake with a simple scenario where we assume a client computer is contacting a server to send it some information.
a) The client sends a packet with the SYN bit set and a sequence number of N.
b) The server sends a packet with an ACK number of N+1, the SYN bit set and a sequence number of X.
c) The client sends a packet with an ACK number of X+1 and the connection is established.
d) The client sends the data.
The first three steps in the above process is called the three way handshake.

22. As the disk space utilization was so high in the server, the Administrator has removed few files from the server but still the disk utilization is showing as high. What would be the reason?

In Linux even if we remove a file from the mounted file system, that will still be in use by some application and for this application, it remains available. Its because file descriptor in /proc/ filesystem is held open..So if there are such open descriptors to files already removed, space occupied by them considered as used. You find this difference by checking them using the "df" and "du" commands. While df is to show the file system usage, du is to report the file space usage. du works from files while df works at filesystem level, reporting what the kernel says it has available.
You can find all unlinked but held open files with:
# lsof | grep '(deleted)'
This will list the filename which is open with the pid in which it is running. We can kill those Pids and which will stop these process and will recover the disk space responsible for this file.

23. What is rDNS and explain its benefits in the Linux Domain Name Systems?

A typical DNS lookup is used to determine which IP address is associated with a hostname, and this is called Forward DNS lookup. A reverse DNS lookup is used for the opposite, to determine which hostname is associated with an IP address. Sometimes reverse DNS lookups are required for diagnostic purposes. Today, reverse DNS lookups are used mainly for security purposes to trace a hacker or spammer. Many modern mailing systems use reverse mapping to provide simple authentication using dual lookup: hostname-to-address and address-to-hostname. The rDNS ( reverse DNS ) is implemented using a specialized zone record for reverse lookups called PTR record. PTR records always resolve to names, never IP addresses.

24. What is sosreport, how do you generate it while working with your Redhat Support Team in production?

Sosreport is a command-line utility in Redhat based linux destros (RHEL / CentOS) which collects system configuration and diagnostic information of your linux box like running kernel version, loaded modules, and system and service configuration files. This command also runs external programs to collect further information, and stores this output in the resulting archive. Sosreport is required when you have open a case with redhat for technical support. Redhat support Engineers will require sosreport of your server for troubleshooting purpose. To run sosreport, sos package should be installed. Sos package is part of default installation in most of linux. If for any reason this package is no installed , then use below yum command to install it manually:

# yum install sos

Generate the report
Open the terminal type sosreport command :

# sosreport

This command will normally complete within a few minutes. Depending on local configuration and the options specified in some cases the command may take longer to finish. Once completed, sosreport will generate a compressed a file under /tmp folder. The file should be provided to Redhat support representative as an attachment to open a support case.

25. What is swappiness in Linux Memory Management and how do we configure that?

The swappiness parameter controls the tendency of the kernel to move processes out of physical memory and onto the swap disk. Because disks are much slower than RAM, this can lead to slower response times for system and applications if processes are too aggressively moved out of memory.
swappiness can have a value of between 0 and 100
swappiness=0 tells the kernel to avoid swapping processes out of physical memory for as long as possible
swappiness=100 tells the kernel to aggressively swap processes out of physical memory and move them to swap cache
The default setting in Redhat/Ubuntu based Linux distros is swappiness=60. Reducing the default value of swappiness will probably improve overall performance for a typical Ubuntu desktop installation.

~$ cat /proc/sys/vm/swappiness
60

If we have enough RAM, we can turn that down to 10 or 15. The swap file will then only be used when the RAM usage is around 80 or 90 percent.
To change the system swappiness value, open /etc/sysctl.conf as root. Then, change or add this line to the file:
vm.swappiness = 10
Reboot for the change to take effect
You can also change the value while your system is still running
sysctl vm.swappiness=10
We can also clear swap by running swapoff -a and then swapon -a as root instead of rebooting to achieve the same effect.

26. What is git ?

Git is a very popular and efficient open source Version Control System. It tracks content such as files and directories. It stores the file content in BLOBs - binary large objects. The folders are represented as trees. Each tree contains other trees (subfolders) and BLOBs along with a simple text file which consists of the mode, type, name and Secure Hash Algorithm of each blob and subtree entry. During repository transfers, even if there are several files with the same content and different names, the GIT software will transfer the BLOB once and then expand it to the different files


Which account is created on Linux installation?
- With the installation of Linux, a super user account is created called as ‘root’.
Which daemon tracks events on your system?
- The syslogd daemon tracks the system information and saves it to specified log files.
Which command would you use if you want to remove the password assigned to a group?
- gpasswd – r removes the password from the group.
- Here, the gpasswd changes the password of the group and when it is accompanied by –r, the password gets removed.
You wish to print a file ‘draft’ with 60 lines to a page. What command would you use?
- The command that I would use is: pr -l60 draft
- The default page length when using pr is 66 lines.
- The -l option specifies a different length.
Which file would you examine to determine the levels of messages written to system log files?
- kernel.h
You are logged on as a regular user. Without logging off and logging on as root, you are required to create a new user account immediately. How would you do it?
- This can be achieved by issuing the su command.
- This will prompt you for the password of the root account.
- Providing the password, logs you in as root. Now, you can perform any administrative duties.
You are required to restore the file memo.ben. It was backed up in the tar file MyBackup.tar. Which command would you use to do it?
- The command that we would use is: tar xf MyBackup.tar memo.ben
- It uses the x switch to extract a file.
What is partial backup?
- When you select only a portion of your file hierarchy or a single partition to back up, it is called partial backup.
What is the fastest way to enter a series of commands from the command-line?
- Write the commands, each separated by a semi-colon. Press enter after the last command.
- The semi-colon would inform the shell that multiple commands are being entered at the command line, to be executed serially.
What are the qualities of soft links?
a.) Soft link files have different inode numbers than source file
b.) The soft link file will be of no use if original file is deleted.
c.) Soft links are not updated
d.) They can create links between directories
e.) They can cross file system boundaries
Differentiate between Cron and Anacron.
a.) Minimum granularity with Cron is minute while it is in days with Anacron.

b.) Cron job can be scheduled by any normal user while Anacron can be scheduled only by the super user.

c.) Cron expects the system to be up and running while the Anacron doesn’t expect the system to be up and running all the time. In case of Anacron, if a job is scheduled and the system is down that time, it will execute the job as soon as the system is up and running.

d.) Cron is ideal for servers while Anacron is ideal for desktops and laptops.

e.) Cron should be used when you want a job to be executed at a particular hour and minute while Anacron should be used in when the job can be executed irrespective of the hour and minute.
What is an INODE?
- It is a structure which has the description of all the files and pointers to the data blocks of files stored in it.
- The information contained is file-size, access and modification time, permission and so on.
Which command is used to check the number of files and disk space used and the each user’s defined quota?
repquota command is used to check the status of the user’s quota along with the disk space and number of files used.

This command gives a summary of the user’s quota that how much space and files are left for the user. Every user has a defined quota in Linux. This is done mainly for the security, as some users have only limited access to files. This provides a security to the files from unwanted access. The quota can be given to a single user or to a group of users.
What is the name and path of the main system log?
By default, the main system log is '/var/log/messages'. This file contains all the messages and the script written by the user. By default all scripts are saved in this file. This is the standard system log file, which contains messages from all system software, non-kernel boot issues, and messages that go to 'dmesg'. dmesg is a system file that is written upon system boot.
How secured is Linux? Explain.
Security is the most important aspect of an operating system. Due to its unique authentication module, Linux is considered as more secured than other operating systems. Linux consists of PAM. PAM is Pluggable Authentication Modules. It provides a layer between applications and actual authentication mechanism. It is a library of loadable modules which are called by the application for authentication. It also allows the administrator to control when a user can log in. All PAM applications are configured in the directory "/etc/pam.d" or in a file "/etc/pam.conf". PAM is controlled using the configuration file or the configuration directory.
Can Linux computer be made a router so that several machines may share a single Internet connection? How?
Yes a Linux machine can be made a router. This is called "IP Masquerade." IP Masquerade is a networking function in Linux similar to the one-to-many (1: Many) NAT (Network Address Translation) servers found in many commercial firewalls and network routers. The IP Masquerade feature allows other "internal" computers connected to this Linux box (via PPP, Ethernet, etc.) to also reach the Internet as well. Linux IP Masquerading allows this functionality even if the internal computers do not have IP addresses.
The IP masquerading can be done by the following steps:

1. The Linux PC must have an internet connection and a connection to LAN. Typically, the Linux PC has two network interfaces-an Ethernet card for the LAN and a dial-up PPP connection to the Internet (through an ISP).

2. All other systems on your LAN use the Linux PC as the default gateway for TCP/IP networking. Use the same ISP-provided DNS addresses on all systems.

3. Enable IP forwarding in the kernel. By default the IP forwarding is not enabled. To ensure that IP forwarding is enabled when you reboot your system, place this command in the /etc/rc.d/rc.local file.

4. Run /sbin/iptables-the IP packet filter administration program-to set up the rules that enable the Linux PC to masquerade for your LAN.
What is the minimum number of partitions that you need to install Linux?
Minimum 2 partitions are needed for installing Linux. The one is "/ or root" which contains all the files and the other is swap. Linux file system is function specific which means that files and folders are organized according to their functionality. For example, all executables are in one folder, all devices in another, all libraries in another and so on. "/ or root" is the base of this file system. All the other folders are under this one. "/" can be consider as "C:". Swap is a partition that will be used as virtual memory. If there is no more available RAM a Linux computer will use an area of the hard disk, called swap, to temporarily store data. In other words it is a way of expanding your computers RAM.
Which command is used to review boot messages?
dmesg command is used to review boot messages. This command will display system messages contained in the kernel ring buffer. We can use this command immediately after booting to see boot messages. A ring buffer is a buffer of fixed size for which any new data added to it overwrites the oldest data in it.

Its basic syntax is:
dmesg [options]
Invoking dmesg without any of its options causes it to write all the kernel messages to standard output. This usually produces far too many lines to fit into the display screen all at once and thus only the final messages are visible. However, the output can be redirected to the less command through the use of a pipe, thereby allowing the startup messages to be viewed on one screen at a time
dmesg | less
Which utility is used to make automate rotation of a log?
logrotate command is used to make automate rotation of log.

Syntax of the command is:
logrotate [-dv] [-f|] [-s|] config_file+
It allows automatic rotation, compression, removal, and mailing of log files. This command is mainly used for rotating and compressing log files. This job is done every day when a log file becomes too large. This command can also be run by giving on command line. We can done force rotation by giving –f option with this command in command line. This command is also used for mailing. We can give –m option for mailing with this command. This option takes two arguments one is subject and other is recipient name.
What are the partitions created on the mail server hard drive?
The main partitions are done firstly which are root, swap and boot partition.

But for the mail server three different partitions are also done which are as follows:

1. /var/spool - This is done so that if something goes wrong with the mail server or spool than the output cannot overrun the file system.
2. /tmp - Putting this on its own partition prevents any user item or software from overrunning the system files.
3. /home - Putting this on its own is useful for system upgrades or reinstalls. It allow not to wipe off the /home hierarchy along with other areas.
What are the fields in the/etc/passwd file?
It contains all the information of the users who log into the system. It contains a list of the system's accounts, giving for each account some useful information like user ID, group ID, home directory, shell, etc. It should have general read permission as many utilities, like ls use it to map user IDs to user names, but write access only for the superuser (root).

The main fields of /etc/passwd file are:

1. Username: It is used when user logs in. It should be between 1 and 32 characters in length.

2. Password: An x character indicates that encrypted password is stored in /etc/shadow file.

3. User ID (UID): Each user must be assigned a user ID (UID). UID 0 (zero) is reserved for root and UIDs 1-99 are reserved for other predefined accounts. Further UID 100-999 are reserved by system for administrative and system accounts/groups.

4. Group ID (GID): The primary group ID (stored in /etc/group file)

5. User ID Info: The comment field. It allow you to add extra information about the users such as user's full name, phone number etc. This field use by finger command.

6. Home directory: The absolute path to the directory the user will be in when they log in. If this directory does not exists then users directory becomes /

7. Command/shell: The absolute path of a command or shell (/bin/bash). Typically, this is a shell.
Which commands are used to set a processor-intensive job to use less CPU time?
nice command is used for changing priority of the jobs.

Syntax:
nice [OPTION] [COMMAND [ARG]...]

Range of priority goes from -20 (highest priority) to 19 (lowest). Priority is given to a job so that the most important job is executed first by the kernel and then the other least important jobs. This takes less CPU times as the jobs are scheduled and are given priorities so the CPU executes fast. The priority is given by numbers like -20 describe the highest priority and 19 describe the least priority.
How to change window manager by editing your home directory?
"/.xinitrc file" allows changing the window manager we want to use when logging into X from that account. The dot in the file name shows you that the file is a hidden file and doesn't show when you do a normal directory listing. For setting a window manager we have to save a command in this file. The syntax of command is: exec windowmanager. After this, save the file. Next time when you run a startx a new window manager will open and become default.

The commands for starting some popular window managers and desktop environments are:
-KDE = startkde
-Gnome = gnome-session
-Blackbox = blackbox
-FVWM = fvwm
-Window Maker = wmaker
-IceWM = icewm
How documentation of an application is stored?
When a new application is installed its documentation is also installed. This documentation is stored under the directory named for application. For example if my application name is App1 then the path of the documentation will be /user/doc/App1. It contains all the information about the application. It contains date of creating application, name of application and other important module of the application. We can get the basic information of application from the documentation.
How shadow passwords are given?
pwconv command is used for giving shadow passwords. Shadow passwords are given for better system security. The pwconv command creates the file /etc/shadow and changes all passwords to ‘x’ in the /etc/passwd file. First, entries in the shadowed file which don't exist in the main file are removed. Then, shadowed entries which don't have `x' as the password in the main file are updated. Any missing shadowed entries are added. Finally, passwords in the main file are replaced with `x'. These programs can be used for initial conversion as well to update the shadowed file if the main file is edited by hand.
How do you create a new user account?
useradd command is used for creating a new user account. When invoked without the -D option, the useradd command creates a new user account using the values specified on the command line and the default values from the system. The new user account will be entered into the system files as needed and initial files copied, depending on the command line options. This command uses the system default as home directory. If –m option is given then the home directory is made.
Which password package is installed for the security of central password?
Shadow password packages are used for security of central passwords. Security is the most important aspect of every operating system. When this package is not installed the user information including passwords is stored in the /etc/passwd file. The password is stored in an encoded format. These encoded forms can be easily identified by the System crackers by randomly encoding the passwords from dictionaries. The Shadow Package solves the problem by relocating the passwords to another file (usually /etc/shadow). The /etc/shadow file is set so that it cannot be read by just anyone. Only root will be able to read and write to the /etc/shadow file.
Which shell do you assign to a POP3 mail-only account?
POP3 mail only account is assigned to the /bin/false shell. However, assigning bash shell to a POP3 mail only gives user login access, which is avoided. /bin/nologin can also be used. This shell is provided to the user when we don’t want to give shell access to the user. The user cannot access the shell and it reject shell login on the server like on telnet. It is mainly for the security of the shells. POP3 is basically used for downloading mail to mail program. So for illegal downloading of emails on the shell this account is assigned to the /bin/false shell or /bin/nologin. These both shells are same they both do the same work of rejecting the user login to the shell. The main difference between these two shells is that false shell shows the incorrect code and any unusual coding when user login with it. But the nologin shell simply tells that no such account is available. So nologin shell is used mostly in Linux.
Which daemon is responsible for tracking events on Linux system?
syslogd is responsible for tracking system information and save it to the desired log files. It provides two system utilities which provide system logging and kernel message trapping. Internet and UNIX domain sockets support enable this utility package to support both local and remote logging. Every logged message contains at least a time and a hostname field, normally a program name field, too. So to track these information this daemon is used.

syslogd mainly reacts to the set of signals given by the user.

These are the signals given to syslogd:

- SIGHUP: This lets syslogd perform a re-initialization. All open files are closed, the configuration file (default is /etc/syslog.conf) will be reread and the syslog facility is started again.

- SIGTERM: The syslogd will die.

- SIGINT, SIGQUIT: If debugging is enabled these are ignored, otherwise syslogd will die.

- SIGUSR1: Switch debugging on/off. This option can only be used if syslogd is started with the - d debug option.

- SIGCHLD: Wait for Childs if some were born, because of waiting messages.
Which daemon is used for scheduling of the commands?
The crontab command is used for scheduling of the commands to run at a later time.

SYNTAX

crontab [ -u user ] file
crontab [ -u user ] { -l | -r | -e }

Options

-l List - display the current crontab entries.

-r Remove the current crontab.

-e Edit the current crontab using the editor specified by the VISUAL or EDITOR environment variables.
When user exits from the editor, the modified crontab will be installed automatically. Each user can have their own crontab, and though these are files in /var, they are not intended to be edited directly.

If the –u option is given than the crontab gives the name of the user whose crontab is to be tweaked. If it is given without this then it will display the crontab of the user who is executing the command.
How environment variable is set so that the file permission can be automatically set to the newly created files?
umask command is used to set file permission on newly created files automatically.

Syntax
umask [-p] [-S] [mode]

It is represented in octal numbers. We can simply use this command without arguments to see the current file permissions. To change the permissions, mode is given in the arguments. The default umask used for normal user is 0002. The default umask for the root user is 0022. For calculating the original values, the values shown by the umask must be subtracted by the default values. It is mainly used for masking of the file and directory permission. The /etc/profile script is where the umask command is usually set for all users. The –S option can be used to see the current default permissions displayed in the alpha symbolic format. For example, umask 022 ensures that new files will have at most 755 permissions (777 NAND 022).

The permissions can be calculated by taking the NAND of original value with the default values of files and directories.


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 Linux and why is it so popular?
Linux is a multiuser, multitask GUI based open source operating system developed by Linus Torvalds Torvalds has invited the community to enhance the Linux kernel and thousands of system programmers worked on to enhance.

Prior to Linux, there is UNIX. The desktop work stations from various companies were based on UNIX. Later a numerous companies entered and each one of them had their own UNIX version. As the proprietary authority is owned by each company and the lack of central authority weaken UNIX. As Linux is free and runs on any PC platform it gained the popularity very quickly. The following are few more reasons for its popularity:

- People who are familiar with UNIX can work on Linux with ease and comfort.
- People who want great control over network security and on operating system

   

Linux LILO - What is LILO?
What is LILO? - LILO is Linux Loader is a boot loader for Linux. It is used to load Linux into the memory and start the Operating system.......
Linux - Difference between home directory and working directory?
Difference between home directory and working directory - Home directory is the default working directory when a user logs in. On the other hand, working directory is the user’s current directory......
Linux - Difference between internal and external commands
Difference between internal and external commands - Internal commands are commands that are already loaded in the system. They can be executed any time and are independent.......





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) What is Linux?

Linux is an operating system based on UNIX, and was first introduced by Linus Torvalds. It is based on the Linux Kernel, and can run on different hardware platforms manufactured by Intel, MIPS, HP, IBM, SPARC and Motorola. Another popular element in Linux is its mascot, a penguin figure named Tux.

2) What is the difference between UNIX and LINUX?

Unix originally began as a propriety operating system from Bell Laboratories, which later on spawned into different commercial versions. On the other hand, Linux is free, open source and intended as a non-propriety operating system for the masses.

3) What is BASH?

BASH is short for Bourne Again SHell. It was written by Steve Bourne as a replacement to the original Bourne Shell (represented by /bin/sh). It combines all the features from the original version of Bourne Shell, plus additional functions to make it easier and more convenient to use. It has since been adapted as the default shell for most systems running Linux.

4) What is Linux Kernel?

The Linux Kernel is a low-level systems software whose main role is to manage hardware resources for the user. It is also used to provide an interface for user-level interaction.

5) What is LILO?

LILO is a boot loader for Linux. It is used mainly to load the Linux operating system into main memory so that it can begin its operations.

6) What is a swap space?

A swap space is a certain amount of space used by Linux to temporarily hold some programs that are running concurrently. This happens when RAM does not have enough memory to hold all programs that are executing.

7) What is the advantage of open source?

Open source allows you to distribute your software, including source codes freely to anyone who is interested. People would then be able to add features and even debug and correct errors that are in the source code. They can even make it run better, and then redistribute these enhanced source code freely again. This eventually benefits everyone in the community.

8 ) What are the basic components of Linux?

Just like any other typical operating system, Linux has all of these components: kernel, shells and GUIs, system utilities, and application program. What makes Linux advantageous over other operating system is that every aspect comes with additional features and all codes for these are downloadable for free.

9) Does it help for a Linux system to have multiple desktop environments installed?

In general, one desktop environment, like KDE or Gnome, is good enough to operate without issues. It’s all a matter of preference for the user, although the system allows switching from one environment to another. Some programs will work on one environment and not work on the other, so it could also be considered a factor in selecting which environment to use.

10) What is the basic difference between BASH and DOS?

The key differences between the BASH and DOS console lies in 3 areas:
– BASH commands are case sensitive while DOS commands are not;
– under BASH, / character is a directory separator and \ acts as an escape character. Under DOS, / serves as a command argument delimiter and \ is the directory separator
– DOS follows a convention in naming files, which is 8 character file name followed by a dot and 3 character for the extension. BASH follows no such convention.

11) What is the importance of the GNU project?

This so-called Free software movement allows several advantages, such as the freedom to run programs for any purpose and freedom to study and modify a program to your needs. It also allows you to redistribute copies of a software to other people, as well as freedom to improve software and have it released to the public.

12) Describe the root account.

The root account is like a systems administrator account, and allows you full control of the system. Here you can create and maintain user accounts, assigning different permissions for each account. It is the default account every time you install Linux.

13) What is CLI?

CLI is short for Command Line Interface. This interface allows user to type declarative commands to instruct the computer to perform operations. CLI offers an advantage in that there is greater flexibility. However, other users who are already accustom with using GUI find it difficult to remember commands including attributes that come with it.

14) What is GUI?

GUI, or Graphical User Interface, makes use of images and icons that users click and manipulate as a way of communicating with the computer. Instead of having to remember and type commands, the use of graphical elements makes it easier to interact with the system, as well as adding more attraction through images, icons and colors.

15) How do you open a command prompt when issuing a command?

To open the default shell (which is where the command prompt can be found), press Ctrl-Alt-F1. This will provide a command line interface (CLI) from which you can run commands as needed.

16) How can you find out how much memory Linux is using?

From a command shell, use the “concatenate” command: cat /proc/meminfo for memory usage information. You should see a line starting something like: Mem: 64655360, etc. This is the total memory Linux thinks it has available to use.

17) What is typical size for a swap partition under a Linux system?

The preferred size for a swap partition is twice the amount of physical memory available on the system. If this is not possible, then the minimum size should be the same as the amount of memory installed.

18) What are symbolic links?

Symbolic links act similarly to shortcuts in Windows. Such links point to programs, files or directories. It also allows you instant access to it without having to go directly to the entire pathname.

19) Does the Ctrl+Alt+Del key combination work on Linux?

Yes, it does. Just like Windows, you can use this key combination to perform a system restart. One difference is that you won’t be getting any confirmation message and therefore, reboot is immediate.

20) How do you refer to the parallel port where devices such as printers are connected?

Whereas under Windows you refer to the parallel port as the LPT port, under Linux you refer to it as /dev/lp . LPT1, LPT2 and LPT3 would therefore be referred to as /dev/lp0, /dev/lp1, or /dev/lp2 under Linux.

21) Are drives such as harddrive and floppy drives represented with drive letters?

No. In Linux, each drive and device has different designations. For example, floppy drives are referred to as /dev/fd0 and /dev/fd1. IDE/EIDE hard drives are referred to as /dev/hda, /dev/hdb, /dev/hdc, and so forth.

22) How do you change permissions under Linux?

Assuming you are the system administrator or the owner of a file or directory, you can grant permission using the chmod command. Use + symbol to add permission or – symbol to deny permission, along with any of the following letters: u (user), g (group), o (others), a (all), r (read), w (write) and x (execute). For example the command chmod go+rw FILE1.TXT grants read and write access to the file FILE1.TXT, which is assigned to groups and others.

23) In Linux, what names are assigned to the different serial ports?

Serial ports are identified as /dev/ttyS0 to /dev/ttyS7. These are the equivalent names of COM1 to COM8 in Windows.

24) How do you access partitions under Linux?

Linux assigns numbers at the end of the drive identifier. For example, if the first IDE hard drive had three primary partitions, they would be named/numbered, /dev/hda1, /dev/hda2 and /dev/hda3.

25) What are hard links?

Hard links point directly to the physical file on disk, and not on the path name. This means that if you rename or move the original file, the link will not break, since the link is for the file itself, not the path where the file is located.

26) What is the maximum length for a filename under Linux?

Any filename can have a maximum of 255 characters. This limit does not include the path name, so therefore the entire pathname and filename could well exceed 255 characters.

27)What are filenames that are preceded by a dot?

In general, filenames that are preceded by a dot are hidden files. These files can be configuration files that hold important data or setup info. Setting these files as hidden makes it less likely to be accidentally deleted.

28) Explain virtual desktop.

This serves as an alternative to minimizing and maximizing different windows on the current desktop. Using virtual desktops, each desktop is a clean slate where you can open one or more programs. Rather than minimizing/restoring all those programs as needed, you can simply shuffle between virtual desktops with programs intact in each one.

29) How do you share a program across different virtual desktops under Linux?

To share a program across different virtual desktops, in the upper left-hand corner of a program window look for an icon that looks like a pushpin. Pressing this button will “pin” that application in place, making it appear in all virtual desktops, in the same position onscreen.

30) What does a nameless (empty) directory represent?

This empty directory name serves as the nameless base of the Linux file system. This serves as an attachment for all other directories, files, drives and devices.

31) What is the pwd command?

The pwd command is short for print working directory command. It’s counterpart in DOS is the cd command, and is used to display the current location in the directory tree.

32) What are daemons?

Daemons are services that provide several functions that may not be available under the base operating system. Its main task is to listen for service request and at the same time to act on these requests. After the service is done, it is then disconnected and waits for further requests.

33) How do you switch from one desktop environment to another, such as switching from KDE to Gnome?

Assuming you have these two environments installed, just log out from the graphical interface. Then at the Log in screen, type your login ID and password and choose which session type you wish to load. This choice will remain your default until you change it to something else.

34) What are the kinds of permissions under Linux?

There are 3 kinds of permissions under Linux:
– Read: users may read the files or list the directory
– Write: users may write to the file of new files to the directory
– Execute: users may run the file or lookup a specific file within a directory

35) How does case sensitivity affect the way you use commands?

When we talk about case sensitivity, commands are considered identical only if every character is encoded as is, including lowercase and uppercase letters. This means that CD, cd and Cd are three different commands. Entering a command using uppercase letters, where it should be in lowercase, will produce different outputs.

36) What are environmental variables?

Environmental variables are global settings that control the shell’s function as well as that of other Linux programs. Another common term for environmental variables is global shell variables.

37) What are the different modes when using vi editor?

There are 3 modes under vi:
– Command mode – this is the mode where you start in
– Edit mode – this is the mode that allows you to do text editing
– Ex mode – this is the mode wherein you interact with vi with instructions to process a file

38) Is it possible to use shortcut for a long pathname?

Yes, there is. A feature known as filename expansion allows you do this using the TAB key. For example, if you have a path named /home/iceman/assignments directory, you would type as follows: /ho[tab]/ice[tab]/assi[tab] . This, however, assumes that the path is unique, and that the shell you’re using supports this feature.

39) What is redirection?

Redirection is the process of directing data from one output to another. It can also be used to direct an output as an input to another process.

40) What is grep command?

grep a search command that makes use of pattern-based searching. It makes use of options and parameters that is specified along the command line and applies this pattern into searching the required file output.

41) What could possibly be the problem when a command that was issued gave a different result from the last time it was used?

One highly possible reason for getting different results from what seems to be the same command has something to do with case sensitivity issues. Since Linux is case sensitive, a command that was previously used might have been entered in a different format from the present one. For example, to lists all files in the directory, you should type the command ls, and not LS. Typing LS would either result in an error message if there is no program by that exact name exist, or may produce a different output if there is a program named LS that performs another function.

42) What are the contents in /usr/local?

It contains locally installed files. This directory actually matters in environments where files are stored on the network. Specifically, locally-installed files go to /usr/local/bin, /usr/local/lib, etc.). Another application of this directory is that it is used for software packages installed from source, or software not officially shipped with the distribution.

43) How do you terminate an ongoing process?

Every process in the system is identified by a unique process id or pid. Use the kill command followed by the pid in order to terminate that process. To terminate all process at once, use kill 0.

44) How do you insert comments in the command line prompt?

Comments are created by typing the # symbol before the actual comment text. This tells the shell to completely ignore what follows. For example: “# This is just a comment that the shell will ignore.”

45) What is command grouping and how does it work?

You can use parentheses to group commands. For example, if you want to send the current date and time along with the contents of a file named OUTPUT to a second file named MYDATES, you can apply command grouping as follows: (date cat OUTPUT) > MYDATES

46) How do you execute more than one command or program from a single command line entry?

You can combine several commands by separating each command or program using a semicolon symbol. For example, you can issue such a series of commands in a single entry:
ls –l cd .. ls –a MYWORK which is equivalent to 3 commands: ls -l cd.. ls -a MYWORK
1
   
ls –l cd .. ls –a MYWORK which is equivalent to 3 commands: ls -l cd.. ls -a MYWORK

**Note that this will be executed one after the other, in the order specified.

47) Write a command that will look for files with an extension “c”, and has the occurrence of the string “apple” in it.

Answer:
 Find ./ -name “*.c” | xargs grep –i “apple”
1
   
 Find ./ -name “*.c” | xargs grep –i “apple”



48) Write a command that will display all .txt files, including its individual permission.

Answer:
ls -a -l *.txt
1
   
ls -a -l *.txt



49) Write a command that will do the following:
-look for all files in the current and subsequent directories with an extension c,v
-strip the,v from the result (you can use sed command)
-use the result and use a grep command to search for all occurrences of the word ORANGE in the files.
Find ./ -name “*.c,v” | sed ‘s/,v//g’ | xargs grep “ORANGE”
1
   
Find ./ -name “*.c,v” | sed ‘s/,v//g’ | xargs grep “ORANGE”



50) What, if anything, is wrong with each of the following commands?
a) ls -l-s
b) cat file1, file2
c) ls – s Factdir

Answers:
a) there should be space between the 2 options: ls -l -s
b) do not use commas to separate arguments: cat file1 file2
c) there should be no space between hyphen and option label: ls –s Factdir

51) What is the command to calculate the size of a folder?

To calculate the size of a folder use the command du –sh folder1.

52) How can you find status of a process?

Use the command

ps ux

53) How can you check the memory status ?

You can use the command

free -m  to display output in MB

free -g  to display output in GB

54) Explain how to color the Git console?

To color the Git console  you can use the command git config—global color.ui auto.  In the command, the color.ui variable sets the default value for variable such as color.diff and color.grep.

55) How can you append one file to another in  Linux?

To append one file to another in Linux you can use command cat file2 >> file 1.  The operator >> appends the output of the named file or creates the file if it is not created.  While another command cat file 1 file 2 > file 3 appends two or more files to one.

56) Explain how you can find a file  using Terminal?

To find a file  you have to use command, find . –name “process.txt” .  It will look for the current directory for a file called process.txt.

57) Explain how you can create a folder using Terminal?

To create a folder, you have to use command mkdir.  It will be something like these :  ~$ mkdir Guru99

58)  Explain how you can view the text file using Terminal?

To view the text file, go to the specific folder where the text files are located by using the command cd and then type less filename.txt.

59) Explain how to enable curl on Ubuntu LAMP stack?

To enable curl on Ubuntu , first install libcurl, once done use following command sudo/etc/init .d /apache2 restart or sudo service apache2 restart.

60) Explain how to enable root loging in Ubuntu?

The command which enables root loging is

#sudo sh-c ‘echo “greater-show-manual-login=true” >>/etc/lightdm/lightdm.conf’

61) How you can run an Linux program in the background simultaneously when you start your Linux Server?

By using nohup.  It will stop the process receiving the NOHUP signal and thus terminating it you log out of the program which was invoked with.  & runs the process in the background.

62) Explain how to uninstall the libraries in Linux?

To uninstall the libraries in Linux, you can use command  sudo apt – get remove library_name

///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
Top Answers to Linux Interview Questions
1. Compare Linux & Windows
Criteria     Linux     Windows
Type of OS     Open Source     Proprietary
Customization     High level of customization     Cannot be changed
Security     Excellent     Cannot be changed
2. What is LILO?

    LILO is Linux Loader is a boot loader for Linux. It is used to load Linux into the memory and start the Operating system. LILO can be configured to boot other operating systems as well. LILO is customizable, which means that if the default configuration is not correct, it can be changed. Config file for LILO is lilo.conf.
    LILO is also a code snippet which loads PC BIOS into the main memory at the time of starting the computer system. It handles the following tasks :
    1. Locating Linux kernel.
    2. Identifying other supporting programs and loading them in the memory.
    3. Starting Kernel.
    The selection of various kernel images and boot routines is supported by LILO. For this reason, it is known as boot manager.

3. You wish to print a file ‘draft’ with 60 lines on a page. What command would you use?
The command used : pr -l60 draft

    The default page length when using pr is 66 lines.
    The -l option specifies a different length.

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4. What is LD_LIBRARY_PATH?

    LD_LIBRARY_PATH is an environment variable. It is used for debugging a new library or a non-standard library. It is also used to identify what directories need to be searched. Path to search for directories needs to given.
    The variable can be set using setenv—LD_LIBRARY_PATH–$PATH.
    It is used to search for the shared objects / dynamic libraries by the operating system for extendable functionality at runtime.Prepare yourself for the Linux Certification with comprehensive Linux Class Online Now!

5. Name a service that you should disable (which acts both as Web and FTP Server) on a Linux Server.

The finger service should be disabled because a remote user can get important information about the system by using that command.
6. What does Sar provide? Where are Sar logs stored?

Sar collects, reports, or saves system activity information, sar serves to log and evaluate a variety of information regarding system activity. With performance problems, sar also permits retroactive analysis of the load values for various sub-systems (CPUs, memory, disks, interrupts, network interfaces and so forth) and limitation of problems in this manner. If CPU utilization is near 100 % (user + nice + system), the workload sampled is CPU-bound.
By default, log files of Sar command is located at /var/log/sa/sadd file, where dd parameter indicates the current day.

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7. How to check Memory stats and CPU stats as a Linux admin?

Using ‘free’ & ‘vmstat’ command, we can display the physical and virtual memory statistics, respectively. With the help of ‘sar’ command, we can see the CPU utilization & other stats.
8. How to reduce or shrink the size of LVM partition?
Below are the logical steps to reduce size of LVM partition :

    Unmount the filesystem using unmount command,
    Use resize2fs command , e.g resiz2fs /dev/mapper/myvg-mylv 10G
    Now use the lvreduce command as lvreduce -L 10G /dev/mapper/myvg-mylv

The above Command will shrink the size LVM partition and fixes the filesystem size to 10GB.Most in-depth, industry-led curriculum in linux. Check the Linux Online Courses now!
9. What are the different modes of Network bonding in Linux?
Below are list of modes used in Network Bonding :

    balance-rr or 0 – round-robin mode for fault tolerance and load balancing.
    active-backup or 1 – Sets active-backup mode for fault tolerance.
    balance-xor or 2 – Sets an XOR (exclusive-or) mode for fault tolerance and load balancing.
    broadcast or 3 – Sets a broadcast mode for fault tolerance. All transmissions are sent on all slave interfaces.
    802.3ad or 4 – Sets an IEEE 802.3ad dynamic link aggregation mode. Creates aggregation groups that share the same speed & duplex settings.
    balance-tlb or 5 – Sets a Transmit Load Balancing (TLB) mode for fault tolerance & load balancing.
    balance-alb or 6 – Sets an Active Load Balancing (ALB) mode for fault tolerance & load balancing.

10. How to check and verify the status of the bond interface?

Using the command ‘cat /proc/net/bonding/bond0,’ we can check what mode is enabled and what LAN cards are used in this bond. In this example, we have one one bond interface. However, we can have multiple bond interfaces like bond1, bond2 and so on.

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11. How can you enhance the security of password file?

Linux keep user account information in a text file called /etc/passwd. This file also stores one way encrypted password. It is accessed by several tools to get user information, which is a security risk, so file need to ‘Word Readable.’ To minimize the security risk, you can use shadow password format. This method saves account information in regular file /etc/passwd.

However, the password is stored as a single “x” character (not actually stored in this file). A second file, called “/etc/shadow” contains encrypted password as well as other information such as account or password expiration values, etc. The /etc/shadow file is readable only by the root account and is therefore less of a security risk
12. What command can you use to make a tape archive file of a /home directory and send it to the /dev/tape device?

The correct command is tar -cvf /dev/tape /home.
The -xvf option is used to extract files from an archive.Expert Linux Professionals are in much demand! Take this linux administration online certification & join the big league!
13. Suppose your FTP Server is hacked and the entire server needs to be restored. How should you restore the original kernel system files?

You cannot restore the entire operating system from tape backup device. So you should reinstall the core operating system, and then restore system configuration files and user data from tape backup device.
14. Why should you avoid Telnet to administer a Linux system remotely?

Telnet uses most insecure method for communication. It sends data across the network in plain text format and anybody can easily find out the password using the network tool. In the case of Telnet, these include the passing of login credentials in plain text, which means anyone running a sniffer on your network can find the information he needs to take control of a device in a few seconds by eavesdropping on a Telnet login session.
15. What is Puppet Server?

Puppet is an open-source & enterprise software for configuration management toll in UNIX like operating system. Puppet is an IT automation software that is used to push configuration to its clients (puppet agents) using code. Puppet code can do multiple tasks from installing new software to checking file permissions to updating user accounts.
16. What is the difference between Cron and Anacron?

    Minimum granularity with Cron is minute while it is in days with Anacron.
    Cron job can be scheduled by any normal user while Anacron can be scheduled only by the super user (the superuser is a special user account used for system administration. Depending on the operating system (OS), the actual name of this account might be root,administrator, admin or supervisor)
    Cron expects the system to be up and running while the Anacron doesn’t expect the system to be up and running all the time. In case of Anacron, if a job is scheduled and the system is down that time, it will execute the job as soon as the system is up and running.
    Cron is ideal for servers while Anacron is ideal for desktops and laptops.
    Cron should be used when you want a job to be executed at a particular hour and minute while Anacron should be used in when the job can be executed irrespective of the hour and minute.

17. What command is used to check the number of files, disk space and each user’s defined quota?

repquota command is used to check the status of the user’s quota along with the disk space and number of files used.
This command gives a summary of the user’s quota that how much space and files are left for the user. Each user has a defined quota in Linux. This is done mainly for the security as some users have only limited access to files. This provides a security to the files from unwanted access. The quota can be given to a single user or to a group of users.
18. What is the name and path of the main system log?

By default, the main system log is ‘/var/log/messages’. This file contains all the messages and the script written by the user. By default, all scripts are saved in this file. This is the standard system log file, which contains messages from all system software, non-kernel boot issues, and messages that go to ‘dmesg’. dmesg is a system file that is written upon system boot.
19. Can Linux computer be made a router so that several machines may share a single Internet connection? How?
Yes, a Linux machine can be made a router. This is called “IP Masquerade.” IP Masquerade is a networking function in Linux similar to the one-to-many (1: Many) NAT (Network Address Translation) servers found in many commercial firewalls and network routers.
The IP Masquerade feature allows other “internal” computers connected to this Linux box (via PPP, Ethernet, etc.) to also reach the Internet as well. Linux IP Masquerading allows this functionality even if the internal computers do not have IP addresses.
The IP masquerading can be done by the following steps :

    The Linux PC must have an internet connection and a connection to LAN. Typically, the Linux PC has two network interfaces-an Ethernet card for the LAN and a dial-up PPP connection to the Internet (through an ISP).
    All other systems on your LAN use the Linux PC as the default gateway for TCP/IP networking. Use the same ISP-provided DNS addresses on all systems.
    Enable IP forwarding in the kernel. By default the IP forwarding is not enabled. To ensure that IP forwarding is enabled when you reboot your system, place this command in the /etc/rc.d/rc.local file.
    Run /sbin/iptables-the IP packet filter administration program-to set up the rules that enable the Linux PC to masquerade for your LAN.

20. How to change window manager by editing your home directory?
“/.xinitrc file” allows changing the window manager we want to use when logging into X from that account. The dot in the file name shows you that the file is a hidden file and doesn’t show when you do a normal directory listing. For setting a window manager we have to save a command in this file.
The syntax of command is : exec window manager. After this, save the file. Next time when you run a startx a new window manager will open and become default.
The commands for starting some popular window managers and desktop environments are :

    KDE = startkde
    Gnome = gnome-session.
    Blackbox = blackbox.
    FVWM = fvwm
    Window Maker = wmaker.
    IceWM = icewm

21. How shadow passwords are given by in Linux?

pwconv command is used for giving shadow passwords. Shadow passwords are given for better system security. The pwconv command creates the file /etc/shadow and changes all passwords to ‘x’ in the /etc/passwd file.

First, entries in the shadowed file which don’t exist in the main file are removed. Then, shadowed entries which don’t have `x’ as the password in the main file are updated. Any missing shadowed entries are added. Finally, passwords in the main file are replaced with `x’. These programs can be used for initial conversion as well to update the shadowed file if the main file is edited by hand.
22. What daemon is used for scheduling of the commands?

The crontab command is used for scheduling of the commands to run at a later time.
SYNTAX

crontab [ -u user ] file
crontab [ -u user ] { -l | -r | -e }

Options
l List – display the current crontab entries.
r Remove the current crontab.
e Edit the current crontab using the editor specified by the VISUAL or EDITOR environment variables.
When user exits from the editor, the modified crontab will be installed automatically. Each user can have their own crontab, and though these are files in /var, they are not intended to be edited directly.
If the –u option is given than the crontab gives the name of the user whose crontab is to be tweaked. If it is given without this then it will display the crontab of the user who is executing the command.
23. What shell does a Linux Administrator assign to a POP3 mail-only account?
POP3 mail only account is assigned to the /bin/false shell. However, assigning bash shell to a POP3 mail only gives user login access, which is avoided. /bin/nologin can also be used. This shell is provided to the user when we don’t want to give shell access to the user. The user cannot access the shell and it rejects shell login on the server as in Telnet. It is mainly for the security of the shells.
POP3 is basically used for downloading mail to mail program. So for illegal downloading of emails on the shell, this account is assigned to the /bin/false shell or /bin/nologin. These both shells are same as they both do the same work of rejecting the user login to the shell.
The main difference between these two shells is that false shell shows the incorrect code and any unusual coding when user login to it. But the nologin shell simply tells that no such account is available. So nologin shell is used often in Linux.
24. If a volume group named VG0 already exists and we need to extend this volume group up to 4GB. How will you do it?

    Firstly create Physical volume (/dev/sda7) of size 4GB.
    Now run following command.
    vgextend VG0 /dev/sda7

25. Is there any relation between modprobe.conf file and network devices?

Yes, this file assigns a kernel module to each network device.

For Example :-

[root@localhost ~]# cat /etc/modprobe.conf
alias eth0 b44
Here, b44 is the kernel module for network device eth0.
We can confirm whether this module “b44” is present or not by the following command
[root@localhost ~]# lsmod |grep b44
b44 29005 0

26. What is YUM?

YUM stands for Yellow dog Updater, Modified because it is based on YUP, the Yellow dog Updater. Where does the name Yellow dog come from? Yellow Dog is a version of Linux for the Power Architecture hardware and is RPM-based, just like Red Hat Enterprise Linux and Fedora. YUP, and later YUM, were written by the Linux community as a way to maintain an RPM-based system
27. What is the role of "Kudzu"?

Kudzu is used to detect new Hardware. RedHat Linux runs a hardware discoverer, named kudzu. When attempting to identify a serial port Kudzu resets the serial port. This stops the serial console. Kudzu is configured from the file/etc/sysconfig/kudzu.
Kudzu can be prevented from resetting hardware, by setting the configuration parameter SAFE to yes.
28. What is the difference between ext2 and ext3 file systems?

    The ext3 file system is an enhanced version of the ext2 file system.
    The most important difference between Ext2 and Ext3 is that Ext3 supports journaling.
    After an unexpected power failure or system crash (also called an unclean system shutdown), each mounted ext2 file system on the machine must be checked for consistency by the e2fsck program. This is a time-consuming process and during this time, any data on the volumes is unreachable.
    The journaling provided by the ext3 file system means that this sort of file system check is no longer necessary after an unclean system shutdown. The only time a consistency check occurs using ext3 is in certain rare hardware failure cases, such as hard drive failures. The time to recover an ext3 file system after an unclean system shutdown does not depend on the size of the file system or the number of files. Rather, it depends on the size of the journal used to maintain consistency. The default journal size takes almost a second to recover, depending on the speed of the hardware.

29. Explain /proc filesystem?

/proc is a virtual file system that provides detailed information about Linux kernel, hardware and running processes. Files under /proc directory named as Virtual files.
Since /proc contains virtual files, it is called virtual file system. These virtual files have unique qualities. Most of them are listed as zero bytes in size.
Virtual files such as /proc/interrupts, /proc/meminfo, /proc/mounts, and /proc/partitions provide an up-to-the-moment glimpse of the system’s hardware. Others: /proc/filesystems file and the /proc/sys/ directory provide system configuration information and interfaces.
30. How do you create ext4 file system?

# mke2fs -t ext4 /dev/DEV
31. How to Enable ACLs for /home partition?
Add following entry in /etc/fstab
LABEL=/home /home ext3 acl 1 2
Now remount /home partition with acl option.
mount -t ext3 -o acl /dev/sda3 /home



/////////////////////////////////////////////////////////////////////////////////////////////////////////////


Linux Interview Questions

A list of top frequently asked Linux interview questions and answers are given below.
1) What is Linux?

Linux is a UNIX based operating system. It was first introduced by Linus Torvalds. It is an open source operating system that was designed to provide free and a low cost operating system for the computer users.
2) What is the difference between UNIX and Linux?

UNIX was originally started as a propriety operating system for Bell laboratories, which later release their commercial version while Linux is a free, open source and a non propriety operating system for the mass uses.
3) What is Linux Kernel?

Linux Kernel is low level system software. It is used to manage the hardware resources for the users. It provides an interface for user-level interaction.
4) Is it legal to edit Linux Kernel?

Yes. You can edit Linux Kernel because it is released under General Public License (GPL) and any one can edit it. It comes under the category of free and open source software.
5) What is LILO?

LILO is a boot loader for Linux. It is used to load the Linux operating system into the main memory to begin its operations.
6) What is the advantage of open source?

Open source facilitates you to distribute your software, including source codes freely to anyone who is interested. So, you can add features and even debug and correct errors of the source code.
7) What are the basic components of Linux?

Just like other operating systems, Linux has all components like kernel, shells, GUIs, system utilities and application programs.
8) What is the advantage of Linux?

Every aspect comes with additional features and it provides a free downloading facility for all codes.
9) What is Swap Space?

Swap space is used to specify a space which is used by Linux to hold some concurrent running program temporarily. It is used when RAM does not have enough space to hold all programs that are executing.
10) What is BASH?

BASH is a short form of Bourne Again SHell. It was a replacement to the original Bourne shell, written bu Steve Bourne.
11) What is the basic difference between BASH and DOS?

    BASH commands are case sensitive while DOS commands are not case sensitive.
    DOS follows a convention in naming files. In DOS, 8 character file name is followed by a dot and 3 characters for the extension. BASH doesn't follow such convention.

12) What is a root account?

The root account is like a system administrator account. It provides you full control of the system. You can create and maintain user accounts, assign different permission for each account etc.
13) What is CLI?

CLI stands for Command Line Interface. It is an interface that allows users to type declarative commands to instruct the computer to perform operations.
14) What is GUI?

GUI stands for Graphical User Interface. It uses the images and the icons which are clicked by the users to communicate with the system. It is more attractive and user-friendly because the use of the images and icons.
15) Which popular office suite is available free for both Microsoft and Linux?

Open Office Suite is available free for both Microsoft and Linux. You can install it on both of them.
16) Suppose your company is recently switched from Microsoft to Linux and you have some MS Word document to save and work in Linux, what will you do?

Install Open Office Suite on Linux. It facilitates you to work with Microsoft documents.
17) What is SMTP?

SMTP stands for Simple Mail Transfer Protocol. It is an internet standard for mail transmission.
18) What is Samba? Why is it used?

Samba service is used to connect Linux machines to Microsoft network resources by providing Microsoft SMB support.
19) What are the basic commands for user management?

    last,
    chage,
    chsh,
    lsof,
    chown,
    chmod,
    useradd,
    userdel,
    newusers etc.

20) What is the maximum length for a file name in Linux?

255 characters.
21) Is Linux Operating system virus free?

No, There is no operating system till date that is virus free but Linux is known to have less number of viruses.
22) Which partition stores the system configuration files in Linux system?

/stc partition.
23) Which command is used to uncompress gzip files?

gunzip command is used to uncompress gzip files.
24) Why do developers use MD5 options on passwords?

MD5 is an encryption method so it is used to encrypt the passwords before saving.
25) What is a virtual desktop?

The virtual desktop is used as an alternative to minimizing and maximizing different windows on the current desktop. Virtual desktop facilitates you to open one or more programs on a clean slate rather than minimizing or restoring all the needed programs.
26) What is the difference between soft and hard mounting points?

In the soft mount, if the client fails to connect the server, it gives an error report and closes the connection whereas in hard mount, if the client fails to access the server, the connection hangs; and once the system is up, it again accesses the server.
27) Does the Alt+Ctrl+Del key combination work in Linux?

Yes, it works like windows.
28) What are the file permissions in Linux?

There are 3 types of permissions in Linux OS that are given below:

    Read: User can read the file and list the directory.
    Write: User can write new files in the directory .
    Execute: User can access and run the file in a directory.

29) What are the modes used in VI editor?

There are 3 types of modes in vi Editor:

    Regular mode or command mode
    Insertion mode or edit mode
    Replacement mode or Ex mode

30) How to exit from vi editor?

The following commands are used to exit from vi editor.

    :wq saves the current work and exits the VI.
    :q! exits the VI without saving current work.

31) How to delete information from a file in vi?

The following commands are used to delete information from vi editor.

    x deletes a current character.
    dd deletes the current line.

32) How to create a new file or modify an existing file in vi?

    vi filename 

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