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Some IT DevOps, SysAdmin, Developer positions require the knowledge of basic linux Operating System. Most of the time, we know the answer but forget them when we don’t practice very often. This refresher will help you prepare for the linux portion of your IT interview by answering some gotcha Linux Questions for IT DevOps and SysAdmin Interviews.

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I- Networking:

1. How many bytes are there in a MAC address?
   48.
   MAC, Media Access Control, address is a globally unique identifier assigned to network devices, and therefore it is often referred to as hardware or physical address. MAC addresses are 6-byte (48-bits) in length, and are written in MM:MM:MM:SS:SS:SS format.
2. What are the different parts of a TCP packet?
   The term TCP packet appears in both informal and formal usage, whereas in more precise terminology segment refers to the TCP protocol data unit (PDU), datagram to the IP PDU, and frame to the data link layer PDU: … A TCP segment consists of a segment header and a data section.
3. Networking: Which command is used to initialize an interface, assign IP address, etc.
   ifconfig (interface configuration). The equivalent command for Dos is ipconfig.
   Other useful networking commands are: Ping, traceroute, netstat, dig, nslookup, route, lsof
4. What’s the difference between TCP and UDP; Between DNS TCP and UDP?
   There are two types of Internet Protocol (IP) traffic. They are TCP or Transmission Control Protocol and UDP or User Datagram Protocol. TCP is connection oriented – once a connection is established, data can be sent bidirectional. UDP is a simpler, connectionless Internet protocol.
   The reality is that DNS queries can also use TCP port 53 if UDP port 53 is not accepted.
   DNS uses TCP for Zone Transfer over port :53.
   DNS uses UDP for DNS Queries over port :53.

5. What are defaults ports used by http, telnet, ftp, smtp, dns, , snmp, squid?
   All those services are part of the Application level of the TCP/IP protocol.
   http => 80
   telnet => 23
   ftp => 20 (data transfer), 21 (Connection established)
   smtp => 25
   dns => 53
   snmp => 161
   dhcp => 67 (server), 68 (Client)
   ssh => 22
   squid => 3128
6. How many host available in a subnet (Class B and C Networks)
   
7. How DNS works?
   When you enter a URL into your Web browser, your DNS server uses its resources to resolve the name into the IP address for the appropriate Web server.
8. What is the difference between class A, class B and class C IP addresses?
   Class A Network (/ 8 Prefixes)
   This network is 8-bit network prefix. IP address range from 0.0.0.0 to 127.255.255.255
   Class B Networks (/16 Prefixes)
   This network is 16-bit network prefix. IP address range from 128.0.0.0 to 191.255.255.255Class C Networks (/24 Prefixes)
   This network is 24-bit network prefix.IP address range from 192.0.0.0 to 223.255.255.255
9. Difference between ospf and bgp?
   The first reason is that BGP is more scalable than OSPF. , and this, normal igp like ospf cannot perform. Generally speaking OSPF and BGP are routing protocols for two different things. OSPF is an IGP (Interior Gateway Protocol) and is used internally within a companies network to provide routing.

II- Operating System
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1. How to find the Operating System version?
   $uname -a
   To check the distribution for redhat for example: $cat /etc/redhat –release
2. How to list all the process running?
   top
   To list java processes, ps -ef | grep java
   To list processes on a specific port:
   netstat -aon | findstr :port_number
   lsof -i:80
3. How to check disk space?
   df shows the amount of disk space used and available.
   du displays the amount of disk used by the specified files and for each subdirectories.
   To drill down and find out which file is filling up a drive: du -ks /drive_name/* | sort -nr | head
4. How to check memory usage?
   free or cat /proc/meminfo
5. What is the load average?
   It is the average sum of the number of process waiting in the queue and the number of process currently executing over the period of 1, 5 and 15 minutes. Use top to find the load average.
6. What is a load balancer?
   A load balancer is a device that acts as a reverse proxy and distributes network or application traffic across a number of servers. Load balancers are used to increase capacity (concurrent users) and reliability of applications.
7. What is the 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.
8. What is the default kill signal?
   There are many different signals that can be sent (see signal for a full list), although the signals in which users are generally most interested are SIGTERM (“terminate”) and SIGKILL (“kill”). The default signal sent is SIGTERM.
   kill 1234
   kill -s TERM 1234
   kill -TERM 1234
   kill -15 1234
9. Describe Linux boot process
   BIOS => MBR => GRUB => KERNEL => INIT => RUN LEVEL
   As power comes up, the BIOS (Basic Input/Output System) is given control and executes MBR (Master Boot Record). The MBR executes GRUB (Grand Unified Boot Loader). GRUB executes Kernel. Kernel executes /sbin/init. Init executes run level programs. Run level programs are executed from /etc/rc.d/rc*.d
   Mac OS X Boot Process:

   Boot ROM	Firmware. Part of Hardware system BootROM firmware is activated
   POST	Power-On Self Test initializes some hardware interfaces and verifies that sufficient memory is available and in a good state.
   EFI	Extensible Firmware Interface EFI does basic hardware initialization and selects which operating system to use.
   BOOTX	boot.efi boot loader load the kernel environment
   Rooting/Kernel	The init routine of the kernel is executed boot loader starts the kernel’s initialization procedure Various Mach/BSD data structures are initialized by the kernel. The I/O Kit is initialized. The kernel starts /sbin/mach_init
   Run Level	mach_init starts /sbin/init init determines the runlevel, and runs /etc/rc.boot, which sets up the machine enough to run single-user. rc.boot figures out the type of boot (Multi-User, Safe, CD-ROM, Network etc.)

10. List services enabled at a particular run level
    chkconfig –list | grep 5:0n
    Enable|Disable a service at a specific run level: chkconfig on|off –level 5
11. How do you stop a bash fork bomb?
    Create a fork bomb by editing limits.conf:
    root hard nproc 512
    Drop a fork bomb as below:
    :(){ :|:& };:
    Assuming you have access to shell:
    kill -STOP
    killall -STOP -u user1
    killall -KILL -u user1
12. What is a fork?
    fork is an operation whereby a process creates a copy of itself. It is usually a system call, implemented in the kernel. Fork is the primary (and historically, only) method of process creation on Unix-like operating systems.
13. What is the D state?
    D state code means that process is in uninterruptible sleep, and that may mean different things but it is usually I/O.

III- File System

1. What is umask?
   umask is “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.
2. What is the role of the 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.

• What is the role of the 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.
• What is the null device in Linux?
  The null device is typically used for disposing of unwanted output streams of a process, or as a convenient empty file for input streams. This is usually done by redirection. The /dev/null device is a special file, not a directory, so one cannot move a whole file or directory into it with the Unix mv command.You might receive the “Bad file descriptor” error message if /dev/null has been deleted or overwritten. You can infer this cause when file system is reported as read-only at the time of booting through error messages, such as“/dev/null: Read-only filesystem” and “dup2: bad file descriptor”.
  In Unix and related computer operating systems, a file descriptor (FD, less frequently fildes) is an abstract indicator (handle) used to access a file or other input/output resource, such as a pipe or network socket.
• What is a inode?
  The inode is a data structure in a Unix-style file system that describes a filesystem object such as a file or a directory. Each inode stores the attributes and disk block location(s) of the object’s data.

IV- Databases

1. What is the difference between a document store and a relational database?
   In a relational database system you must define a schema before adding records to a database. The schema is the structure described in a formal language supported by the database and provides a blueprint for the tables in a database and the relationships between tables of data. Within a table, you need to define constraints in terms of rows and named columns as well as the type of data that can be stored in each column.In contrast, a document-oriented database contains documents, which are records that describe the data in the document, as well as the actual data. Documents can be as complex as you choose; you can use nested data to provide additional sub-categories of information about your object. You can also use one or more document to represent a real-world object.
2. How to optimise a slow DB?
   • Rewrite the queries
   • Change indexing strategy
   • Change schema
   • Use an external cache
   • Server tuning and beyond
3. How would you build a 1 Petabyte storage with commodity hardware?
   Using JBODs with large capacity disks with Linux in a distributed storage system stacking nodes until 1PB is reached.
   JBOD (which stands for “just a bunch of disks”) generally refers to a collection of hard disks that have not been configured to act as a redundant array of independent disks (RAID) array.
   

V- Scripting

1. What is @INC in Perl?
   The @INC Array. @INC is a special Perl variable that is the equivalent to the shell’s PATH variable. Whereas PATH contains a list of directories to search for executables, @INC contains a list of directories from which Perl modules and libraries can be loaded.
2. Strings comparison – operator – for loop – if statement
   
3. Sort access log file by http Response Codes
   Via Shell using linux commands
   cat sample_log.log | cut -d ‘”‘ -f3 | cut -d ‘ ‘ -f2 | sort | uniq -c | sort -rn
4. Sort access log file by http Response Codes Using awk
   awk ‘{print $9}’ sample_log.log | sort | uniq -c | sort -rn
5. Find broken links from access log file
   awk ‘($9 ~ /404/)’ sample_log.log | awk ‘{print $7}’ sample_log.log | sort | uniq -c | sort -rn
6. Most requested page:
   awk -F\” ‘{print $2}’ sample_log.log | awk ‘{print $2}’ | sort | uniq -c | sort -r
7. Count all occurrences of a word in a file
   grep -o “user” sample_log.log | wc -w

Learn more at http://career.guru99.com/top-50-linux-interview-questions/

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