# Linux Interview Questions for DevOps Interview 1. When you login you get "$" prompt, what is the prompt for root? * `ans: #` 2. Explain the difference between grep and egrep? * `Search online` 3. What is the port # for DNS, NTP and NFS? * `53,123 and 111/2049` 4. What is the configuration file name of DNS and where is it located? * `/etc/named.conf` 5. How many new directories will be created after running the following command ```sh mkdir {a..c}{1..3} # ans: 9 ``` 6. Your PC is configured with a DNS server address but not the default gateway. Can the PC access internet? * `No` 7. What is the difference between IP and Gateway? * `Search online` 8. Can you assign one static IP to 2 computers, if not then why? * `No because it will create IP conflict` 9. How to change IPs address to static? * `ifconfig x.x.x.x` 10. You are trying to ping a server by hostname and you get an error message, `"ping: unknown host …"`. What could be the reason and how to solve the problem so you can ping it by hostname? * `Check for /etc/hosts or DNS to see if it has hostname to IP entry` 11. Explain the difference between relative and absolute path? * `Absolute path starts from / where relative path is your current directory` 12. List 3 different methods of adding user? * `Search online` 13. What is the command to change file/directory ownership and group? * `chown and chgrp` 14. List any 3 type of filesystem? * `ext4,NTFS and FAT` 15. When you login you get a message on the screen. What is the name of that file and where is it located? * `/etc/motd` 16. What is /bin directory used for? * `Search online` 17. What are the different types of DNS Server * `Master and secondary` 18. How to change a user password? * `passwd username` 19. What is the version of Redhat Linux you have experience with? * `7.4` 20. List any 4 linux distributions? * `Redhat, CentOS, Ubuntu and SUSE` 21. How to logoff from the system? * `exit` 22. Give any 3 examples of operating systems? * `Windows, Linux and MAC` 23. How to create a directory? * `mkdir` 24. Where are the zone files located for DNS service? * `/var/named/zonefiles` 25. How to check kernel version? * `uname -a` 26. Which directory has all the configuration files? * `/etc` 27. How to become root user from a regular user? * `su –` 28. How many mega bytes in 1 giga bytes? * `Search online` 29. What is the purpose of having different network ports? * `So the communication of each application goes through a dedicated port` 30. How to display first column of a file? * `cat filename | awk ‘{print $1}’` 31. What is the name of DNS rpm package? * `bind` 32. What is the difference between nslookup and dig commands? * `Search online` 33. How to check your user id and group id? * `id` 34. How to check a file’s permission? * `ls –l` 35. What is the difference between “kill” and “kill -9” command? * `Search online` 36. What is subnet? * `Search online` 37. You are troubleshooting an issue with Redhat support and they have asked you to send the contents of /etc directory. How and which method you will use to transfer the contents? * `tar (compress) the entire /etc directory and ftp` 38. What is root home directory? * `/root` 39. What is rsyslogd deamon and its purpose? * `Search online` 40. Your company has terminated a server administrator. What is first thing as an administrator you should do to enhance the security? * `Change root password` 41. How to check the computer name or host name in Linux? * `hostname` 42. Which permission allows a user to run an executable with the permissions of the owner of that file? * `First 3 bits should have x` 43. What is the command to untar a tarred file? * `untar` 44. What is /proc directory used for? * `Search online` 45. What is the purpose of nsswitch.conf file * `It tells the system where to go to resolve hostnames` 46. List 3 basic commands to navigate the filesystem? * `cd, pwd and ls` 47. Which service/daemon should be running on the server that allows you to connect remotely? * `sshd` 48. What is the purpose of firewall? * `Search online` 49. List any 3 IT components? * `Hardware, OS and Applications` 50. Which directory has all the commands we use, e.g. ls, cd etc.? * `/usr/bin or /bin` 51. What is the difference between memory, virtual memory and cache? * `Search online` 52. Which of the following is correct? * \[x\] `a. Hardware  Operating System  Users` * \[ \] `b. Operating System  Users  Hardware` * \[ \] `c. Database  Hardware  Users` 53. Which of the following is a communication command? * `o grep` * `o mail` * `o touch` * `o cd` 54. How to rename a file or directory? * `mv` 55. How to change a hostname in Linux? * `Search online` 56. How to check network interfaces in Linux? * `ifconfig` 57. Why is “tail –f logfilename” command used most often and what does it do? * `It will output all incoming logs in real time` 58. What type of hardware have you worked on? * `You should get yourself familiar with Dell, HP and UCS hardware by going online and check the vendor websites` 59. How to sort a file in reverse order? * `cat filename | sort –r` 60. What is the name of operating system that runs Unix? * Solaris, HP-UX etc. 61. List all byte sizes from smallest to largest? * `Search online` 62. How to check the total number of partition in Linux? * `fdisk -l` 63. How to access a linux system from a linux system? * `ssh` 64. Explain the procedure of bonding 2 NICs or interfaces together? * `Search online` 65. What is the exact command syntax to list the 5th column of a file and cut the first 3 letters? * `cat filename | awk ‘{print $5}’ | cut –c1-3` 66. What is /etc/hosts file used for? * `To resolve hostnames with IP address` 67. List any 3 options of ‘df’ command and what they are used for? * `Search online` 68. What is the command to change file/directory permissions? * `chmod` 69. What is the purpose of pipe (|)? * `To combine multiple commands` 70. What is /etc directory used for? * `For configuration files` 71. Which command is used to list files in a directory? * `ls -l` 72. There is a command which gives you information about other commands, please explain that command and what is it used for? * `man` 73. How to delete a file and a directory? * `rm filename and rmdir dirname` 74. What is the difference between “tail” and “tail -10”? * `None` 75. List 4 commands to display or read a file contents? * `cat, more, less, vi` 76. Which command is used to read the top 5 lines of a file? * `head -5 filename` 77. What are the different commands or methods to write to a file? * `echo > filename and vi filename` 78. What is swap space and how to check swap space? * `Search online` 79. What is inode and how to find an inode of a file? * `Search online` 80. Which file to edit for kernel tuning? * `Search online` 81. What is the latest version of Redhat? * `Search online` 82. Name the command to find specific word from a file? * `grep word filename` 83. You have scheduled a job using crontab but it does not run at the time you specified, what could be the reason and how would you troubleshoot? * `Check your system time` * `Check your crontab entry` * `Check /var/log/messages` 84. How to check system hardware information? * `dmidecode` 85. How to check network interface MAC address? * `ifconfig` 86. If I don’t want others to read my file1, how to do that? * `Remove r from the last 3 bits of file permission` 87. What is the purpose of “uniq” and “sed” command? * `Search online` 88. Which command is used to list the contents of a directory in the most recent time and in reverse order, meaning the most updated file should be listed on the bottom? * `ls –ltr` 89. What is the difference between tar, gzip and gunzip? * `Search online` 90. What are the different ways to install and OS? * `DVD, DVD iso and network boot` 91. How to view difference between two files? * `diff file1 and file2` 92. You noticed that one of the Linux servers has no disk space left, how would you troubleshoot that issue? * `If running LVM then add more disk and extend LVM If not running LVM then add more disk, create a new partition and link the new partition to an existing filesystem` 93. How to check Redhat version release? * `uname –a or /etc/redhat-release` 94. What is the difference between TCP and UDP? * `Search online` 95. What is a zombie process? * `Search online` 96. How do you search for a pattern/word in a file and then replace it in an entire file? * `sed command` 97. Explain the purpose of “touch” command? * `To create an empty file` 98. If a command hangs, how to stop it and get the prompt back? * `Ctrl C` 99. Which command is used to count words or lines? * `wc` 100. How to check the number of users logged in? * `who` 101. What is the command to view the calendar of 2011? * `cal 2011` 102. Which command is used to view disk space? * `df –h` 103. How to create a new group in Linux? * `groupadd` 104. What is the command to send a message to everyone who is logged into the system? * `wall` 105. Which command is used to check total number of disks? * `fdisk –l` 106. What is an mail server record in DNS? * `MX` 107. What does the following command line do? ```sh ps -ef | awk '{print $1}' | sort | uniq ``` ANS: `List the first column of all running processes, sort them andremove duplicates` 108. You get a call that when a user goes to [www.yourwebsite.com](http://www.yourwebsite.com) it fails and gets an error, how do you troubleshoot? * `Check for user internet` * `Check to see if user computer has DNS for hostname lookup` * `Check to see if the server is up that is running that website` * `Check to see if the server’s web service is running` * `Check for DNS availability which is resolving that website` 109. List 4 different directories in /? * `/etc, /bin, /tmp, /home` 110. What is the output of the following command: * `$tail -10 filename | head -1` ANS: `It will show the first line from the last 10 lines of a file` 111. What are the different fields in /etc/passwd file? * `Search online` 112. Which command is used to list the processes? * `ps –ef` 113. What is the difference between “hostname” and “uname” commands? * `Hostname will give you system name and uname will give you OS information` 114. How to check system load? * `top and uptime command` 115. How to schedule jobs? * `crontab and at` 116. What is the 3rd field when setting up crontab? * `Day of the month` 117. What is the command to create a new user? * `useradd` 118. What is the “init #” for system reboot? * `6` 119. How to restart a service? * `systemctl restart servicename` 120. How to shutdown a system? * `shutdown or init 0` 121. What is “ftp” command used for? * `To transfer files from one computer to another` 122. Explain cron job syntax? First is minute, second is..? * `Min, house, day of the month, month, day of the week and command` 123. How to delete a package in Linux? * `rpm –e packagename` 124. What is the file name where user password information is saved? * `/etc/shadow` 125. Which command you would use to find the location of chmod command? * `which chmod` 126. Which command is used to check if the other computer is online? * `ping othercomputer` 127. Please explain about LAN, MAN and WAN? * `Search online` 128. How to list hidden files in a directory? * `ls –la` 129. What is the difference between telnet and ssh? * `ssh is secure where telnet is not` 130. How to run a calculator on Linux and exit out of it? * `bc and quit` 131. List any 4 commands to monitor system? * `top, df –h, iostat, dmesg` 132. You are notified that your server is down, list the steps you will take to troubleshoot? * `Check the system physically` * `Login through system console` * `Ping the system` * `Reboot or boot if possible` 133. What is difference between static and DHCP IP? * `Search online` 134. How to write in vi editor mode? * `i = insert, a = insert in next space, o = insert in new line` 135. What is the difference between “crontab” and “at” jobs? * `crontab is for repetitive jobs where at is for one time job` 136. What is vCenter server in VMWare? * `Search online` 137. What is “dmidecode” command used for? * `To get system information` 138. What is the difference between SAN and NAS? * `Search online` 139. What is the location of system logs? E.g. messages * `/var/log directory` 140. How to setup an alias and what is it used for? * `alias aliasname=”command”` * `It is used to created short-cuts for long commands` 141. What is the purpose of “netstat” command? * `Search online` 142. What are terminal control keys, list any 3? * `Crtl C, D and Z` 143. Which command(s) you would run if you need to find out how many processes are running on your system? * `ps –ef | wc –l` 144. What are the different types of shells? * `sh, bash, ksh, csh etc.` 145. How to delete a line when in vi editor mode? * `dd` 146. Which is the core of the operating system? * \[ \] `a) Shell` * \[x\] `b) Kernel` * \[ \] `c) Commands` * \[ \] `d) Script` 147. Which among the following interacts directly with system hardware? * \[ \] `a) Shell` * \[ \] `b) Commands` * \[x\] `c) Kernel` * \[ \] `d) Applications` 148. How to save and quit from vi editor? * `Shift ZZ or :wq!` 149. What is the difference between a process and daemon? * `Search online` 150. What is the process or daemon name for NTP? * `ntpd` 151. What are a few commands you would run if your system is running slow? * `top, iostat, df –h, netstat etc.` 152. How to install a package in Redhat Linux? * `yum install packagename` 153. What is the difference between “ifconfig” and “ipconfig” commands? * `ifconfig for Linux and ipconfig for Windows` 154. What is the first line written in a shell script? * `Define shell e.g. #!/bin/bash` 155. Where is the network (Ethernet) file located, please provide exact directory location and file name? * `/etc/sysconfig/network-scripts/ifcfg-nic` 156. Why do we use “last” command? * `To see who has logged in the system whether active or logged off` 157. What is RHEL Linux stands for? * `Search online` 158. To view your command history, which command is used and how to run a specific command? * `history and history #` 159. What is NTP and briefly explain how does it work and where is the config files and related commands of NTP? * `Search online` 160. How to disable firewall in Linux? * `Search online` 161. How to configure mail server relay for sendmail service? * `Edit /etc/mail/sendmail.mc file and add SMART_HOST entry` 162. Where is samba log file located? * `/var/log/samba` 163. What is mkfs command used for? * `To create a new filesystem` 164. If you create a new group, which file does it get created in? * `/etc/group` 165. Which file has DNS server information (e.g. DNS resolution)? * `/etc/resolv.conf` 166. What are the commands you would run if you need to find out the version and build date of a package (e.g. http)? * `rpm –qi http` 167. On the file permissions? What are the first 3 bits for and who is it for? * `Read, write and execute. They are used for the owner of the file` 168. How to create a soft link? * `ln –s` 169. How to write a script to delete messages in a log file older than 30 days automatically? * `Search online` 170. How to quit out of “man” command? * `q` 171. Which command is used to partition disk in Linux? * `fdisk` 172. What is the difference between “shutdown” and “halt” command? * `Search online` 173. What is the exact syntax of mounting NFS share on a client and also how to un-mount? * `Search online` 174. What experience do you have with scripting, explain? * `if-the, do-while, case, for loop scripts` 175. How to get information on all the packages installed on the system? * `rpm –qa` 176. Explain VMWare? * `Search online` 177. You are tasked to examine a log file in order to find out why a particular application keep crashing. Log file is very lengthy, which command can you use to simplify the log search using a search string? * `grep for error, warning, failure etc. in /var/log/messages file` 178. What is /etc/fstab file and explain each column of this file? * `Search online` 179. What the latest version of Windows server? * `Search online` 180. What is the exact command to list only the first 2 lines of history output? * `history | head -2` 181. How to upgrade Linux from 7.3 to 7.4? * `yum install update` 182. How to tell which shell you are in or running? * `$0` 183. You have tried to “cd” into a directory but you have been denied. You are not the owner of that directory, what permissions do you need and where? * `- - - - - - - r – x` 184. What is CNAME record in DNS? * `Entry for hostname to hostname` 185. What is the name of VMWare operating system? * `ESXi` 186. What is the client name used to connect to ESXi or vCenter server? * `vSphere client` 187. You get a call from a user saying that I cannot write to a file because it says, permission denied. The file is owned by that user, how do you troubleshoot? * `Give write permission on the first 3 bits` 188. What is the latest version of VMWare? * `Search online` 189. What is the name of firewall daemon in Linux? * `firewalld` 190. Which command syntax you can use to list only the 20th line of a file? * `Search online` 191. What is the difference between run level 3 and 5? * `3 = Boot system with networking, 5 = boot system with networking and GUI` 192. List a few commands that are used in troubleshooting network related issue? * `netstat, tcpdump etc.` 193. What is the difference between domain and nameserver? * `Search online` 194. You open up a file and it has 3000 lines and it scrolled up really fast, which command you will use to view it one page at a time? * `more or less` 195. How to start a new shell. E.g. start a new ksh shell? * `Simply type ksh, or bash` 196. How to kill a process? * `kill processID` 197. How to check scheduled jobs? * `crontab –l` 198. How to check system memory and CPU usage? * `free and top` 199. Which utility could you use to repair the corrupted file system? * `fsck` 200. What is the command to make a service start at boot? * `systemctl enable servicename` 201. How to combine 2 files into 1? E.g. you 3 lines in file “A” and 5 lines in file “B”, which command syntax to use that will combine into one file of 3+5 = 8 lines * `cat fileA >> fileB` 202. What is echo command used for? * `To output to a screen` 203. What does the following command do? `echo This year the summer will be great > file1` * `It will create a new file “file1” with the content as “This year the summer will be great”` 204. Which file to modify to allow users to run root commands * `/etc/sudoers` 205. You need to modify httpd.conf file but you cannot find it, Which command line tool you can use to find file? * `find / -name “httpd.conf”` 206. Your system crashed and being restarted, but a message appears, indicating that the operating system cannot be found. What is the most likely cause of the problem? * `The /boot file is most likely corrupted`