Linux System Administrator/DevOps Interview Questions

General Questions:

• What did you learn yesterday/this week?
• Talk about your preferred development/administration environment. (OS, Editor, Browsers, Tools etc.)
• Tell me about the last major Linux project you finished.
• Tell me about the biggest mistake you've made in [some recent time period] and how you would do it differently today. What did you learn from this experience?
• Why must we choose you?
• What function does DNS play on a network?
• What is HTTP?
• What is an HTTP proxy and how does it work?
• Describe briefly how HTTPS works.
• What is SMTP? Give the basic scenario of how a mail message is delivered via SMTP.
• What is RAID? What is RAID0, RAID1, RAID5, RAID10?
• What is a level 0 backup? What is an incremental backup?
• Describe the general file system hierarchy of a Linux system.
• Which difference have between public and private SSH key?

Simple Linux Questions:

• What is the name and the UID of the administrator user?
• How to list all files, including hidden ones, in a directory?
• What is the Unix/Linux command to remove a directory and its contents?
• Which command will show you free/used memory? Does free memory exist on Linux?
• How to search for the string "my konfu is the best" in files of a directory recursively?
• How to connect to a remote server or what is SSH?
• How to get all environment variables and how can you use them?
• I get "command not found" when I run ifconfig -a. What can be wrong?
• What happens if I type TAB-TAB?
• What command will show the available disk space on the Unix/Linux system?
• What commands do you know that can be used to check DNS records?
• What Unix/Linux commands will alter a files ownership, files permissions?
• What does chmod +x FILENAME do?
• What does the permission 0750 on a file mean?
• What does the permission 0750 on a directory mean?
• How to add a new system user without login permissions?
• How to add/remove a group from a user?
• What is a bash alias?
• How do you set the mail address of the root/a user?
• What does CTRL-c do?
• What does CTRL-d do?
• What does CTRL-z do?
• What is in /etc/services?
• How to redirect STDOUT and STDERR in bash? (> /dev/null 2>&1)
• What is the difference between UNIX and Linux.
• What is the difference between Telnet and SSH?
• Explain the three load averages and what do they indicate. What command can be used to view the load averages?
• Can you name a lower-case letter that is not a valid option for GNU ls?
• What is a Linux kernel module?
• Walk me through the steps in booting into single user mode to troubleshoot a problem.
• Walk me through the steps you'd take to troubleshoot a 404 error on a web application you administer.
• What is ICMP protocol? Why do you need to use?

Medium Linux Questions:

• What do the following commands do and how would you use them?
• tee
• awk
• tr
• cut
• tac
• curl
• wget
• watch
• head
• tail
• less
• cat
• touch
• sar
• netstat
• tcpdump
• lsof
• What does an & after a command do?
• What does & disown after a command do?
• What is a packet filter and how does it work?
• What is Virtual Memory?
• What is swap and what is it used for?
• What is an A record, an NS record, a PTR record, a CNAME record, an MX record?
• Are there any other RRs and what are they used for?
• What is a Split-Horizon DNS?
• What is the sticky bit?
• What does the immutable bit do to a file?
• What is the difference between hardlinks and symlinks? What happens when you remove the source to a symlink/hardlink?
• What is an inode and what fields are stored in an inode?
• How to force/trigger a file system check on next reboot?
• What is SNMP and what is it used for?
• What is a runlevel and how to get the current runlevel?
• What is SSH port forwarding?
• What is the difference between local and remote port forwarding?
• What are the steps to add a user to a system without using useradd/adduser?
• What is MAJOR and MINOR numbers of special files?
• Describe the mknod command and when you'd use it.
• Describe a scenario when you get a "filesystem is full" error, but 'df' shows there is free space.
• Describe a scenario when deleting a file, but 'df' not showing the space being freed.
• Describe how 'ps' works.
• What happens to a child process that dies and has no parent process to wait for it and what’s bad about this?
• Explain briefly each one of the process states.
• How to know which process listens on a specific port?
• What is a zombie process and what could be the cause of it?
• You run a bash script and you want to see its output on your terminal and save it to a file at the same time. How could you do it?
• Explain what echo "1" > /proc/sys/net/ipv4/ip_forward does.
• Describe briefly the steps you need to take in order to create and install a valid certificate for the site https://foo.example.com.
• Can you have several HTTPS virtual hosts sharing the same IP?
• What is a wildcard certificate?
• Which Linux file types do you know?
• What is the difference between a process and a thread? And parent and child processes after a fork system call?
• What is the difference between exec and fork?
• What is "nohup" used for?
• What is the difference between these two commands?
• myvar=hello
• export myvar=hello
• How many NTP servers would you configure in your local ntp.conf?
• What does the column 'reach' mean in ntpq -p output?
• You need to upgrade kernel at 100-1000 servers, how you would do this?
• How can you get Host, Channel, ID, LUN of SCSI disk?
• How can you limit process memory usage?
• What is bash quick substitution/caret replace(^x^y)?
• Do you know of any alternative shells? If so, have you used any?
• What is a tarpipe (or, how would you go about copying everything, including hardlinks and special files, from one server to another)?
• How can you tell if the httpd package was already installed?
• How can you list the contents of a package?
• How can you determine which package is better: openssh-server-5.3p1-118.1.el6_8.x86_64 or openssh-server-6.6p1-1.el6.x86_64 ?
• Can you explain to me the difference between block based, and object based storage?

Hard Linux Questions:

• What is a tunnel and how you can bypass a http proxy?
• What is the difference between IDS and IPS?
• What shortcuts do you use on a regular basis?
• What is the Linux Standard Base?
• What is an atomic operation?
• Your freshly configured http server is not running after a restart, what can you do?
• What kind of keys are in ~/.ssh/authorized_keys and what it is this file used for?
• I've added my public ssh key into authorized_keys but I'm still getting a password prompt, what can be wrong?
• Did you ever create RPM's, DEB's or solaris pkg's?
• What does :(){ :|:& };: do on your system?
• How do you catch a Linux signal on a script?
• Can you catch a SIGKILL?
• What's happening when the Linux kernel is starting the OOM killer and how does it choose which process to kill first?
• Describe the linux boot process with as much detail as possible, starting from when the system is powered on and ending when you get a prompt.
• What's a chroot jail?
• When trying to umount a directory it says it's busy, how to find out which PID holds the directory?
• What's LD_PRELOAD and when it's used?
• You ran a binary and nothing happened. How would you debug this?
• What are cgroups? Can you specify a scenario where you could use them?
• How can you remove/delete a file with file-name consisting of only non-printable/non-type-able characters?
• How can you increase or decrease the priority of a process in Linux?

Expert Linux Questions:

• A running process gets EAGAIN: Resource temporarily unavailable on reading a socket. How can you close this bad socket/file descriptor without killing the process?
• What do you control with swapiness?
• How do you change TCP stack buffers? How do you calculate it?
• What is Huge Tables? Why isn't it enabled by default? Why and when use it?
• What is LUKS? How to use it?

Networking Questions:

• What is localhost and why would ping localhost fail?
• What is the similarity between "ping" & "traceroute" ? How is traceroute able to find the hops.
• What is the command used to show all open ports and/or socket connections on a machine?
• Is 300.168.0.123 a valid IPv4 address?
• Which IP ranges/subnets are "private" or "non-routable" (RFC 1918)?
• What is a VLAN?
• What is ARP and what is it used for?
• What is the difference between TCP and UDP?
• What is the purpose of a default gateway?
• What is command used to show the routing table on a Linux box?
• A TCP connection on a network can be uniquely defined by 4 things. What are those things?
• When a client running a web browser connects to a web server, what is the source port and what is the destination port of the connection?
• How do you add an IPv6 address to a specific interface?
• You have added an IPv4 and IPv6 address to interface eth0. A ping to the v4 address is working but a ping to the v6 address gives you the response sendmsg: operation not permitted. What could be wrong?
• What is SNAT and when should it be used?
• Explain how could you ssh login into a Linux system that DROPs all new incoming packets using a SSH tunnel.
• How do you stop a DDoS attack?
• How can you see content of an ip packet?
• What is IPoAC (RFC 1149)?
• What will happen when you bind port 0?

[⬆] MySQL questions:

• How do you create a user?
• How do you provide privileges to a user?
• What is the difference between a "left" and a "right" join?
• Explain briefly the differences between InnoDB and MyISAM.
• Describe briefly the steps you need to follow in order to create a simple master/slave cluster.
• Why should you run "mysql_secure_installation" after installing MySQL?
• How do you check which jobs are running?
• How would you take a backup of a MySQL database?

DevOps Questions:

• Can you describe your workflow when you create a script?
• What is GIT?
• What is a dynamically/statically linked file?
• What does "./configure && make && make install" do?
• What is puppet/chef/ansible used for?
• What is Nagios/Zenoss/NewRelic used for?
• What is Jenkins/TeamCity/GoCI used for?
• What is the difference between Containers and VMs?
• How do you create a new postgres user?
• What is a virtual IP address? What is a cluster?
• How do you print all strings of printable characters present in a file?
• How do you find shared library dependencies?
• What is Automake and Autoconf?
• ./configure shows an error that libfoobar is missing on your system, how could you fix this, what could be wrong?
• What are the advantages/disadvantages of script vs compiled program?
• What's the relationship between continuous delivery and DevOps?
• What are the important aspects of a system of continuous integration and deployment?
• How would you enable network file sharing within AWS that would allow EC2 instances in multiple availability zones to share data?

Fun Questions:

• A careless sysadmin executes the following command: chmod 444 /bin/chmod - what do you do to fix this?
• I've lost my root password, what can I do?
• I've rebooted a remote server but after 10 minutes I'm still not able to ssh into it, what can be wrong?
• If you were stuck on a desert island with only 5 command-line utilities, which would you choose?
• You come across a random computer and it appears to be a command console for the universe. What is the first thing you type?
• Tell me about a creative way that you've used SSH?
• You have deleted by error a running script, what could you do to restore it?
• What will happen on 19 January 2038?
• How to reboot server when reboot command is not responding?

Demo Time:

• Unpack test.tar.gz without man pages or google.
• Remove all "*.pyc" files from testdir recursively?
• Search for "my konfu is the best" in all *.py files.
• Replace the occurrence of "my konfu is the best" with "I'm a linux jedi master" in all *.txt files.
• Test if port 443 on a machine with IP address X.X.X.X is reachable.
• Get http://myinternal.webserver.local/test.html via telnet.
• How to send an email without a mail client, just on the command line?
• Write a get_prim method in python/perl/bash/pseudo.
• Find all files which have been accessed within the last 30 days.
• Explain the following command (date ; ps -ef | awk '{print $1}' | sort | uniq | wc -l ) >> Activity.log
• Write a script to list all the differences between two directories.
• In a log file with contents as <TIME> : [MESSAGE] : [ERROR_NO] - Human readable text display summary/count of specific error numbers that occurred every hour or a specific hour.