Linux System Administrator/DevOps Interview Questions
A collection of linux sysadmin/devops interview questions. Feel free to contribute via pull requests, issues or email messages.
Table of Contents
- Contributors
- General Questions
- Simple Linux Questions
- Medium Linux Questions
- Hard Linux Questions
- Expert Linux Questions
- Networking Questions
- MySQL Questions
- DevOps Questions
- Fun Questions
- Demo Time
- Other Great References
####[⬆] Contributors:
- 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 we must 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.
####[⬆] 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 konfi 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 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
?
####[⬆] 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
- 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 ?
-
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?
-
What are run-levels 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 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 yout 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)?
####[⬆] 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?
####[⬆] 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?
####[⬆] 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.
####[⬆] Other Great References:
Some questions are 'borrowed' from other great references like: