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.

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? Answer:0
How to list all files, including hidden one, in a directory? Answer:ls -la
What is the Unix/Linux command to remove a directory and its contents? Answer:rm -rf <directory_name>
Which command will show you free/used memory? Does free memory exist on Linux? Answer:htop or top or vmstat
How to search for the string "my konfi is the best" in files of a directory recursively? Answer:grep -r "my konfi is the best" <directory_name>
How to connect to a remote server or what is SSH? Answer:ssh <username>@<host_ip> more
How to get all environment variables and how can you use them? Answer:env using them echo $PATH
I get "command not found" when I run ifconfig -a. What can be wrong? Answer:/sbin is not part of PATH environment variable, do export PATH=$PATH:/sbin
What happens if I type TAB-TAB? Answer:Would show Display all 1234 possiblilities (y/n)
What command will show the available disk space on the Unix/Linux system? Answer:df -h
What commands do you know that can be used to check DNS records?Answer: nslookup -type=A google.com
What Unix/Linux commands will alter a files ownership, files permissions? Answer:chmod
What does chmod +x FILENAMEdo? Answer: sets execute permission on the file.
What does the permission 0750 on a file mean? Answer
What does the permission 0750 on a directory mean? Answer
How to add a new system user without login permissions? Answer
How to add/remove a group from a user? Answer
What is a bash alias? Answer: alias ll="ls -l" this will execute ls -l on the prompt when we execute ll
How do you set the mail address of the root/a user? Answer
What does CTRL-c do? Answer: sends SIGINT and CTRL-z sends SIGTSTP more
What is in /etc/services? Answer: Server to Port Translation. more
How to redirect STDOUT and STDERR in bash? Answer: (> /dev/null 2>&1)
What is the difference between UNIX and Linux. Answer: UNIX commercial proprietary OS, Linux is Free and Opensourve more
What is the difference between Telnet and SSH? Answer: Both network Communication protocol, telnet unsecure (plain-text), ssh is secure shell. more
Explain the three load averages and what do they indicate. Answer

####[⬆] Medium Linux Questions:

What do the following commands do?
tee
awk
tr
cut
tac
curl
wget
watch
tail
What does a & after a command do? Runs the command in background.
What does & disown after a command do? Answer
What is a packet filter and how does it work? Answer
What is Virtual Memory? Answer
What is swap and what is it used for? Answer
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? Answer
What is the sticky bit? Answer
What does the immutable bit to a file? A file with an immutable attribute can not be -> Modified, Deleted, Renamed, No soft or hard link created by anyone including root user :) more.
What is the difference between hardlinks and symlinks? What happens when you remove the source to a symlink/hardlink? Answer
What is an inode and what fields are stored in an inode? Answer Answer
How to force/trigger a file system check on next reboot? Answer: All you need to do is just create empty file called /forcefsck. more
What is SNMP and what is it used for? Answer: Simple Network Management Protocol. more
What is a runlevel and how to get the current runlevel? Answer
What is SSH port forwarding? Answer
What is the difference between local and remote port forwarding? Answer
What are the steps to add a user to a system without using useradd/adduser? Answer
What is MAJOR and MINOR numbers of special files? <<<<<<< HEAD
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 a scenario when you get a "filesystem is full" error, but 'df' shows there is free space? Answer: inode Table is full, no space to create new file inode, but there is Disk Space Available. more more
Describe a scenario when deleting a file, but 'df' not showing the space being freed? Answer: While deleting a file, if another process is holding the file then the file will not be deleted, we need to kill the process or reboot the system.
Describe how 'ps' works. Answer: man ps
What happens to a child process that dies and has no parent process to wait for it and what’s bad about this? Answer: Creates a orphan child with init as its parent.
Explain briefly each one of the process states. Answer
How to know which process listens on a specific port? Answer: sudo netstat -plant
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? Answer: using tee. Ex: sh bash_script.sh | tee logger
Explain what echo "1" > /proc/sys/net/ipv4/ip_forward does. Answer: Enables IP Forwarding, can also be done in sysctl.conf using the parameter net.ipv4.ip_forward = 1. more

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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 Answer: assigns value hello to myvars, accessible only in the current script.
export myvar=hello Answer: sets environment variable, which can be accessed by the current session.
How many NTP servers would you configure in your local ntp.conf? Answer: Atleast 3.
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?

####[⬆] 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?

####[⬆] 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?

####[⬆] 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 yout the response sendmsg: operation not permitted. What could be wrong?
What is SNAT and when should be used?
Explain how could you ssh login into a Linux system that DROPs all new incomming packets using a SSH tunnel.
How do you stop a DDoS?
How can you see content of ip packet?

####[⬆] 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?
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 continous integration and deployment?

####[⬆] Fun Questions:

A careless sysadmin executes the following command: chmod 444 /bin/chmod - what do you do to fix this? busybox chmod +x /bin/chmod
I've lost my root password, what can I do? Answer: Login as single user mode and recover the password.
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? pwd
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?

####[⬆] Demo Time:

Unpack test.tar.gz without man pages or google. tar -xvzf test.tar.gz
Remove all "*.pyc" files from testdir recursively? find . -name "*.pyc" -exec rm -rf {} +
Search for "my konfu is the best" in all *.py files. grep --color -r "my konfu is the best" *.py
Replace the occurrence of "my konfu is the best" with "I'm a linux jedi master" in all *.txt files. find . -name "*.py" -exec sed -i 's/my\ konfu\ is\ the\ best/I'm\ a\ linux\ jedi\ master/g' {} +
Test if port 443 on a machine with IP address X.X.X.X is reachable. telnet X.X.X.X 443
Get http://myinternal.webserver.local/test.html via telnet.

step 1: telnet myinternal.webserver.local 80 step 2: GET /test.html HTTP/1.1
How to send an email without a mail client, just on the command line? echo "This is my Mail" | mailx -v -A gmail_account_in_mailrc -s "This is my Subject" [email protected]
Write a get_prim method in python/perl/bash/pseudo. SEE END OF README
Find all files which have been accessed within the last 30 days. find <src_directory> -mmin 43200 (43200 minutes).
Explain the following command (date ; ps -ef | awk ‘{print $1}’ | sort | uniq | wc -l ) >> Activity.log - Number of different users activity (processes) in the server.
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 occured every hour or a specific hour. for item in ERRORCODE1 ERRORCODE2 ERRORCODE3 ERRORCODE4; do echo ERROR CODE $item : `find <src_dir> -cmin -60 -exec grep $item {} + | wc -l`; done

####[⬆] Other Great References:

Some questions are 'borrowed' from other great references like:

`get_prime` function in python.

def get_prime(num):
    print num

    if num <= 1:
        print "Not a Prime Number"
    else:
        for i in range(2, num):
            if num%i == 0:
                print num, "is Not Prime"
                print i, "x", num//i, "=", num
                break
        else:
            print "Prime"

get_prime(int(input("Enter a Number:")))

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