linuxqa

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

[⬆] 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 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?
    00 - root
  • How to list all files, including hidden ones, in a directory?
    ls -al
  • What is the Unix/Linux command to remove a directory and its contents?
    rm -r -f
  • Which command will show you free/used memory? Does free memory exist on Linux?
    df -h  
    du 
  • How to search for the string "my konfu is the best" in files of a directory recursively?
    grep -r "my kk"
  • How to connect to a remote server or what is SSH?
    ssh remote_username@remote_host
  • How to get all environment variables and how can you use them?
    printenv
  • I get "command not found" when I run ifconfig -a. What can be wrong?
    apt install net-tools
  • What happens if I type TAB-TAB?
    . /etc/bash_completion
  • What command will show the available disk space on the Unix/Linux system?
    df -k df -h
  • What commands do you know that can be used to check DNS records?
    nslookup     dig
  • What Unix/Linux commands will alter a files ownership, files permissions?
    chmod
  • What does chmod +x FILENAME do?
    make executable
  • What does the permission 0750 on a file mean?
    https://chmodcommand.com/chmod-750/
  • What does the permission 0750 on a directory mean?
    https://chmodcommand.com/chmod-750/
  • How to add a new system user without login permissions?

    You can use the -M switch (make sure it's a capital) to ensure no home directory will be created:

    useradd -M subversion
    then lock the account to prevent logging in:

    usermod -L subversion

  • How to add/remove a group from a user?

    When adding a user to a group, it can be done with:

    usermod -a -G group user   

    gpasswd -d user group

  • What is a bash alias?

    A Bash alias is essentially nothing more than a keyboard shortcut, an abbreviation, a means of avoiding typing a long command sequence.
    If, for example, we include alias lm="ls -l | more" in the ~/.bashrc file, then each lm [1] typed at the command-line will automatically be replaced by a ls -l | more.
    This can save a great deal of typing at the command-line and avoid having to remember complex combinations of commands and options.
    Setting alias rm="rm -i" (interactive mode delete) may save a good deal of grief, since it can prevent inadvertently deleting important files.

    In a script, aliases have very limited usefulness. It would be nice if aliases could assume some of the functionality of the C preprocessor,
    such as macro expansion, but unfortunately Bash does not expand arguments within the alias body.
    [2] Moreover, a script fails to expand an alias itself within "compound constructs," such as if/then statements, loops, and functions.
    An added limitation is that an alias will not expand recursively.
    Almost invariably, whatever we would like an alias to do could be accomplished much more effectively with a function.

  • How do you set the mail address of the root/a user?
    Simply create /root/.forward and place your email address in this file. It will be forwarded to your external mail address.
  • What does CTRL-c do?
    Ctrl + C is used to kill a process with signal SIGINT , by other words it is a polite kill . Ctrl + Z is used to suspend a process by sending it the signal SIGSTOP ,
    which is like a sleep signal, that can be undone and the process can be resumed again
  • What is in /etc/services?
    UNIX operating systems store what's called aservices file at /etc/services. It stores information about numerous services that client applications might use on the computer. Within the file is theservice name, port number and protocol it uses, and any applicable aliases.
  • How to redirect STDOUT and STDERR in bash? (> /dev/null 2>&1)

    There are two main output streams in Linux (and other OSs), standard output (stdout) and standard error (stderr). Error messages, like the ones you show, are printed to standard error. The classic redirection operator (command > file) only redirects standard output, so standard error is still shown on the terminal. To redirect stderr as well, you have a few choices:

    1. Redirect stdout to one file and stderr to another file:

      command > out 2>error
      
    2. Redirect stderr to stdout (&1), and then redirect stdout to a file:

      command >out 2>&1
      
    3. Redirect both to a file:

      command &> out
      

    For more information on the various control and redirection operators, see here.

  • What is the difference between UNIX and Linux?
    The primary difference is that Linux and Unix are two different Operating Systems though they both have some common commands
  • What is the difference between Telnet and SSH?
    Summary:
    1. SSH and Telnet commonly serves the same purpose
    2. SSH is more secure compared to Telnet
    3. SSH encrypts the data while Telnet sends data in plain text
    4. SSH uses a public key for authentication while Telnet does not use any authentication
    5. SSH adds a bit more overhead to the bandwidth compared to Telnet
    6. Telnet has been all but replaced by SSH in almost all uses

    Read more: Difference Between Telnet and SSH | Difference Between http://www.differencebetween.net/technology/internet/difference-between-telnet-and-ssh/#ixzz5QgLF7JEG
  • Explain the three load averages and what do they indicate. What command can be used to view the load averages?

    So, Bridge Operator, what numbering system are you going to use? How about:

    • 0.00 means there's no traffic on the bridge at all. In fact, between 0.00 and 1.00 means there's no backup, and an arriving car will just go right on.
    • 1.00 means the bridge is exactly at capacity. All is still good, but if traffic gets a little heavier, things are going to slow down.
    • over 1.00 means there's backup. How much? Well, 2.00 means that there are two lanes worth of cars total -- one lane's worth on the bridge, and one lane's worth waiting. 3.00 means there are three lane's worth total -- one lane's worth on the bridge, and two lanes' worth waiting. Etc.

     = load of 1.00

     = load of 0.50

     = load of 1.70

  • Can you name a lower-case letter that is not a valid option for GNU ls?
  • What is a Linux kernel module?
    Loadable kernel modules in Linux are loaded (and unloaded) by the modprobe command. They are located in /lib/modules and have had the extension .ko
    ("kernel object") since version 2.6 (previous versions used the .o extension).[3] The lsmod command lists the loaded kernel modules.
    In emergency cases, when the system fails to boot due to e.g. broken modules, specific modules can be enabled or disabled by modifying the kernel boot parameters list
    (for example, if using GRUB, by pressing 'e' in the GRUB start menu, then editing the kernel parameter line).
  • Walk me through the steps in booting into single user mode to troubleshoot a problem.
    1. Hold down the left Shift key while rebooting to bring up GRUB menu
    2. Select (highlight) the GRUB boot menu entry you wish to use.
    3. Press e to edit the GRUB boot commands for the selected boot menu entry.
    4. Look near the bottom of the list of commands for lines similar to

      linux /boot/vmlinuz-3.2.0-24-generic root=UUID=bc6f8146-1523-46a6-8b\
      6a-64b819ccf2b7 ro  quiet splash
      initrd /boot/initrd.img-3.2.0-24-generic
      
    5. Change the middle line in (4) by adding the kernel boot parameter single to the end of the line (i.e. after ro quiet splash).

      For this example you would change:

      6a-64b819ccf2b7 ro  quiet splash
      

      to

      6a-64b819ccf2b7 ro  quiet splash single
      
    6. Press either Ctrl+X or F10 to boot using these kernel options.

  • Walk me through the steps you'd take to troubleshoot a 404 error on a web application you administer.

[⬆] 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 ?
  • 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?
  • 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 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.

[⬆] Other Great References:

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