Linux System Administrator/DevOp Interview Questions

A collection of linux sysadmin/devop interview questions. Feel free to contribute via pull requests, issues or email messages.

Table of Contents

1. Contributors
2. General Questions
3. Simple Linux Questions
4. Medium Linux Questions
5. Hard Linux Questions
6. Expert Linux Questions
7. Networking Questions
8. DevOp Questions
9. Fun Questions
10. Demo Time
11. Other Great References

####[⬆] Contributors:

• moregeek
• typhonius
• martin
• negesti
• peter
• andreashappe
• quatrix
• biyanisuraj

The majority of the questions were collected from:

• https://github.com/gurmeet1109/docgurmeet/tree/master/InterviewQuestionsSamples
• https://github.com/kylejohnson/linux-sysadmin-interview-questions/blob/master/test.md

####[⬆] 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?
• What function does DNS play on a network? Mappings of addresses to names and vice versa (known as records) are stored in a database
• What is HTTP? HyperText Transfer Protocol (HTTP) is the underlying protocol used by the World Wide Web to define how messages are formatted and transmitted.
• What is an HTTP proxy and how does it work? A HTTP proxy speaks the HTTP protocol, it's especially made for HTTP connections but can be abused for other protocols as well
• What is SMTP? Give the basic scenario of how a mail message is delivered via SMTP! Simple Mail Transfer Protocol (SMTP) is an Internet standard for electronic mail (e-mail) transmission.
• What is RAID? What is RAID0, RAID1, RAID5, RAID10? RAID(redundant array of independent disks). RAID0: Blocks striped, no mirror. no parity. RAID1: Blocks mirrored, no stripe. no parity. RAID5: min 3 disks. blocks striped, distributed parity. RAID10: min 4 disks, blocks mirrored.
• What is a level 0 backup? What is an incremental backup? level 0: full backup. Incremental: identifying, recording and, thus, preserving only those files that have changed since the last backup.
• Describe the general file system hierarchy of a Linux system. central concepts are superblock, inode, data block, directory block, and indirection block.Linux chooses to have a single hierarchical directory structure. Everything starts from the root directory, represented by /, and then expands into sub-directories instead of having so-called 'drives'. /bin Essential command binaries /boot Static files of the boot loader /dev Device files /etc Host-specific system configuration /lib Essential shared libraries and kernel modules /media Mount point for removeable media /mnt Mount point for mounting a filesystem temporarily /opt Add-on application software packages /sbin Essential system binaries /srv Data for services provided by this system /tmp Temporary files /usr Secondary hierarchy /var Variable data

####[⬆] Simple Linux Questions:

• What is the name and the UID of the administrator user? Root, UID: 0.

• How to list all files, including hidden one, in a directory? ls -a

• What is the Unix/Linux command to remove a directory and its contents? rm -rf directoryname

• Which command will show you free/used memory? Does free memory exist on Linux? free

• How to search for the string "my konfi is the best" in files of a directory recursively? cd /path/to/dir find . -type f -exec grep -l "word" {} +

• How to connect to a remote server or what is SSH?

• How to get all environment variables and how can you use them? printenv

• I get "command not found" for ifconfig -a. What can be wrong? as root?

• What happens if I type TAB-TAB? display all methods

• What command will show the available disk space on the Unix/Linux system? df -h #human readable

• What command is used to lookup DNS records? dig

• What Unix/Linux commands will alter a files ownership, files permissions? chown, chmod

• What does chmod +x FILENAMEdo? exe

• What does the permission 0750 on a file mean? Owner may read, write and execute. The Group may read and execute. (but not write) The world may not do anything with this file. 0750 = User:rwx Group:r-x World:---

• What does the permission 0750 on a directory mean? 0755 (drwxr-xr-x)

• How to add a new system user without login permissions?

• How to add/remove a group from a user? deluser 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 typed at the command-line will automatically be replaced by a ls -l.

• How do you set the mail address of the root/a user? /etc/alias

• 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 Load average is a gauge of how many processes are on average, concurrently demanding CPU attention. the load average of the system over the last 1, 5, and 15 minutes.

####[⬆] Medium Linux Questions:

• What do the following commands do?
• tee Tee command is used to store and view (both at the same time) the output of any other command.
• awk Awk is both a programming language and text processor that can be used to manipulate text data in very useful ways.
• tr tr abbreviated as translate or transliterate. It takes as parameters two sets of characters, and replaces occurrences of the characters in the first set with the corresponding elements from the other set i.e. it is used to translate characters.
• cut cut is used for text processing
• tac Concatenate and print files in reverse.
• curl transfer a URL
• wget wget stands for "web get". It is a command-line utility which downloads files over a network.
• watch execute a program periodically, showing output fullscreen
• tail
• What does a & after a command do? makes the command run in the background.
• What does & disown after a command do?
• What is a packet filter and how does it work?
• 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?

An A record matches up a domain (or subdomain) to an IP address. NS records or name server records set the nameservers for a domain (or subdomain). A PTR record or pointer record matches up an IP address to a domain (or subdomain), allowing reverse DNS queries to function. An MX record or mail exchange record sets the mail delivery destination for a domain (or subdomain). A CNAME record or Canonical Name record matches up a domain (or subdomain) to a different domain.

• Are there any other RRs and what are they used for?

• What is the sticky bit?

• 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?

• Howto 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? ID Name Description 0 Halt Shuts down the system. 1 Single-user Mode Mode for administrative tasks.[2][2] 2 Multi-user Mode Does not configure network interfaces and does not export networks services.[3] 3 Multi-user Mode with Networking Starts the system normally.[4] 4 Not used/User-definable For special purposes. 5 Start the system normally with appropriate display manager. ( with GUI ) Same as runlevel 3 + display manager. 6 Reboot Reboots the system.

• What is SSH port forwarding?

• What is the difference between local and remote port forwarding?

• What steps to add a user to a system without using useradd/adduser? Edit /etc/passwd with vipw and add a new line for the new account. Be careful with the syntax. Do not edit directly with an editor. vipw locks the file, so that other commands won't try to update it at the same time. You should make the password field be `*', so that it is impossible to log in. Similarly, edit /etc/group with vigr, if you need to create a new group as well. Create the home directory of the user with mkdir. Copy the files from /etc/skel to the new home directory. Fix ownerships and permissions with chown and chmod

• What is MAJOR and MINOR numbers of special files? major number identifies the driver associated with the device. minor number is used by the kernel to determine exactly which device is being referred to.

• 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?

• How to know which process listens on a specific port? netstat -tulpn

####[⬆] Hard Linux Questions:

• What is the difference between processes and threads?

• What is a tunnel and how you can bypass a http proxy?

• What is the difference between IDS and IPS? Firewall - A device or application that analyzes packet headers and enforces policy based on protocol type, source address, destination address, source port, and/or destination port. Packets that do not match policy are rejected. Intrusion Detection System - A device or application that analyzes whole packets, both header and payload, looking for known events. When a known event is detected a log message is generated detailing the event. Intrusion Prevention System - A device or application that analyzes whole packets, both header and payload, looking for known events. When a known event is detected the packet is rejected.

• What shortcuts do you use on a regular basis?

• What is the Linux Standard Base? The Linux Standard Base (LSB) is a joint project by several Linux distributions under the organizational structure of the Linux Foundation to standardize the software system structure, including the filesystem hierarchy used in the GNU/Linux operating system.

• What is an atomic operation? execute atomically without interruption

• 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 and why you would care about that?

• How trace system call and signal? strace

• What's happening when the Linux kernel is starting the OOM killer, how does it choose which process to kill first. ideal candidate for liquidation is a recently started, non privileged process which together with its children uses lots of memory, has been nice'd, and does no raw I/O

• 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. BIOS, MBR, BOOTLOADER, KERNEL, INIT, RUNLEVEL

• What's a chroot jail? A chroot on Unix operating systems is an operation that changes the apparent root directory for the current running process and its children.

• When trying to umount a directory it says it's busy, how to find out which PID holds the directory? lsof

• What's LD_PRELOAD and when it's used? A list of additional, user-specified, ELF shared libraries to be loaded before all others. The items of the list can be separated by spaces or colons. This can be used to selectively override functions in other shared libraries.

• You run a binary and nothing happens, how do you debug what's doing?

####[⬆] Expert Linux Questions:

• A running process gets EAGAIN: Resource temporarily unavailable on reading a socket. How you can 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 command is 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 command is used to show the route table for a machine?
• 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?

####[⬆] DevOp 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"?
• What is puppet/chef/ansible used for?
• How do you create a new mysql user?
• How do you create a new postgres user?
• What is a virtual IP address? What is a cluster?
• How print the strings of printable characters in files?
• How look 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?
• Advantages/disadvantages of script vs compiled program.
• What is the difference between fork and thread? And parent and child process in fork system call?
• 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?
• 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?

####[⬆] 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.
• ⁉️ more on files ... cut, tr, awk ...
• 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.
• Write a program in any language you choose, to reverse a file.
• 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

####[⬆] Other Great References:

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

• https://github.com/darcyclarke/Front-end-Developer-Interview-Questions
• https://github.com/kylejohnson/linux-sysadmin-interview-questions/blob/master/test.md
• https://github.com/gurmeet1109/docgurmeet/tree/master/InterviewQuestionsSamples
• http://slideshare.net/kavyasri790693/linux-admin-interview-questions