Top 10 Linux Job Interview Questions

Hey, what's going on, guys? In this video,we're going to cover the top ten Linuxinterview questions. These are the mostcommon Linux system administration jobinterview questions that you'll find outin the wild. So these are what you'll getin your first phone screen. Generally,they'll obviously be maybe more specificinterviews after this. It also depends onthe position you're actuallyinterviewing for. Any given positionmight involve lots of other tech whilethis covers the majority of thetypes of things that are asked duringjunior Linux admin interviews. You may

also want to know similartypes of questions for databases, TCP/IP,and networking programming or a specificprogramming language. There may also beinfrastructure or architecture questionsif you're doing a more mid-level orsenior type position. Likewise, you may beasked about specific applications orservices. You can get mad, you can get sad,you can have lots of opinions about this;whatever, this is just the reality fortech phone screens. Your interviewer onlyhas a couple of minutes to figure out ifyou're worth the time investment.

The interview you're preparing for, if itinvolves Linux in some capacity, shouldmake you really comfortable with these basicthings. If you haven’t yet, make sure tocheck out my basic Linux systemadministration playlist, because itcovers a lot of the theory and alot of how Linux works behind what we’regoing to talk about. As a last piece ofcaution or word of advice, do not justmemorize these things, because even ifyou make it through the first phonescreen, they’re going to be pretty pissedif it turns out that you actually don’tunderstand what you’re doing after youmake it through by memorizing answers tothese basic questions. Use thesequestions as a jumping-off point forfurther exploration. Okay?So let’s jump right in. The firstquestion you should immediately know theanswer to is, "How would you check thekernel version of the currently runningsystem that you’re on?" The answer is younameyou name prints out system information.

Specifically, the -a flag younamed -a will give you everything it’sgot. Soyou’ve got your hostname, OS, kernelTo release the kernel version, architecture, etc.,there are a couple of small permutations ofthis that you can use. So, kernel versionand kernel release, if you're looking forjust those specific chunks ofinformation, but that’s unique. So, ifsomeone asks you how to check thekernel version of the system, or evenwhat operating system is running, this isa fairly quick way to find out. Anotherfavorite question is how would you goabout checking what your current IPaddress is, and this may come in a coupleof permutations. Like, what's the currentaddress on the easy road of a command,or what’s your current IPv6 address, orwhat have you? There are a couple of ways to dothis socially, sort of the old and thenew way. The old way is using ifconfig for interfaceconfiguration. I've got a bunch of littledevices here; don’t worry about that. Thisis the loopback device, and this is ourEthernet device. You can see our IPv4address here along with the broadcast andsubnet mask. Our IPv6 address informationcan be found here. Now, if someone asks youfor a specific interface, in general, youshould be using the newer IP tools.

The command is IP a, then you want the addressshow, which gives you everything.

Your devices can be given anotherargument: the device name. To use IP address orshow 0, as you can see, that gives you allthe info for Ethernet 0, including IPv4and CIDR notation. How would you checkfor free disk space? Other versions ofthis question are: how would youcheck how much disk space is left, orwhat percentage of disk space you'reusing? Don't get tripped up by differentways of asking these. DF is your friend.

DF shows all file systems in a human-readableformat, so using "DF -h" will give you a readout ofall of your file systems, includingvirtual file systems. You can see ourhard drive partitions are here, and ourroot partition is mounted on SDA1.You can see it's 5.8 GBof capacity; we've used 5 GB of that,so we've got about half a GB left.We're using 91%. So, remember that managingdisk space is a completelynormal part of being a Linux system adminor working with Linux in general.

Don't be surprised if someone asks youhow would you see if a service isrunning, or how would you start or stopa service, or how would you reload aservice on any newer system.

Linux distributions have switched tosystemd. However, on an older system, youwould have said either "service" or "I'll use"at your dev just as the service name. Thismight be nginx, MySQL, or what haveyou.Service name goes here. Service you'dhave status checks, status of a service.Start, stop, etc. are also arguments youcan put at the end. So, "service service"name and then what you want to do to iton a newer systemd system. So, somethingthat is not running this, "five minute," orupstart or another init system, you woulduse systemctl. This is the new version, "quote unquote."If that's allyou remember, "systemctl status service."Now, on an old system, I'm not usingsystemd, so this is a command not found.

But the important thing is it's"systemctl status service name." You'llnotice that this is actually reversed;the order of the service or unit nameargument. Systemd calls this a unitinstead of a service, but that's prettymuch all you need to know. It takes thesame start/stop and able to save. It willreload, etc. You can see I've got somefiles and directories here. HowEverything insideof a given directory, I'm going to say, dosh code, will check the code directory.

Disk usage, this is going to go andtally that up, so you can see I've got ateam's worth of stuff in my codedirectory.

Nice and easy, a little bit morenetworking focused.How would you check for open ports? Let'ssay this question can be asked in acouple of different ways.Check listening network sockets, openports. I could say TCP services that arelistening on TCP or UDP ports or sockets.So, this question can come in a coupleof different versions, but if you hear,you know network socket listening ports, TCPand UDP services, start thinking netstat.

Because that is what shows youthat stuff. So, by default, it gives youbasically a uselessly big output; it'smeant to be filtered down withother Linux and UNIX tools. So, you'llprobably want to run something likenetstat. I like to use the -tuln -pLPand listening token. Just remember that,and it's pretty good shorthand for most times when you need this.

The command "man netstat" will get you the manual page,which I will elaborate on in a second.This command shows you the address and portthat different services are listening on.I'm actually going to run this commandas root because you will see in a second thaton the right, this PID or program name isactually blank for everything becausewe are not root and only root can see that.So please say "su" to execute asroot; you can see we now have all thatextra information. Okay, so from left toright, what type of service is it? Well,it’s TCP, IPv4, TCP, IPv6, UDP, UDP, AVP, IPv6. Excuse me, I get a little excited here.

The local address that it's listening on,the "0.0.0.0" meansall public addresses. Port 80,foreign addresses, is it listening, and theprocess ID and name of the process. So itactually sort of looks up the processname from the PID that owns this socket.You can see that nginx, our web server, islistening on port 80, and anyone on ourlocal network that can reach ourmachine can check out what we’re hostingon our web server. "127.0.0.1" is our localmachine, so this is actually notaccessible from outside of our machine.

Again, for more on that, just learn alittle bit more about networking.This netstat -tulpn is extremelyuseful, and again, remember to run it asroot. It just gives you thatextra insight into who is actually listening onthat port. It’s very interesting to know.How would you check the CPU usage of a givenprocess? You should run PS ax, then grep foryour process name. This is a pipe. Ifyou’re not familiar with pipes, I highlysuggest that you watch some of my basicshell videos. It’s like a building blockof Linux, and you should not be asysadmin if that doesn’t make sense toyou. So, run PS ox, pipe it into grep, and lookfor nginx. You can see these are all theprocesses right now in the process table thathave nginx in their name, and you can getsome process information this way, such as theuser it’s running as. As you can see, themaster process is running as root, while theworker processes are running as www-data.

You kind of need to knowhow to parse this. Read up on PS. Anotherway of doing this is using top. You’ll find topjust about everywhere. This is updatedby default every second or two.This gives you a lot more information. By default,It's ordered in descending order by CPUtime percentage of CPU that it's using,so this will be updated with your mostCPU intensive processes. Although you canchange what's displayed.

Likewise, it also gives you abunch of other stats about memory,swapping, use, load averages, etc. Nicealternative to this, although you won'tfind it installed by default, is Htop.

That's interesting to you. I like this;you can search it easily with theslash key and search for some process, whichgives you a nice little graphicalrepresentation of the cores it sees,memory, swap, etc. Nice and intuitive. Sothat's how you check what'sgoing on with processes: what users own aprocess, what a process is runningas, and things like CPU memory usage, etc.

How would you mount a newvolume, let's say a new hard drivepartition that you've just plugged in,or a USB stick? How would youmount that in your system?Well, Linux will generally have adirectory called "mount" in root, and thatis sort of the canonical place tomount things. So to mount a new volume,you would say "mount" is the command, andThen the absolute path to the volume, soif it's a device, like let's see, it's ahard drive partition SDA — it's thesecond partition on the SDA hard drive.

You could say "mount address," and then themount point. I just wanted to mount it onmount MNT. This would be the command.Likewise, checking for existing mounts isjust the mount command without anyarguments, so you can see, here's what'smounted and the mount options it's got.You also get the filesystem type, so X4got a ButterFS here. On the last littletrick on mount, just because it's fast.

If you automatically need to mount avolume at boot, what file would you lookin? The answer is /etc/fstab. We take aquick look at that file; you can see thisis our disk partitions that we want, sothis is SDA 1, and it's UUID, a uniqueuser ID of the unique device ID, and our swappartSJ5. It's unique ID so that we can findthat at boot. When I generally makechanges to config files that are notpart of a larger system, like just on mydesktop, so not as part of a businessconfiguration management setup, I like tojust leave my name in there, just so Ican quickly grep for files that have.

This string in them so that I can seeeverything that I've changed on thesystem, you know, that isn’t in some kindof configuration management or automaticprovisioning script. Finally, the verymost important thing: man pages. Youwill get asked how you look stuff up. Howdo you find answers to questions? Yourfirst answer to this should be, "It's acommand. I look at the man pages, themanual pages." So, "man command" gets youthere. For example, "man ps." You want toknow all the options you could pass tops; it’s kind of a big command. Here areall the interesting things you can dowith it. Generally, man pages have a sort

of name of the command, basic synopsis,the way you call it, so the syntax forcalling it, a description of the command,sort of many, many very dense tutorials,usually, and some examples. BSDs are a lotbetter than most Linux distros for this,but you hope to find an example sectionat the bottom. In this case, you've got anexample section here, although this isoften missing depending on what thecommand is, and then a listing of all theoptions in detail. See, this is a very oldand well-documented command. Other thanman: Google, Server Fault, Stack Overflow.

These are good answers in an interview.You should know multiple ways of findinganswers to questions. If you haven't readit yet, I will link it here with the"How to Ask Smart Questions" or "How to AskQuestions the Smart Way" article. This isrequired reading for anybody in tech.I hope that's been useful. If you wantmore of these, whether for Linux, forhigher-level Linux questions, or forother skill silos like databases,networking, programming, infrastructure,cloud stuff, applications, and services,I'd be happy to make more of thesevideos. If they're helpful, leave acomment with topics you'd like to seecovered. Subscribe for more if youhaven't already. Check out the basicsysadmin skills playlist because there'sjust a whole lot of background stuff youneed to know in there. So good luck onyour job hunt and in your interviews, andmaybe we'll run across each other inreal life sometime. Thanks for watching,guys.