10 Linux and UNIX Interview Questions and Answers asked in Wipro TCS Capegemini

UNIX and Linux Interview Questions and Answers
Questions from various UNIX operating systems e.g. Solaris, Linux, IBM AIX or any other UNIX operating system is asked on different support and programming interviews. I have always seen few interview questions from Linux and UNIX along with SQL in almost every Java programming interviews. You just can not afford not to prepare questions from UNIX and Linux until your Job absolutely doesn't require any work in UNIX operating system. I have collected many UNIX command interview questions and already shared them but I found that except system admin jobs, many programming job interviews only focus on general UNIX commands e.g. chmod, find or grep and fundamentals like finding files and directories, managing file space, networking commands, checking process status and managing file permissions.

In this article, we will see such kind of frequently asked interview questions and answers from UNIX and Linux operating System. Questions are very fundamental in nature and not limited to Linux only and equally applicable to other UNIX operating systems e.g. Solaris, IBM AIX etc. 

Many of these UNIX questions are asked during various interviews on companies like TCS, Infosys, Citibank , Wipro, Capgemini and Tech Mahindra. As I said they are very fundamental and can be asked in any company.  If you come across any other frequently asked Linux questions which are not on this list feel free to suggest.




UNIX and Linux Interview Questions and Answers

Linux command interview questions and answers for programmersHere is my list of frequently asked UNIX and Linux interview questions and answers. All these questions are based upon fundamental commands and concepts which are a must for working on any UNIX operating system e.g. Solaris.


1) How to find all the links in a folder in UNIX or Linux ?
This is a tricky UNIX question as there is no specific command to find all symbolic links. Though you have ln command for creating and updating soft links but nothing which gives you all the links in a directory. You need to use ls command which lists everything in the directory and then you need to list all the links, as they start with "l" as first characters, as shown in above article .

here is the actual UNIX command to find all links in a directory :

linux@nyj872:~ ls -lrt
total 2.0K
-rw-r--r--  1 Linux Domain Users  0 Dec  6  2011 a
drwxr-xr-x+ 1 Linux Domain Users  0 Sep 19 12:30 java/
lrwxrwxrwx  1 Linux Domain Users  4 Sep 19 12:31 version_1.0 -> java/

linux@nyj872:~ ls -lrt | grep '^l'
lrwxrwxrwx  1 Linux Domain Users  4 Sep 19 12:31 version_1.0 -> java/



2) How to find a process and kill that?
Another classic UNIX interview questions. Answer of this question is simple if you are familiar with ps, grep and kill command. by using "ps -ef" you can get a list of all process and then use grep to find your process and get the PID of that process. Once you got PID you can use the kill command to kill that process as shown in this example of kill command in UNIX.



3) How to run a program in the background in UNIX or Linux ?
an easy UNIX or Linux interview question, only when you know. You can use & to run any process in the background and then you can use jobs to find the job id for that process and can use  fg and bg command to bring that process into foreground and background. See how Linux works to learn more about running a process in the background.


Linux interview questions and answers




4) How to sort the output of a command in reverse order in Linux or UNIX ?
One more Linux commands interview question which checks knowledge of frequently used command. you can use sort command in UNIX to sort the output of any command by using PIPE. By using -r option with sort command you can sort the output of any command in reverse order. See these sort command examples for more details.



5) How to create an archive file in UNIX or Linux Operating System ?
Another interview question based on knowledge of UNIX or Linux command. you can use the tar command to great archives in UNIX or Linux. you can even combine tar and gzip to create a compressed archive in UNIX.



6) What is the meaning of a file has 644 permission ?
To answer this UNIX or Linux interview question, you must know basics of files and directories in UNIX.  The 644 represents permission 110 for the owner, permission 100 for group and 100 for others which means read + write for an owner who creates that file and read only permission for group and others. 

You can also see The Linux Command Line: A Complete Introduction, a book to learn more about essential shell commands in Linux.


Linux Interview Questions for 2 year experienced




7) How will you remove empty files or directories from /tmp ?
You can use the find command to find all the empty files and directory and then combine the output with xargs command to remove all empty files from /tmp or any other directory in UNIX. See how to delete empty directory and files in UNIX  for exact command and to answer this UNIX command interview questions in detail.



8) I have read permission on a directory but I am not able to enter it why ?
One more tricky UNIX questions. In order to get into a directory, you need to execute permission. if your directory does not have to execute permission then you can not go into that directory by using the cd command. read UNIX files and directory permissions for more information.



9) How do you find all files which are modified 10 minutes before ?
This is another the Linux interview questions from frequently used command e.g. find and grep. you can use -mtime option of the find command to list all the files which are modified 10 or m minutes before. see these find command examples for more details.



10) How to do you find the size of directory in UNIX or Linux ?
This is another tricky and bit tough Linux interview question as popular ls command doesn't show the complete size of directories in UNIX. you need to use du command to get a full size of directories including all subdirectories in UNIX. See How to find directory size in UNIX for exact command and detailed explanation.

You can also check out Practical Guide to Linux Commands, Editors, and Shell Programming to learn more about essential Linux commands. One of the best book on Linux.


UNIX interview questions with answers




11) How do  you find all the process listening on a particular port in Linux?
You can use the netstat command to find all the process which are listening on a particular port as shown below:


$ netstat -nap | grep 8084
(Not all processes could be identified, non-owned process info will not be shown, you would have to be root to see it all.)
tcp 0 0 0.0.0.0:8084 0.0.0.0:* LISTEN 25314/java

Here 25314 is the process id of a tomcat server running on Linux. Since tomcat is a Java web application it started with java command and that's why you see 25314/java. See here to learn more about netstat command in UNIX.



12) How to you find all the process which has opened a file in Linux?
You can use the lsof (list open files) command to find out the process which has a file handle on the particular file. It's very useful command to check which process are reading a file. You can use lsof command as shown below to find out all the process:


$ lsof /home/someuser/somefile


will list all the process which has opened this file. you can see the command, PID, user and full file path to find out the process. See here to learn more about the lsof command in Linux.


Linux and UNIX Interview Questions with Answers


13) How to send HTTP request from a Linux server?
You can use either wget or curl command to send HTTP request, both GET and POST from a Linux machine as shown below:


$ curl http://api.openweathermap.org/data/2.5/weather?q=London,uk&appid=bd82977b86bf27fb59a04b61b657fb6f
{"coord":{"lon":-0.13,"lat":51.51},"weather":[{"id":804,"main":"Clouds",
"description":"overcast clouds","icon":"04n"}],"base":"stations",
"main":{"temp":282,"pressure":1022,"humidity":87,"temp_min":277.15,
"temp_max":285.15},"visibility":10000,"wind":{"speed":1.5},
"clouds":{"all":90},"dt":1445577409,"sys":{"type":1,"id":5093,
"message":0.0201,"country":"GB","sunrise":1445582275,"sunset":1445619056},
"id":2643743,"name":"London","cod":200}

Here we have sent a GET request to http://api.openweathermap.org and received a JSON response. You can also specify timeout using curl -m option, see here to learn more about curl command in UNIX.



14) How do you create a full directory structure e.g. /parent/child/grandchild in UNIX?
Well, you can use the mkdir command with option -p to create all parent directories in one go as shown below:


[# ~]$ mkdir -p software/java/app/config
[# ~]$
[# ~]$ pwd
/home/john
[# ~]$ cd software/java/app/config/
[# ~/software/java/app/config]$

You can see here software/java/app/config directory is created in just one command. See here to learn more about mkdir command in UNIX



15) How do you edit a large text file in UNIX without opening it?
You can use the sed command to edit a large file in UNIX without opening it. The sed stands for stream editor and you can do find replace in a file as shown below:


$ sed 's/Apple/Microsoft/' bestphone.txt


This command will find "Apple" in line and replace it with "Microsoft" string in the file bestphone.txt.


These were some of the frequently asked UNIX and Linux command interview questions and answers which appear in many IT Job interview which requires knowledge of UNIX operating system, Including programming job interviews e.g. core Java and J2EE interviews. Questions from UNIX and Linux is also very popular during C and C++ programming interviews.



13 comments:

  1. My putty keep disconnecting when working over SSH and travelling, is there a way to solve that ?

    ReplyDelete
    Replies
    1. increase the seconds between keepalive

      Delete
    2. Enter the command below, it will not disconnecting:

      export TMOUT=3600000000

      Delete
  2. 1) How to find all the links in a folder in UNIX or Linux ?

    Why can't we use find instead of ls?

    Example,

    -bash-4.1# find . -type l -ls | head
    405327 0 lrwxrwxrwx 1 root root 21 Jul 15 12:25 ./gdm/Xsession -> ../X11/xinit/Xsession
    263317 0 lrwxrwxrwx 1 root root 33 Jul 15 12:58 ./skel/.xinputrc -> /etc/X11/xinit/xinput.d/ibus.conf
    261742 0 lrwxrwxrwx 1 root root 10 Oct 9 20:32 ./rc4.d -> rc.d/rc4.d
    261948 0 lrwxrwxrwx 1 root root 36 Jul 15 12:14 ./alternatives/mkisofs-mkisofsman -> /usr/share/man/man1/genisoimage.1.gz
    261949 0 lrwxrwxrwx 1 root root 20 Jul 15 12:14 ./alternatives/mkisofs-mkhybrid -> /usr/bin/genisoimage
    262127 0 lrwxrwxrwx 1 root root 33 Oct 9 20:33 ./alternatives/jre_1.6.0 -> /usr/lib/jvm/jre-1.6.0-ibm.x86_64
    262809 0 lrwxrwxrwx 1 root root 18 Oct 9 20:32 ./alternatives/print-lpc -> /usr/sbin/lpc.cups
    263444 0 lrwxrwxrwx 1 root root 32 Sep 24 00:46 ./alternatives/jaxp_parser_impl -> /usr/share/java/libgcj-4.4.6.jar
    263449 0 lrwxrwxrwx 1 root root 37 Sep 24 00:47 ./alternatives/javah -> /usr/lib/jvm/java-1.5.0-gcj/bin/javah
    263452 0 lrwxrwxrwx 1 root root 44 Sep 24 00:47 ./alternatives/appletviewer -> /usr/lib/jvm/java-1.5.0-gcj/bin/appletviewer
    -bash-4.1#

    ReplyDelete
    Replies
    1. Good One!!! Expecting this type of answers ;-)

      --
      Manoj Kumar
      http://linuxinterviewquetions.blogspot.no/2011/11/linux-interview-questions.html

      Delete
  3. It is only good for junior developers. Not good for experienced

    ReplyDelete
    Replies
    1. I agree, I would prefer more task based questions than fact based question in any UNIX or Linux Interviews. I would rather ask candidate to

      find a text in 3GB file? (based on SED command)
      finding a file which may exist in sub directory? (based on find command)
      sorted values from 5th column of a CSV file? (cut, sort, or awk command based)
      Finding how much swap space is free? (memory based)
      finding which thread from a particular process is hogging CPU(prstat command based).

      Let me know what do you think about these Linux questions?

      Delete
  4. How do you find open files for a process in Linux?
    Answer : you can either use lsof command which lists open file descriptor or you can do ls /proc/pid/fd

    ReplyDelete
  5. What is difference between ps -ef and ps -auxwww command? when to use which ps command?

    ReplyDelete
    Replies
    1. ps -auxwww gives the detailed description of active processes

      Delete
  6. normally, we use ps to get process details,, so using when you run "ps -aux" this would print details using Unix format and when we run "ps aux" this would print details in BSD format. both command would mostly perform the same job,, u would need to check the man page of "ps" for more details and use options accordingly ....

    ReplyDelete
  7. i was asked to tell the boot process in linux

    ReplyDelete
    Replies
    1. 1. BIOS(POST check)
      2. MBR(partition Information and it will execute the GRUB)
      3. GRUB (Bootloader and it will execute the kernel)
      4. Kernel (It will execute the /sbin/init)
      5. init (execute the runlevel programs)
      6. Runlevel(X-windows)

      Delete