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DeepakPrabakar / unix-linux-interview-questions

My Unix / Linux Interview Questions

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Unix / Linux Interview Questions

This repo contains questions that have been taken from various sources. Most of these involve the use of two or more commands. I have used GIT Bash to execute the below scripts.

Difficulty Level: Beginner to Intermediate

All the Best !!!

Test Commands

Tip n Hints

Option Description
data path to data files to supply the data that will be passed into templates.
engine engine to be used for processing templates. Handlebars is the default.
ext extension to be used for dest files.
First Header Second Header Second Header centre
Content cell 1 Content cell 2 RitghtContent cell 2 centreContent cell 2
Content column 1 Content column 2 engine to be used for processing templates. Handlebars is the default. engine to be used for processing templates. Handlebars is the default.

References

Question List

1. Delete blank lines in file

$ sed '^\s*$'/d' file.txt

To save changes back to file

$ sed -i '^\s*$'/d' file.txt

Also

$ grep -v '^\s*$' file.txt

2. Print number of times each word appears in a file

$ cat file | tr '[:space:]' '[\n*]' | grep -v '^\s*$' | sort | uniq -c | sort -bnr

tr just replaces spaces woth newlines
grep -v '^\s*$' trims out empty lines
sort to prepare as input for uniq
uniq -c to count occurrences
sort -bnr sorts in numeric reverse order while ignoring whitespace

3. List all the files in directory except .txt and .pdf

$ ls -I "*.txt" -I "*.pdf"

If you want to iterate across all the subdirectories:

$ ls -I "*.txt" -I "*.pdf" -R

Also

$ find -not -iname "*.txt"
$ find . ! '(' -name '*.txt' -o -name '*.pdf' ')'

Reference: https://unix.stackexchange.com/questions/47151/how-do-i-list-every-file-in-a-directory-except-those-with-specified-extensions

4. Rename all .jpg files to .jpeg

$ vi image.sh

for x in `ls ./images/`;
do
    a=`echo $x | cut -d'.' -f1`
    mv ./images/$x ./images/$a".jpeg";
done

$ ./image.sh

5. Delete files of size more than 100mb in a folder which are older than 90 days

$ find /folder -size +204800 -mtime +90 -exec rm {} \;

6. Reverse the words in a sentence

$  echo "Have a good day" | awk '{ for(i=0;i<NF;++i){ x=NF-i; printf "%s ",$x;}  }'

Output: day good a Have

7. Given an input file having lines with alphabets and numbers. Print only the distinct alpha-numberic lines.

Input

$ cat input.txt

1234567890
0987654321
ABCDEFGH
123456789X
1234567890
123456789X

Output

123456789X

$ vi script.sh

#!/bin/bash
awk '
BEGIN {
i=0; 
}
{ 
    if (($1 ~ /[0-9]/) && ($1 ~ /[A-Z]/)) { 
        flg=0; 
        for (x = 0; $x < $i; x=$x+1) { 
            if($arr[$x] == $1) {
                flg=1;
                break;
            } 
        }
        if(flg==0) {
            arr[$i] = $1; i=$i+1;
        } 
    } 
}
END { 
    for (x = 0; x < $i; x++)
        print $arr[0]
}' input.txt | uniq

$ ./script.sh

8. Given a comma separated input file with items, category and cost. Display the category and its sum.

Input

$ cat input.txt

item1,category1,200
item2,category2,100
item3,category3,300
item4,category1,400
item5,category2,500
item6,category1,600

Output

category1 - 1200
category2 - 600
category3 - 300

$ vi script.sh

awk '
BEGIN{
x=0;
flag=0
}
{ 
    split($0,arr,",")           # split - arr[1] has item, arr[2] has category, arr[3] has cost
    flag=0;
    for(i=0;i<=x;i++){
        if(arr[2]==cat_arr[i]){
            flag=1;
            sum_arr[i]=sum_arr[i]+arr[3]
        }
    }
    if(flag==0){
        cat_arr[x]=arr[2]
        sum_arr[x]=arr[3]
        x++;
    }
}
END {
    for(i=0;i<x;i++){
        print(cat_arr[i],"-",sum_arr[i])
    }
}' input.txt

$ ./script.sh

9. Output of following command ps -ef | awk '{ print $1}' | sort | uniq | wc -l

Answer 2

$ ps -ef
     UID     PID    PPID  TTY        STIME COMMAND
prabakad   20464       1 ?          Jul 13 /usr/bin/mintty
prabakad    4940   18944 pty2       Jul 14 /usr/bin/bash
prabakad   18944       1 ?          Jul 14 /usr/bin/mintty
prabakad   19944   20464 pty1       Jul 13 /usr/bin/bash
prabakad   11492    4940 pty2       Jul 14 /usr/bin/winpty
prabakad   20708   16868 pty0     20:38:09 /usr/bin/ps
prabakad   16868   13672 pty0       Jul 12 /usr/bin/bash
prabakad   13672       1 ?          Jul 12 /usr/bin/mintty

$ ps -ef | awk '{ print $1}'
UID
prabakad
prabakad
prabakad
prabakad
prabakad
prabakad
prabakad
prabakad
prabakad

$ ps -ef | awk '{ print $1}' | sort | uniq
prabakad
UID

$ ps -ef | awk '{ print $1}' | sort | uniq | wc -l
2

10. Kill process based on user and process.

  • Using grep to search
$ ps -ef | grep "bash" | grep "prabakad"| awk 'BEGIN{print $2}{kill -9 $2}

$ ps -ef | grep "bash" | grep "prabakad"| awk '{print $2}' | xargs kill -9
  • Using ps command with -u and -p
$ ps -ef -u prabakad -p 8552 | tail -n +2 | awk '{print $2}' | xargs kill -9

Explanation

$ ps -ef
     UID     PID    PPID  TTY        STIME COMMAND
prabakad   19208   18800 pty0     21:46:39 /usr/bin/bash
prabakad   11440   19208 pty0     21:46:43 /usr/bin/ps
prabakad   18800       1 ?        21:46:38 /usr/bin/mintty

$ ps -ef -u prabakad -p 19208
     UID     PID    PPID  TTY        STIME COMMAND
prabakad   19208   18800 pty0     21:46:39 /usr/bin/bash

$ ps -ef | grep "bash" | grep "prabakad"
prabakad   19208   18800 pty0     21:46:39 /usr/bin/bash
prabakad   19484   19208 pty0     21:47:52 /usr/bin/bash

grep command does not give header.
ps command gives header, so use tail command to remove it.
The PIDs are sent as arguments using xargs and killed using kill -9

11. Print contents of a file starting from the nth line.

For example, print the give file starting from the 4th line

Input

$ cat lines.txt
line1
line2
line3
line4
line5
line6
line7
line8
line9
line10
line11
  • Using tail command
$ cat lines.txt | tail -n +4
line4
line5
line6
line7
line8
line9
line10
line11
  • Using sed command
$ sed '1,3d' lines.txt
line4
line5
line6
line7
line8
line9
line10
line11

Additional Information

head command displays first n lines.
tail command displays last n lines. If tail is used with +, displays lines starting from nth line as shown above.
The sed command above deletes the first 3 lines and prints output starting from 4th line.

12. Write a command to calculate the total size of all pdf files in the directory.

$ du -hc *.pdf | tail -1
25K     total

13. Write a command to delete the first and last line in every file in the directory and rename it.

Input

$ ls ./sample
abc_20171607.txt      rocket_20171503.txt   xyz_20171507.txt
mno xyz_20171507.txt  sample1_20171607.txt

$ cat ./sample/abc_20171607.txt
header
line1
line2
line3
footer

Output

$ ls sample
renamed_abc_20171607.log      renamed_sample1_20171607.log
renamed_mno xyz_20171507.log  renamed_xyz_20171507.log
renamed_rocket_20171503.log

$ cat ./sample/renamed_abc_20171607.log
line1
line2
line3
$ vi sample.sh

IFS=$'\n'
for filename in `ls ./sample -I '*.sh'`;                        # ls in sample folder searching file other than .sh
do
    newfilename=`echo $filename | cut -d '.' -f1`
    sed -i -e '1d' -e '$d' ./sample/$filename                   # -i to save to file. '1d' and '$d' delete first and last line
    mv ./sample/$filename ./sample/"renamed"_$newfilename".log"
done

$ ./sample.sh

Additional

If IFS=$'\n' (Internal Field Separator) is missing, file names having space in them are not recognised.

$ ./sample.sh
abc_20171607.txt
mno
xyz_20171507.txt
rocket_20171503.txt
sample1_20171607.txt
xyz_20171507.txt

14. Given a file with records. Print the sum of m where name=aman

Input

$ cat input.txt
t=20
m=10
name=aman
--xx--
t=30
m=20
name=deep
--xx--
t=40
m=30
name=aman
--xx--
t=40
m=300
name=aman
--xx--
t=40
m=10
name=rahul

Output

sum of aman is 340

$ vi script.awk

BEGIN {
	RS="--xx--\n";
	FS="\n";
}
{
	split($3,data,"=")
	if(data[2]=="aman"){
        split($2,mdata,"=")
        sum=sum+mdata[2]
	}
}
END{
	print ("sum of aman is",sum)
}

$ awk -f script.awk input.txt

Reference: http://www.thegeekstuff.com/2010/01/8-powerful-awk-built-in-variables-fs-ofs-rs-ors-nr-nf-filename-fnr/

15. Given a file with numbers. Sort each line.

Input

$ cat input.txt
4 6 8 1 7
2 12 9 6 10
6 1 14 5 7

Output

1 4 6 7 8
2 6 9 10 12
1 5 6 7 14

$ awk ' {split( $0, a, " " ); asort( a ); for( i = 1; i <= length(a); i++ ) printf( "%s ", a[i] ); printf( "\n" ); }' input.txt

Reference: http://www.unix.com/shell-programming-and-scripting/180835-sort-each-row-horizontally-awk-any.html

16. Bash shell script to reverse a string / word.

$ vi reverse.sh

#!/bin/bash
input="$1"
reverse="" 
len=${#input}
for (( i=$len-1; i>=0; i-- ))
do 
	reverse="$reverse${input:$i:1}"
done 
echo "$reverse"

$ ./reverse.sh Hello
olleH

Also

$ echo "Hello" | rev
olleH

$ rev<<<"This is a test"
tset a si sihT

Reference: https://www.cyberciti.biz/faq/how-to-reverse-string-in-unix-shell-script/

17. Reverse last 4 digits of a string.

Input

123456****7890

Output

123456****0987

$ vi rev.sh

input="$1"
reverse=""

len=${#input}

for((i=0;i<$len-4;i++))
do
{
reverse="$reverse${input:$i:1}"
}
done
for((i=len-1;i>=len-4;i--))
do
{
reverse="$reverse${input:$i:1}"
}
done

echo $reverse

$ ./rev.sh 123456****7890

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My Unix / Linux Interview Questions