Linux Shell Scripting Interview Questions and Answers

1) How to pass argument to a script ?
Ans.  ./script argument
Example : Script will show filename
./show.sh file1.txt
cat show.sh
#!/bin/bash
cat $1

2) How to use argument in a script ?
Ans. First argument: $1,
Second argument : $2
Example : Script will copy file (arg1) to destination (arg2)
./copy.sh file1.txt /tmp/
cat copy.sh
#!/bin/bash
cp $1 $2

3) How to calculate number of passed arguments ?
Ans. $#

4) How to get script name inside a script ?
Ans. $0

5) How to check if previous command run successful ?
Ans. $?

6) How to get last line from a file ?
Ans. tail -1

7) How to get first line from a file ?
Ans. head -1

8) How to get 3rd element from each line from a file ?
Ans. awk ‘{print $3}’

9) How to get 2nd element from each line from a file, if first equal FIND
Ans. awk ‘{ if ($1 == “FIND”) print $2}’

10) How to debug bash script
Ans. Add -xv to #!/bin/bash
Example
#!/bin/bash –xv

11) Give an example how to write function ?
Ans. function example {
echo “Hello world!”
}

12) How to add string to string ?
Ans. V1=”Hello”
V2=”World”
let V3=$V1+$V2
echo $V3
Output
Hello+World

13) How to add two integers ?
Ans. V1=1
V2=2
V3=$V1+$V2
echo $V3
Output
3
Remember you need to add “let” to line V3=$V1+$V2
then echo $V3 will give 3
if without let , then it will be
echo $V3 will give 1+2

14) How to check if file exist on filesystem ?
Ans. if [ -f /var/log/messages ]
then
echo “File exists”
fi

15) Write down syntax for all loops in shell scripting ?
Ans. for loop :
for i in $( ls ); do
echo item: $i
done
while loop :
#!/bin/bash
COUNTER=0
while [ $COUNTER -lt 10 ]; do
echo The counter is $COUNTER
let COUNTER=COUNTER+1
done
until loop :
#!/bin/bash
COUNTER=20
until [ $COUNTER -lt 10 ]; do
echo COUNTER $COUNTER
let COUNTER-=1
done

16) What it means by #!/bin/sh or #!/bin/bash at beginning of every script ?
Ans. That line tells which shell to use. #!/bin/bash script to execute using /bin/bash. In case of python script there there will be #!/usr/bin/python

17) How to get 10th line from the text file ?
Ans. head -10 file|tail -1

18) What is the first symbol in the bash script file
Ans. #

19) What would be the output of command: [ -z “” ] && echo 0 || echo 1
Ans. 0

20) What command “export” do ?
Ans. Makes variable public in subshells

21) How to run script in background ?
Ans. add “&” to the end of script

22) What “chmod 500 script” do ?
Ans. Makes script executable for script owner

23) What “>” do ?
Ans. Redirects output stream to file or another stream.

24) What difference between & and &&
Ans. & – we using it when want to put script to background
&& – when we wand to execute command/script if first script was finished successfully

25) When we need “if” before [ condition ] ?
Ans. When we need to run several commands if condition meets.

26) What would be the output of the command: name=John && echo ‘My name is $name’
Ans. My name is $name

27) Which is the symbol used for comments in bash shell scripting ?
Ans. #

28) What would be the output of command: echo ${new:-variable}
Ans. variable

29) What difference between ‘ and ” quotes ?
Ans. ‘ – we use it when do not want to evaluate variables to the values
” – all variables will be evaluated and its values will be assigned instead.

30) How to redirect stdout and stderr streams to log.txt file from script inside ?
Ans. Add “exec >log.txt 2>&1” as the first command in the script

31) How to get part of string variable with echo command only ?
Ans. echo ${variable:x:y}
x – start position
y – length
example:
variable=”My name is Petras, and I am developer.”
echo ${variable:11:6} # will display Petras

32) How to get home_dir with echo command only if string variable=”User:123:321:/home/dir” is given ?
Ans. echo ${variable#*:*:*:}
or
echo ${variable##*:}

33) How to get “User” from the string above ?
Ans. echo ${variable%:*:*:*}
or
echo ${variable%%:*}

34) How to list users which UID less that 100 (awk) ?
Ans. awk -F: ‘$3<100’ /etc/passwd

35) Write the program which counts unique primary groups for users and displays count and group name only
Ans. cat /etc/passwd|cut -d: -f4|sort|uniq -c|while read c g
do
{ echo $c; grep :$g: /etc/group|cut -d: -f1;}|xargs -n 2
done

36) How to change standard field separator to “:” in bash shell ?
Ans. IFS=”:”

37) How to get variable length ?
Ans. $