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. ${#variable}

38) How to print last 5 characters of variable ?
Ans. echo ${variable: -5}

39) What difference between ${variable:-10} and ${variable: -10} ?
Ans. ${variable:-10} – gives 10 if variable was not assigned before
${variable: -10} – gives last 10 symbols of variable

40) How to substitute part of string with echo command only ?
Ans. echo ${variable//pattern/replacement}

41) Which command replaces string to uppercase ?
Ans. tr ‘[:lower:]’ ‘[:upper:]’

42) How to count local accounts ?
Ans. wc -l /etc/passwd|cut -d” ” -f1
or
cat /etc/passwd|wc -l

43) How to count words in a string without wc command ?
Ans. set ${string}
echo $#

44) Which one is correct “export $variable” or “export variable” ?
Ans. export variable

45) How to list files where second letter is a or b ?
Ans.ls -d ?[ab]*

46) How to add integers a to b and assign to c ?
Ans. c=$((a+b))
or
c=`expr $a + $b`
or
c=`echo “$a+$b”|bc`

47) How to remove all spaces from the string ?
Ans. echo $string|tr -d ” ”

48) Rewrite the command to print the sentence and converting variable to plural: item=”car”; echo “I like $item” ?
Ans. item=”car”; echo “I like ${item}s”

49) Write the command which will print numbers from 0 to 100 and display every third (0 3 6 9 …) ?
Ans. for i in {0..100..3}; do echo $i; done
or
for (( i=0; i<=100; i=i+3 )); do echo “Welcome $i times”; done

50) How to print all arguments provided to the script ?
Ans. echo $*
or
echo [email protected]

51) What difference between [ $a == $b ] and [ $a -eq $b ]
Ans. [ $a == $b ] – should be used for string comparison
[ $a -eq $b ] – should be used for number tests

52) What difference between = and ==
Ans. = – we using to assign value to variable
== – we using for string comparison

53) Write the command to test if $a greater than 12 ?
Ans. [ $a -gt 12 ]

54) Write the command to test if $b les or equal 12 ?
Ans. [ $b -le 12 ]
55) How to check if string begins with “abc” letters ?
Ans. [[ $string == abc* ]]

56) What difference between [[ $string == abc* ]] and [[ $string == “abc*” ]]
Ans. [[ $string == abc* ]] – will check if string begins with abc letters
[[ $string == “abc*” ]] – will check if string is equal exactly to abc*

57) How to list usernames which starts with ab or xy ?
Ans. egrep “^ab|^xy” /etc/passwd|cut -d: -f1

58) What $! means in bash ?
Ans. Most recent background command PID

59) What $? means ?
Ans. Most recent foreground exit status.

60) How to print PID of the current shell ?
Ans. echo $$

61) How to get number of passed arguments to the script ?
Ans. echo $#

62) What difference between $* and [email protected]
Ans. $* – gives all passed arguments to the script as a single string
[email protected] – gives all passed arguments to the script as delimited list. Delimiter $IFS

63) How to define array in bash ?
Ans. array=(“Hi” “my” “name” “is”)

64) How to print the first array element ?
Ans. echo ${array[0]}

65) How to print all array elements ?
Ans. echo ${array[@]}

66) How to print all array indexes ?
Ans. echo ${!array[@]}

67) How to remove array element with id 2 ?
Ans. unset array[2]

68) How to add new array element with id 333 ?
Ans. array[333]=”New_element”

69) How shell script get input values ?
Ans. a) via parameters
./script param1 param2
b) via read command
read -p “Destination backup Server : ” desthost

70) How can we use “expect” command in a script ?
Ans. /usr/bin/expect << EOD
spawn rsync -ar ${line} ${desthost}:${destpath}
expect “*?assword:*”
send “${password}\r”
expect eof
EOD

 

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