Linux Shell Scripting Interview Questions and Answers

Q1. 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

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

Q3. How to calculate number of passed arguments ?

Ans. $#

Q4. How to get script name inside a script ?

Ans. $0

Q5. How to check if previous command run successful ?

Ans. $?

Q6. How to get last line from a file ?

Ans. tail -1

Q7. How to get first line from a file ?

Ans. head -1

Q8. How to get 3rd element from each line from a file ?

Ans. awk '{print $3}'

Q9. How to get 2nd element from each line from a file, if first equal FIND

Ans. awk '{ if ($1 == "FIND") print $2}'

Q10. How to debug bash script

Ans. Add -xv to #!/bin/bash
Example
#!/bin/bash –xv

Q11. Give an example how to write function ?

Ans. function example {
echo "Hello world!"
}

Q12. How to add string to string ?

Ans. V1="Hello"
V2="World"
let V3=$V1+$V2
echo $V3
Output
Hello+World

Q13. 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

Q14. How to check if file exist on filesystem ?

Ans. if [ -f /var/log/messages ]
then
echo "File exists"
fi

Q15. 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

Q16. 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

Q17. How to get 10th line from the text file ?

Ans. head -10 file|tail -1

Q18. What is the first symbol in the bash script file

Ans. #

Q19. What would be the output of command: [ -z "" ] && echo 0 || echo 1

Ans. 0

Q20. What command "export" do ?

Ans. Makes variable public in subshells

Q21. How to run script in background ?

Ans. add "&" to the end of script

Q22. What "chmod 500 script" do ?

Ans. Makes script executable for script owner

Q23. What ">" do ?

Ans. Redirects output stream to file or another stream.

Q24. 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

Q25. When we need "if" before [ condition ] ?

Ans. When we need to run several commands if condition meets.

Q26. What would be the output of the command: name=John && echo 'My name is $name'

Ans. My name is $name

Q27. Which is the symbol used for comments in bash shell scripting ?

Ans. #

Q28. What would be the output of command: echo ${new:-variable}

Ans. variable

Q29. 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.

Q30. 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

Q31. 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

Q32. 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##*:}

Q33. How to get “User” from the string above ?

Ans. echo ${variable%:*:*:*}
or
echo ${variable%%:*}

Q34. How to list users which UID less that 100 (awk) ?

Ans. awk -F: '$3<100' /etc/passwd

Q35. 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

Q36. How to change standard field separator to ":" in bash shell ?

Ans. IFS=":"

Q37. How to get variable length ?

Ans. ${#variable}

Q38. How to print last 5 characters of variable ?

Ans. echo ${variable: -5}

Q39. 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

Q40. How to substitute part of string with echo command only ?

Ans. echo ${variable//pattern/replacement}

Q41. Which command replaces string to uppercase ?

Ans. tr '[:lower:]' '[:upper:]'

Q42. How to count local accounts ?

Ans. wc -l /etc/passwd|cut -d" " -f1
or
cat /etc/passwd|wc -l

Q43. How to count words in a string without wc command ?

Ans. set ${string}
echo $#

Q44. Which one is correct "export $variable" or "export variable" ?

Ans. export variable

Q45. How to list files where second letter is a or b ?

Ans.ls -d ?[ab]*

Q46. 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`

Q47. How to remove all spaces from the string ?

Ans. echo $string|tr -d " "

Q48. 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"

Q49. 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

Q50. How to print all arguments provided to the script ?

Ans. echo $*
or
echo $@

Q51. 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

Q52. What difference between = and ==

Ans. = - we using to assign value to variable
== - we using for string comparison

Q53. Write the command to test if $a greater than 12 ?

Ans. [ $a -gt 12 ]

Q54. Write the command to test if $b les or equal 12 ?

Ans. [ $b -le 12 ]

Q55. How to check if string begins with "abc" letters ?
Ans. [[ $string == abc* ]]

Q56. 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*

Q57. How to list usernames which starts with ab or xy ?

Ans. egrep "^ab|^xy" /etc/passwd|cut -d: -f1

Q58) What $! means in bash ?

Ans. Most recent background command PID

Q59. What $? means ?

Ans. Most recent foreground exit status.

Q60. How to print PID of the current shell ?

Ans. echo $$

Q61. How to get number of passed arguments to the script ?

Ans. echo $#

Q62. What difference between $* and $@

Ans. $* - gives all passed arguments to the script as a single string
$@ - gives all passed arguments to the script as delimited list. Delimiter $IFS

Q63. How to define array in bash ?

Ans. array=("Hi" "my" "name" "is")

Q64. How to print the first array element ?

Ans. echo ${array[0]}

Q65. How to print all array elements ?

Ans. echo ${array[@]}

Q66. How to print all array indexes ?

Ans. echo ${!array[@]}

Q67. How to remove array element with id 2 ?

Ans. unset array[2]

Q68. How to add new array element with id 333 ?

Ans. array[333]="New_element"

Q69. How shell script get input values ?

Ans. a) via parameters
./script param1 param2
b) via read command
read -p "Destination backup Server : " desthost

Q70. 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