Shell Scripting Interview Questions

Shell Scripting Interview Questions

Following are some of the most frequently asked Shell Scripting interview questions in the interview, here are the answers to them.

Q1) 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

Q2) What it means by #!/bin/sh or #!/bin/bash at the 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 will be #!/usr/bin/python.

Q3) What command "export" do?

Ans: Makes variable public in subshells.

Q4) How to run the script in the background?

Ans: add "&" to the end of the script.

Q5) What "chmod 500 script" do?

Ans: Makes script executable for script owner.

Q6) What ">" do?

Ans: Redirects output stream to file or another stream.

Q7) What difference between & and &&

Ans:

& - we use it when we want to put the script to the background

&& - when we want to execute a command/script if the first script was finished successfully

Q8) When do we need "if" before [ condition]?

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

Q9) What would be the output of the command: name=John && echo 'My name is $name'

Ans: My name is $name

Q10) Which is the symbol used for comments in bash shell scripting?

Ans: #

Q11) What would be the output of the command: echo ${new:-variable}

Ans: variable

Q12) 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 their values will be assigned instead.

Q13) How to print all arguments provided in the script?

Ans: echo $* or echo $@

Q14) 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

Q15) What difference between = and ==

Ans:

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

Q16) Write the command to test if $a greater than 12?

Ans: [ $a -gt 12 ]

Q17) Write the command to test if $b is less or equal to 12?

Ans: [ $b -le 12 ]

Q18) How to redirect stdout and stderr streams to the log.txt file from the script inside?

Ans: Add "exec >log.txt 2>&1" as the first command in the script.

Q19) How to get part of the string variable with the 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

Q20) 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##*:}

Q21) How to get “User” from the string above?

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

Q22) How to list users which UID less than 100 (awk)?

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

Q23) Compare Numeric Operators

Ans:

Q24) Syntax to define an array in bash?

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

Q25) Print the first array element.

Ans: echo ${array[0]}

Q26) Print all array elements.

Ans: echo ${array[@]}

Q27) Print all array indexes syntax?

Ans: echo ${!array[@]}

Q28) Syntax to remove array element with id 123?

Ans: unset array[123]

Q29) Syntax to add a new element to the array with id 1980?

Ans: array[1980]="New_element"