Linux Interview Questions – Shell Scripting Part 1

1. Write a shell script to get current date, time, user name and current working directory.
Answer : The commands logname, date, who I am and pwd will output username, current date and time, and current working directory. Just implementing these commands in the script and making it a bit interactive.

Now create a file called ‘userstats.sh‘ and add the following code to it.

#!/bin/bash 
echo "Hello, $LOGNAME" 
echo "Current date is `date`" 
echo "User is `who i am`" 
echo "Current directory `pwd`"

Place execute permission and run the script as shown below.

# chmod 755 userstats.sh 
# ./userstats.sh
Sample Output
Hello, avi 
Current date is Sat Jun  7 13:05:29 IST 2014 
User is avi      pts/0        2014-06-07 11:59 (:0) 
Current directory /home/avi/Desktop
2. Write a Shell Script that adds two numbers if provided as the command Line Argument and if the two numbers are not entered it outputs an Error Message along with a one-Line of how-to use description.
Answer : Here is the simple Shell Script along with description which adds two number if provided as Command-Line argument, if not it throws error with single line on how to use the script.

Again create a file called ‘two-numbers.sh‘ and add the following content to it.

#!/bin/bash 
# The Shebang

if [ $# -ne 2 ] 
# If two Inputs are not received from Standard Input

then 
# then execute the below statements

    echo "Usage - $0   x    y" 
    # print on standard output, how-to use the script (Usage - ./1.sh   x    y )

    echo "        Where x and y are two nos for which I will print sum" 
    # print on standard output, “Where x and y are two nos for which I will print sum ”

    exit 1 
    # Leave shell in Error Stage and before the task was successfully carried out.

fi 
# End of the if Statement.

    echo "Sum of $1 and $2 is `expr $1 + $2`"
    # If the above condition was false and user Entered two numbers as a command Line Argument,   
       it will show the sum of the entered numbers.

Set executer permission on the file and run the script as shown below.

# chmod 755 two-numbers.sh

Condition 1: Running the script without entering two numbers as command line argument, you will get the following output.

Sample Output
# ./two-numbers.sh

Usage - ./two-numbers.sh   x    y 
        Where x and y are two nos for which I will print sum

Condition 2: When Numbers are entered as command line argument you will get the result as shown.

$ ./two-numbers.sh 4 5 

Sum of 4 and 5 is 9

Hence the above shell script fulfills the condition as suggested in the question.

3. You needs to print a given Number say 10572, in reverse order using a Shell script such that the input is provided using command Line Argument only. If the input data is not provided as Command Line Argument, it should throw and error and should suggest, how to use the script. Write the script but before that tell me the algorithm that needs to be implemented here.

Algorithm

  1. 1. Let the Input Number = n
  2. 2. Set rev=0, sd=0 (Reverse and single digitis set to 0)
  3. 3. n % 10, will find and give single left most digit
  4. 4. reverse number is generated as rev * 10 + sd
  5. 5. Decrease Input Number (n) by 1.
  6. 6. if n > 0, then goto step 3 else goto setp 7
  7. 7. Print rev

Now again, create a file called ‘numbers.sh‘ and add the following given code it.

#!/bin/bash 
if [ $# -ne 1 ] 
then 
    echo "Usage: $0   number" 
    echo "       I will find reverse of given number" 
    echo "       For eg. $0 0123, I will print 3210" 
    exit 1 
fi 

n=$1 
rev=0 
sd=0 

while [ $n -gt 0 ] 
do 
    sd=`expr $n % 10` 
    rev=`expr $rev \* 10  + $sd` 
    n=`expr $n / 10` 
done 
    echo  "Reverse number is $rev"

Grant a execute permission on the file and run the script as shown below.

# chmod 755 numbers.h

Condition 1: When Input is not provided as command line argument, you will get the following output.

Sample Output
./numbers.sh

Usage: ./numbers.sh  number 
       I will find reverse of given number 
       For eg. ./2.sh 123, I will print 321

Condition 2: When Input was provided as command line Argument.

$ ./numbers.sh 10572 

Reverse number is 27501

The above script worked perfectly and the output was just what we needed.

4. You are supposed to calculate a real number calculation directly from terminal and not any shell script. What will you do (say the real numbers are 7.56 and 2.453)?
Answer : We need to use bc command in a special way as described below. The output of echo 7.56 + 2.453 should be pipelined to bc.

For example, run the following command to calculate numbers in real time using bc command as shown.

$ echo 7.56 + 2.453 | bc

10.013
5. You are supposed to find the value of pi upto 100 places of decimal, what is the easiest way to get the result done.
Answer : The easiest way to find the value of pi, correct upto 100 places of decimal we just need to issue the below command.
# pi 100 3.141592653589793238462643383279502884197169399375105820974944592307816406286208998628034825342117067

Obviously! We must have package ‘pi‘ installed. Just do a apt or yum to get a required package to install ‘pi‘ onto the distribution you are using.

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