20 Shell Scripting Questions and Answers

What could possibly make the <cat filename> command return the following error?
```
 --bash: cat: Command not found
```
Ans.

The command not found error occurs when either the PATH variable is corrupt or has some issue in configuration.

Hence, it’ll throw the error message as mentioned in the question.
How can we pass arguments to a script in Linux? And how to access these arguments from within the script?

Ans.

We can write a bash script that can accept arguments from the command line in the following manner.
```
 $ ./scriptName "arg1" "arg2"..."argn"
```
To access these arguments, we can use the positional parameters ($1 to $9) in the script. Each parameter corresponds to the position of the argument on the command line.
What are the differences between $* and $@?

Ans.

In Linux, $* and $@ are both represent command line arguments passed to a shell script, but they behave differently:

| $* | $@ | | --- | --- | | $* treats all the arguments as a single entity and doesn’t preserve whitespace or quote characters. | $@ treats each argument as a separate entity while it ensures to preserve the whitespace and quotes. | | $*combines all the arguments into a single string with spaces as separators. | $@ treats each argument as a separate entity while ensuring to preserve the whitespace and quotes. | | If you have arguments with spaces or special characters, they may not be handled correctly. | It maintains the integrity of each argument as it is passed, making it suitable for iterating through arguments safely. |

consider this example to understand the $*.
```
 $ ./script.sh arg1 "arg 2" arg3
 $*
 Output: arg1 "arg 2" arg3
```
consider this example to understand the $@.
```
 $ ./script.sh arg1 "arg 2" arg3
```
What is the command to display only the regular files in a directory?

Ans.

The following command filters and displays only the regular files in a directory.
```
 $ ls -lrt | grep ^-
```

Write a shell script to display the last updated file or the newest file in a directory.

Ans.

Following is the test script to list the most recently changed file.

#!/bin/bash 

ls -lrt | grep ^- | awk 'END{print $NF}'

Another way to achieve the same behavior is as follows:

#!/bin/bash

# Use ls to list files in the directory by modification time (newest first),
# then use head to display the first (newest) file in the list.

ls -t "$directory_path" | head -n 1

Q-6. Print the current date, time, username, and current working directory.

Ans.

Let’s create a file named <stats.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'"

First of all, assign the execute permission for the script and then execute it. It’ll display the required information.

Q-7. Write a shell script that adds an extension “.new” to all the files in a directory.

Ans.

Please use the following script to change all the files in a directory to a “.new” extension. Kindly make sure to supply the directory name as an argument while running the test script.

#!/bin/bash 
dir=$1
for file in `ls $1/*`
do
    mv $file $file.new
done

Q-8. Write a shell script to print a number in reverse order. It should support the following requirements.

1. The script should accept the input from the command line.
2. If you don’t input any data, then display an error message to execute the script correctly.

Ans.

Let’s first, jot down the steps involved in the test script.

1. Suppose the input number is n.
2. Set reverse and single digits to 0 (i.e. rev=0, sd=0).
3. The expression (n % 10) will give the single leftmost digit i.e. sd.
4. To reverse the number, use this expression <rev * 10 + sd>.
5. Decrease the input number (n) by 1.
6. If n is greater than 0, then go to step no. 3. Else, execute step no. 7.
7. Print the result.

The script code is as follows.

#!/bin/bash 
if [ $# -ne 1 ] 
then 
    echo "Please provide the correct input in the below format."
    echo "Usage: $0 number" 
    echo "       This script will reverse the given number."
    echo "       For eg. $0 1234, will print 4321"
    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"

There are two ways in which we can run the above script.

Instance 1:

If there is no input, you will get the following output.

$ ./testscript.sh

Please provide the correct input in the below format.
Usage: ./testscript.sh number 
This script will reverse the given number.
For eg. ./testscript.sh 1234, will print 4321

Instance 2:

When the input is available in the command line Argument, you will get the following output.

$ ./testscript.sh 12345
Reverse number is 54321

Q-9. How will you delete a file that has special characters in its file name?

Ans.

In Linux or Unix-like systems, you may come across file names including the following special characters, white spaces, backslashes, and more.

-
--
;
&
$
?
*

Bash shell considers most of the above special characters as commands. Thus, the “rm” command may not be able to delete such files. The simplest way to delete files having special characters in their name is by using the inode number.

Step-1.

The <-i> option of the <ls> command displays the index number (inode) of each file.

$ ls -li
9966571 -rw-r--r-- 1 techbeamers users 0 Sep 16 10:05

Step-2.

Use the <find> command given below to delete the file, if it has an inode number such as 9966571.

$ find . -inum 9966571 -exec rm -i {} \;

Q-10. How to ask for input in a shell script from the terminal?

Ans:

In Linux, we can use the <read> command to take input from the user. It reads data from the terminal as the user enters it from the keyboard. And stores into a variable.

Let’s see a sample test script featuring the <read> command.

#!/bin/bash
echo ‘Please enter your name’
read name
echo “My Name is $name”

Upon running the above script, it’ll prompt for the name and assign the input value to the variable <name>.

$ ./test.sh
Please enter your name
TechBeamers
My Name is TechBeamers

Q-11. How do we do numeric comparisons in Linux?

Ans.

The test command can perform various types of numeric comparisons using the following operators.

1. <eq>

Syntax: INTEGER1 -eq INTEGER2
Description: INTEGER1 is equal to INTEGER2
Example Code:

#!/bin/bash
read -p "Please enter number 20 from the keyboard : " n
if test $n -eq 20
then
    echo "Thanks for entering the number 20."

else

   echo “You are not following my instructions.”
fi

2. <ge>

Syntax: INTEGER1 -ge INTEGER2
Description: INTEGER1 is greater than or equal to INTEGER2
Example Code:

#!/bin/bash
read -p "Enter a number >= 10 : " n
if test $n -ge 10
then
    echo "Correct value entered."

else

    echo “You are not following my instructions.”
fi

3. <gt>

Syntax: INTEGER1 -gt INTEGER2
Description: INTEGER1 is greater than INTEGER2
Example Code:

#!/bin/bash
read -p "Enter number > 20 : " n
if test $n -gt 20
then
    echo "$n is greater than 20."

else

    echo “You are not following my instructions.”
fi

4. <le>

Syntax: INTEGER1 -le INTEGER2
Description: INTEGER1 is less than or equal to INTEGER2
Example Code:

#!/bin/bash
read -p "Enter backup level : " n
if test $n -le 6
then
    echo "Incremental backup requested."
fi
if test $n -eq 7
then
    echo "Full backup requested."

else

    echo “You are not following my instructions.”
fi

5. <lt>

Syntax: INTEGER1 -lt INTEGER2
Description: INTEGER1 is less than INTEGER2
Example Code:

#!/bin/bash
read -p "Do not enter negative number here : " n

if test $n -lt 0
then
    echo "You entered a negative number!!"
else
    echo “You are not following my instructions.”
fi

6. <ne>

Syntax: INTEGER1 -ne INTEGER2
Description: INTEGER1 is not equal to INTEGER2
Example Code:

#!/bin/bash
read -p "Do not enter number 5 here : " n
if test $n -ne 5
then
    echo "Thanks for not entering number 5."

else

    echo “You are not following my instructions.”
fi

Q-12. What is the syntax for different types of loops available in shell scripting?

Ans.

Following are the loops supported in shell scripting.

1. <for loop>.

#!/bin/bash

for i in $( ls ); do
    echo item: $i
done

2. <while loop>.

#!/bin/bash

COUNT=0
while [ $COUNT -lt 10 ]; do
    echo The counter value is $COUNT
    let COUNT+=1
done

3. <until loop>.

#!/bin/bash

COUNT=20
until [ $COUNT -lt 10 ]; do
    echo The counter value is $COUNT
    let COUNT-=1
done

4. <select loop>.

#!/bin/bash

PS3="Select a week day (1-7): "
select i in mon tue wed thur fri sat sun exit
do
  case $i in
     mon) echo "Monday";;
     tue) echo "Tuesday";;
     wed) echo "Wednesday";;
     thur) echo "Thursday";;
     fri) echo "Friday";;
     sat) echo "Saturday";;
     sun) echo "Sunday";;
     exit) exit;;
  esac
done

Q-13. How will you find the sum of all numbers in a file in Linux?

Ans.

See also How to Use Bash to Replace Character in String

Following is the script which computes the sum of all numbers stored in a file.

#!/bin/bash

awk '{x+=$0}END{print x}' filename

Q-14. Write a shell script to delete the lines containing a word <dd> if it appears between the 5th and 7th position.

Ans.

Let’s take a fixed-width file as:

ff1 ff2 ff3
sds dd fd
sd ss ew
dd dd se

The expected output after script execution is.

ff1 ff2 ff3
sd ss ew

The test script to do the expected task is as follows.

for i in 'cat test.txt'
do
    j = 'expr length $i'
    if [[ ( "$j" == 5 || "$j" == 7 ) ]]
    then
        v = 'echo $i | grep dd'
        if [ "$v" != "" ]
        then
            echo $v
            sed -i "s/$v/lalat/g" test.txt
        fi
    fi
done

Q-15. Find the unique words in a file and also count the occurrence of each of these words.

Ans.

$ cat animal.txt
tiger bear
elephant tiger bear
bear

The following test script/command will count the unique words.

$ awk '{for(i=1;i<=NF;i++)a[$i]++;}END{for(i in a){print i, a[i];}}' animal.txt

Output:
elephant 1
bear 3
tiger 2

Q-16. Get the total count of the word “Linux” in all the “.txt” files and also across files present in subdirectories.

Ans.

The following is the test script/command which recursively searches for the “.txt” files and returns the total occurrences of a word <Linux>.

$ find  . -name *.txt -exec grep -c Linux '{}' \; | awk '{x+=$0;}END{print x}'

Q-17. Write a shell script to validate password strength.

Here are a few assumptions for the password string.

Length – minimum of 8 characters.
Contain both alphabet and number.
Include both the small and capital case letters.

If the password doesn’t comply with any of the above conditions, then the script should report it as a <Weak Password>.

Ans.

#Password Validation Script
echo "Enter your password"
read password
len="${#password}"

if test $len -ge 8 ; then
    echo "$password" | grep -q [0-9]
     if test $? -eq 0 ; then
           echo "$password" | grep -q [A-Z]
                if test $? -eq 0 ; then
                    echo "$password" | grep -q [a-z]   
                      if test $? -eq 0 ; then
                       echo "Strong Password"
                   else
                       echo "Weak Password -> Should include a lower case letter."
                   fi
            else
               echo "Weak Password -> Should include a capital case letter." 
            fi
     else
       echo "Weak Password -> Should use numbers in your password."   
     fi
else
    echo "Weak Password -> Password length should have at least 8 characters."
fi

Q-18. Write a shell script to print the count of files and subdirectories in the specified directory.

Ans.

#!/bin/bash

if [ -d "$@" ]; then
    echo "Files found: $(find "$@" -type f | wc -l)"
    echo "Folders found: $(find "$@" -type d | wc -l)"
else
    echo "[ERROR] Please retry with a folder."
    exit 1
fi

Q-19. Print the reverse of an input number using a script.

Ans.

#!/bin/bash

if [ $# -eq 1 ]; then
    if [ $1 -gt 0 ]; then
        num=$1
        revNum=0
        while [ $num -ne 0 ]
        do
            testnum=$(( $num %  10 ))
            revNum=$(( $revNum * 10 + $testnum ))
            num=$(( $num / 10 ))
        done
        echo "Reverse Number:  $revNum of $1"
    else
        echo "Input is less than 0, retry with a different number."
    fi
else
    echo "ERROR: Retry with one parameter."
fi

Q-20. Reverse the list of strings and reverse each string further in the list.

Ans.

#!/bin/bash

if [ $# != 0 ]; then
    len=`echo $@ | wc -c`
    len=`expr $len - 1`
    strrev=""
    while test $len -gt 0
    do

Summary – Essential Shell Scripting Questions and Answers

We are hopeful th strrev1=`echo $@ | cut -c$len` strrev=$strrev$strrev1 len=`expr $len - 1` done echo $strrev else echo "ERROR: Retry with a list of strings." fi

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