Linux commands Interview Questions

Basics

How to find the no of arguments passed

$# --> will give the no of arguments passed while invoking the shellscript

How to get the name of the shell script file inside the script

$0 will give the script name

How to know whether the last command is executed successfully

$? --> will return the status code of the last command executed

How to get the last line from the file

tail -1 file_name (By default tail file_name will return last 10 lines of file, if we pass -3 it will show the last three lines)

How to get the first line from the file

head -1 file_name (By default tail file_name will return first 10 lines of file, if we pass -3 it will show the first 3 lines)

How to get third element from each line

awk '{print $3}' filename
ps -aux | awk '{print $3}'

How to write a function in shell script

#!/bin/sh

# Define your function here
Hello () {
  echo "Hello World $1 $2"
  return 10
}

# Invoke your function
Hello Zara Ali

# Capture value returned by last command
ret=$?  # For function it will return return_value, for others it will return status code
echo "Return value is $ret"

Intermediate:

The history command cannot be executed from bash shell script, but it will work in c shell

How would you compare the strings in script

The test command is used to compare the text strings. The text command compares text strings by comparing each character in each string

How to redirect both the standard output and standard error to the same location

command1 > out.txt 2> err.txt

what are the default permissions of the file when it is created

-rw-r--r-- 1

Determine the output of the command

echo ${new:-variable}
output: variable

Here new is the variable name, if this variable name is already defined, it will take that value, else it will pass this default value “variable” to the
variable name “new”

How to get part of the string with echo command only

echo ${variable_name:2:6}

where 2 is the staring index and 6 is the count of characters after the starting index

Rewrite the command to print the sentence and convert the variable to plural

variable="plane"; echo "I like ${variable}s"

OUTPUT:

I like planes

To print the first array element

#!/bin/bash
array={"Hi", "my", "name", "is", "vignesh"}
echo ${array[0]}

To print all elements of array and its index

echo ${array[@]} — >Printing all array elements
echo ${!array[@]} –> printing array index

Experienced

what is crontab

The crontab is a list of commands that you want to run on a regular schedule and also the name of the command used to manage the list.

Crontab stand for “cron table” because it uses the job scheduler cron to execute the tasks
The schedule is called the crontab, which is also the name of the program used to edit that schedule.

How many fields are present in a crontab file and what does each field specify?

The crontab file has fix fields,

The first five fields tell cron when to execute the command : minute(0-59), hour(0-23, day(1-31),)

month(1-12), and the day of the week (0-6) 0-sunday
The sixth field contains the command to be executed

what are the two files of crontab command ?

cron.allow – It decides which users need to be permitted from using the crontab command

cron.deny – It decides which users need to be prevented from using the crontab command

Redhat/centos --> /var/spool/cron directory<br>DEbian/ubuntu --> /cron/crontab directory

what command needs to be used to take the backup?

tar -> tape archive
tar -czvf folder_name -> for compressing and creating the package for particular folder
tar -xzvf package_name -> for extracting the package

What are the different commands available to check the disk usage

df – This command is used to check the free disk space
du – This command is used to check the directory wise disk space
dfspace – This command is used to check the free disk space in term of MB

what are the different communication commands available in the shell?

mail – which will call the mail binary to send and read mails
news – which is used to read the system wide news to standard output
wall(write to all) – It is used to send out shutdown messages to all users
motd(message of the day) –

How to find out the total disk space used by a specific user

du -s /home/user_name

How to debug the problems encountered in shell script/program?

By adding logs
By set -x

what is the difference between = and == ?

= -> used for assigning value to the variable
== -> used for string comparison

The = operator in test command or within [] will used for comparison

How to open a read only file in a shell

vi -R file_name

How can the contents of a file inside the jar be read without extracting in a shell script?

tar -tvf package_name.tar

t -> table 0f contents
v -> verbose
f -> file

How can you pass parameters to the shell script ?

./script.sh param1 param2

How to find all the files modified in less than 3 days and print the record count odf each?

find . -type f -mtime -3 -exec ls -l {} ; > last3days.txt

-type f –> search only for files
-mtime -3 –> for last three days

Throw a error message if 2 arguments are not passed

if [ $# -ne 2]
    echo "Enter two parameters"

Here $# will get the no of parameters passed

Calculate a real number calculation directly from terminal and not any shell script

echo 7.56 + 4.432 | bc

How can you get the value of pi till a 100 decimal places

echo “scale=100; 4*a(1)” | bc -l

For debugging use set -x

will print the actual script and also the errors in the script

Convert this format using shell script (YYYYMMDD -> 1st jan 2021)

date +'%d %b %Y'
12 Jan 2021