Linux Commands Interview questions

1.What is Linux and Why use it ?

Linux is an open-source operating system whose source code is available for modification and commercial and non-commercial distribution under the guidelines of the GNU General Public License.

Linux has a number of advantages over the Windows operating system and is used widely because of these advantages Below are a few listed:

• The major advantage of the Linux Operating System is that it is a freely available open-source OS.
• Privacy and Security are very important for the users, especially nowadays, when hacking and other cyber malpractices have become so common. Linux maintains the privacy of the user and is a lot more secure than Windows OS.
• Linux Operating System is highly stable and does not hang or requires to be restarted again and again.
• Linux is fast and easy to install and can be installed very easily from the internet.
• Linux is network friendly.

2.File and Directory CRUD Navigation Commands

ls(list all directories)
# list all the files and directories inside the current directory in which you are.
 
ls -R
# List all the files and directories inside the current directory as well as all the files and directories of the sub-directories as well.
 
ls -a 
# List all the files and directories in the current directory and also lists the hidden files.
 
ls -al
# Lists files and directories of the current directory along with the details like permissions(read,write,execute),owner,file/dir size,etc.
 
cd 
# This command is used to move to the root directory
 
cd ~
# move to the root/home directory.
 
cd dirName
# Move to a particular directory from the current directory.
 
mkdir
# This command creates a directory
 
cat > fileName
# This command creates a file in the current directory.
 
cat fileName
# This command displays the content in a file.
 
cat f1 f2 > f3
# This command joins the content of two files and store it in the third file.
 
rmdir dirName
# This command is the remove directory command. It deletes a directory
 
mv fileName "new file path"
# This command is the move file command. It moves the file to the new path specified.
 
mv fileName newName
# This command changes the name of the file from the old name.
 
find    
# This command is used for walking a file hierarchy. It is used to find #files/directories and perform operations on them.
 
grep  pattern fileName
# The full form of this command is a global search for regular expression and  
# printout. This command searches a file for a particular pattern of characters and
# displays all the lines that contain that pattern. The pattern being searched is   
# called a regular expression (regex). There are a lot of  available.

2. System Information Commands

$ history
$ # This command displays the list of all the typed commands in the current terminal session.
$ clear
$ # Clear the terminal screen
$ hostname
$ # shows the name of the system host.
 
$ hostid
$ # Displays the id of the host of the system.
 
$ sudo
$ # Allows  a regular user to run the programs with the security privileges of a superuser or root
 
$ apt-get  # This command is used to install and add new packages.
 
$ date  # This command is used to show the current date and time.
 
$ cal  # Show the calendar of the current month.
 
$ whoami  # This command displays the name with which you are logged in.
 
$ whereis [options] fileName # This command is used to find the location of source/binary file of a command and manuals sections for a specified file in Linux System.

3. File Permission Commands

There are 3 types of people who can use a file and each type has 3 types of access to the file.

The diagram shows that there are 3 types of people accessing a file and they are:

1. User (u)
2. Group (g)
3. Others (o)

Also, the access that we want to give to each of them is of three types:

1. Read (r)
2. Write (w)
3. Execute (x)

• Numerical Method for granting file permissions

There are numeric codes for each permission. They are as follows:

1. r (read) = 4
2. w (write) = 2
3. x (execute) = 1
4. No permissions  = 0

4.Hardware Information Commands.

$ cpu-info # This command is used to display the information about your CPU
$ free -h   # This command is used to display the free and used memory. -h is used for converting the information(to be displayed) to human-readable form.
$ lsusb -tv # List all the USB connected devices.
$
$ du # This command stands for disk usage and is used to estimate the space usage for a file or directory.
$

5.File and Directory Compression Commands

gzip fileName
 
# This command is used to compress a file with gzip compression.
 
gunzip fileName.gz
 
# This command is used to unzip a file that has gzip compression.
 
tar cf myDir.tar myDir
 
# This command is used to create an uncompressed tar archive.
 
tar cfz myDir.tar myDir
 
# This command is used to create a tar archive with gzip compression
 
tar xf file
 
# This command is used to extract the contents of any type of tar archive.
 

6. Enviroment Variable Commands

env 
# This command displays all the environment variables.
 
echo $variable
 
# This command displays the environment variable.
 
unset
 
# This command removes a variable.

7.User Management Commands

sudo adduser username
# This command is used to add a user
 
sudo passwd -l 'username'
# This command is used to change the password of a user.
 
sudo userdel -r 'username'
# This command is used to remove a newly created user.
 
sudo usermod -a -G GROUPNAME USERNAME
# This command is used to add a user to a particular group.
 
Sudo deluser USER GROUPNAME
# This command is used to remove a user from a group.
 
finger
# This command shows the information of all the users logged in.
 
finger username
 
# This command gives information about a particular user.

8. Networking Commands

dir 
 
# This command is used to display files in the current directory of a remote computer.
 
put file
 
# This command is used to upload 'file' from local to the remote computer.
 
get file
 
# This file is used to download 'file' from remote to the local computer.
 
quit
 
# This command is used to log out.
 

9. Process Commands

bg
 
# This command is used to send a process to the background.
 
fg
 
# This command is used to run a stopped process in the background
 
top 
# This command is used to get the details of all active processes.
 
ps 
# This command is used to give the status of running for a user.
 
ps PID 
# This command gives the status of a particular process.
 
pidof 
 
# This command is used to give the process ID of a particular process.