Linux

 

Learn basic commands for Linux

What Is Linux?

Linux is an operating system's kernel. You might have heard of UNIX. Well, Linux is a UNIX clone. But it was actually created by Linus Torvalds from Scratch. Linux is free and open-source, that means that you can simply change anything in Linux and redistribute it in your own name! There are several Linux Distributions, commonly called “distros”.Ubuntu Linux

  • Red Hat Enterprise Linux
  • Linux Mint
  • Debian
  • Fedora

Linux is Mainly used in servers. About 90% of the internet is powered by Linux servers. This is because Linux is fast, secure, and free! The main problem of using Windows servers are their cost. This is solved by using Linux servers. The OS that runs in about 80% of the smartphones in the world, Android, is also made from the Linux kernel. Most of the viruses in the world run on Windows, but not on Linux!

Tips and Tricks for Using Linux Command Line

  • You can use the clear command to clear the terminal if it gets filled up with too many commands.
  • TAB can be used to fill up in terminal. For example, You just need to type “cd Doc” and then TAB and the terminal fills the rest up and makes it “cd Documents”.
  • Ctrl+C can be used to stop any command in terminal safely. If it doesn't stop with that, then Ctrl+Z can be used to force stop it.
  • You can exit from the terminal by using the exit command.
  • You can power off or reboot the computer by using the command sudo halt and sudo reboot.

1. pwd command

When you first open the terminal, you are in the home directory of your user. To know which directory you are in, you can use the “pwd” command. It gives us the absolute path, which means the path that starts from the root. The root is the base of the Linux file system. It is denoted by a forward slash( / ). The user directory is usually something like "/home/username".

2. ls command

 Use the "ls" command to know what files are in the directory you are in. You can see all the hidden files by using the command “ls -a”.

3. cd command

Use the "cd" command to go to a directory. For example, if you are in the home folder, and you want to go to the downloads folder, then you can type in “cd Downloads”. Remember, this command is case sensitive, and you have to type in the name of the folder exactly as it is. But there is a problem with these commands. Imagine you have a folder named “Raspberry Pi”. In this case, when you type in “cd Raspberry Pi”, the shell will take the second argument of the command as a different one, so you will get an error saying that the directory does not exist. Here, you can use a backward slash. That is, you can use “cd Raspberry\ Pi” in this case. Spaces are denoted like this: If you just type “cd” and press enter, it takes you to the home directory. To go back from a folder to the folder before that, you can type “cd ..” . The two dots represent back.

4. mkdir & rmdir command

Use the mkdir command when you need to create a folder or a directory. For example, if you want to make a directory called “DIY”, then you can type “mkdir DIY”. Remember, as told before, if you want to create a directory named “DIY Hacking”, then you can type “mkdir DIY\ Hacking”. Use rmdir to delete a directory. But rmdir can only be used to delete an empty directory. To delete a directory containing files, use rm.

5. rm command

Use the rm command to delete files and directories.  Use "rm -r" to delete just the directory. It deletes both the folder and the files it contains when using only the rm command.

6. touch command

The touch command is used to create a file. It can be anything, from an empty txt file to an empty zip file. For example, “touch new.txt”.

7. man & --help command

To know more about a command and how to use it, use the man command. It shows the manual pages of the command. For example, “man cd” shows the manual pages of the cd command. Typing in the command name and the argument helps it show which ways the command can be used (e.g., cd –help).

8. cp command

Use the cp command to copy files through the command line. It takes two arguments: The first is the location of the file to be copied, the second is where to copy.

9. mv command

Use the mv command to move files through the command line. We can also use the mv command to rename a file. For example, if we want to rename the file “text” to “new”, we can use “mv text new”. It takes the two arguments, just like the cp command.

10. locate command

The locate command is used to locate a file in a Linux system, just like the search command in Windows. This command is useful when you don't know where a file is saved or the actual name of the file. Using the -i argument with the command helps to ignore the case (it doesn't matter if it is uppercase or lowercase). So, if you want a file that has the word “hello”, it gives the list of all the files in your Linux system containing the word "hello" when you type in “locate -i hello”. If you remember two words, you can separate them using an asterisk (*). For example, to locate a file containing the words "hello" and "this", you can use the command “locate -i *hello*this”.

11. alias command

alias is a useful shell built-in command for creating aliases (shortcut) to a Linux command on a system. It is helpful for creating new/custom commands from existing Shell/Linux commands (including options):

$ alias home='cd /home/tecmint/public_html'

12. arp command

ARP (Address Resolution Protocol) is a protocol that maps IP network addresses of a network neighbor with the hardware (MAC) addresses in an IPv4 network.

You can use it as below to find all alive hosts on a network:

$ sudo arp-scan --interface=enp2s0 --localnet

13. clear command

clear command lets you clear the terminal screen, simply type.

$ clear

14. cmp command

cmp performs a byte-by-byte comparison of two files like this.

$ cmp file1 file2

15. history command

history command is used to show previously used commands or to get info about command executed by a user.

$ history

16. echo command

The "echo" command helps us move some data, usually text into a file. For example, if you want to create a new text file or add to an already made text file, you just need to type in, “echo hello, my name is alok >> new.txt”. You do not need to separate the spaces by using the backward slash here, because we put in two triangular brackets when we finish what we need to write.

17. cat command

Use the cat command to display the contents of a file. It is usually used to easily view programs.

18. nano, vi, jed command

nano and vi are already installed text editors in the Linux command line. The nano command is a good text editor that denotes keywords with color and can recognize most languages. And vi is simpler than nano. You can create a new file or modify a file using this editor. For example, if you need to make a new file named "check.txt", you can create it by using the command “nano check.txt”. You can save your files after editing by using the sequence Ctrl+X, then Y (or N for no). In my experience, using nano for HTML editing doesn't seem as good, because of its color, so I recommend jed text editor. We will come to installing packages soon.

19. sudo command

A widely used command in the Linux command line, sudo stands for "SuperUser Do". So, if you want any command to be done with administrative or root privileges, you can use the sudo command. For example, if you want to edit a file like viz. alsa-base.conf, which needs root permissions, you can use the command – sudo nano alsa-base.conf. You can enter the root command line using the command “sudo bash”, then type in your user password. You can also use the command “su” to do this, but you need to set a root password before that. For that, you can use the command “sudo passwd”(not misspelled, it is passwd). Then type in the new root password.

20. df command

Use the df command to see the available disk space in each of the partitions in your system. You can just type in df in the command line and you can see each mounted partition and their used/available space in % and in KBs. If you want it shown in megabytes, you can use the command “df -m”.


21. du command

Use du to know the disk usage of a file in your system. If you want to know the disk usage for a particular folder or file in Linux, you can type in the command df and the name of the folder or file. For example, if you want to know the disk space used by the documents folder in Linux, you can use the command “du Documents”. You can also use the command “ls -lah” to view the file sizes of all the files in a folder.

22. tar command

Use tar to work with tarballs (or files compressed in a tarball archive) in the Linux command line. It has a long list of uses. It can be used to compress and uncompress different types of tar archives like .tar, .tar.gz, .tar.bz2,etc. It works on the basis of the arguments given to it. For example, "tar -cvf" for creating a .tar archive, -xvf to untar a tar archive, -tvf to list the contents of the archive, etc. Since it is a wide topic,

  Extract a tar.gz archive

# tar -xvzf bigfile.tar.gz

Create a tar/tar.gz archive

# tar -cvf Videos.tar ./Latest/

23. zip, unzip command

Use zip to compress files into a zip archive, and unzip to extract files from a zip archive.

Install Zip on Ubuntu and Debian

#sudo apt install zip

Install Zip on CentOS and Fedora

#sudo yum install zip

Syntax:

#zip [options] zipfile files_list

Syntax for  Creating a zip file:

$zip myfile.zip filename.txt

Unzip

$unzip myfile.zip

24. uname command

Use uname to show the information about the system your Linux distro is running. Using the command “uname -a” prints most of the information about the system. This prints the kernel release date, version, processor type, etc.

25. apt-get command

Use apt to work with packages in the Linux command line. Use apt-get to install packages. This requires root privileges, so use the sudo command with it. For example, if you want to install the text editor jed (as I mentioned earlier), we can type in the command “sudo apt-get install jed”. Similarly, any packages can be installed like this. It is good to update your repository each time you try to install a new package. You can do that by typing “sudo apt-get update”. You can upgrade the system by typing “sudo apt-get upgrade”. We can also upgrade the distro by typing “sudo apt-get dist-upgrade”. The command “apt-cache search” is used to search for a package. If you want to search for one, you can type in “apt-cache search jed”(this doesn't require root).

26. chmod command

Use chmod to make a file executable and to change the permissions granted to it in Linux. Imagine you have a python code named numbers.py in your computer. You'll need to run “python numbers.py” every time you need to run it. Instead of that, when you make it executable, you'll just need to run “numbers.py” in the terminal to run the file. To make a file executable, you can use the command “chmod +x numbers.py” in this case. You can use “chmod 755 numbers.py” to give it root permissions or “sudo chmod +x numbers.py” for root executable. 

27. hostname command

Use hostname to know your name in your host or network. Basically, it displays your hostname and IP address. Just typing “hostname” gives the output. Typing in “hostname -I” gives you your IP address in your network.

28. ping

Use ping to check your connection to a server. Wikipedia says, "Ping is a computer network administration software utility used to test the reachability of a host on an Internet Protocol (IP) network". Simply, when you type in, for example, “ping google.com”, it checks if it can connect to the server and come back. It measures this round-trip time and gives you the details about it. The use of this command for simple users like us is to check your internet connection. If it pings the Google server (in this case), you can confirm that your internet connection is active!

29. wget command

You can use the wget command if you want to download a file from a server to your local computer:

The command will attempt to download the resource you supplied and it will automatically detect the file extension. For example: If you point it to a website, it’ll download an HTML file. If you’re transferring a large file, you can simply log out and the tool will finish the task in the background.

30. ssh command

SSH command is used to login into remote host. For example the below ssh command will connect to remote host (192.168.50.2) using user as narad.

# ssh narad@192.168.50.2

To check the version of ssh use option -V (uppercase) shows version of ssh.

# ssh -V 
OpenSSH_5.3p1, OpenSSL 1.0.0-fips 29 Mar 2010

31. service Command

Service command call script located at /etc/init.d/ directory and execute the script. There are two ways to start the any service. For example, we start the service called httpd with service command.

# service httpd start
OR
# /etc/init.d/httpd start

he basic syntax of Linux service is:

$ service [Script] [Action] [Option]

If you don’t specify the “[Action]” parameter, you’ll see all allowed actions in the command line.

What options and actions does Linux service have?

Strictly speaking, Linux service only has the default options “-h” or “–help”, “-v” or “–version”, and “-status-all”. However, you can specify the command with various actions. These are the most important ones:

  • start: Starts the script.
  • stop: Stops the script.
  • status: Shows the current status of the script.
  • restart: Ensures that the script is restarted.

Examples of using the Linux service command

To help you understand how the service command is used in practice, we’ll conclude with some examples of its main use.

$ sudo service sshd start

This starts the sshd script.

$ sudo service sshd stop

This forces the sshd script to stop.

$ sudo service sshd restart

This command restarts the corresponding script.

$ sudo service sshd status

This command is used to query the status of the script.

32. tar Command

tar command is used to compress files and folders in Linux. For example the below command will create a archive for /home directory with file name as archive-name.tar.

# tar -cvf archive-name.tar /home

To extract tar archive file use the option as follows.

# tar -xvf archive-name.tar

33. grep Command

grep search for a given string in a file. Only tecmint user displays from /etc/passwd file. we can use -i option for ignoring case sensitive.

# grep tecmint /etc/passwd
 
tecmint:x:500:500::/home/tecmint:/bin/bash

34. yum command (RHEL/CentOS/Fedora)

The Yellowdog Updater, Modified (YUM) is a free and open-source command-line package-management utility for computers running the Linux operating system using the RPM Package Manager.

RPM Package Manager (RPM) (originally Red Hat Package Manager, now a recursive acronym) is a free and open-source package management system.[5] The name RPM refers to .rpm file format and the package manager program itself. RPM was intended primarily for Linux distributions; the file format is the baseline package format of the Linux Standard Base.

YUM can perform operations such as:

· installing packages

· deleting packages

· updating existing installed packages

· listing available packages

· listing installed packages

Install a Package with YUM

# yum install firefox   

Removing a Package with YUM

# yum remove firefox

Updating a Package using YUM

# yum update mysql

List a Package using YUM

# yum list openssh

Search for a Package using YUM

# yum search vsftpd

Get Information of a Package using YUM

# yum info firefox

List all Available Packages using YUM

# yum list | less

List all Installed Packages using YUM

# yum list installed

Install a Group Packages

# yum groupinstall 'MySQL Database'

35. ifconfig command

ifconfig command is used to configure a Linux systems network interfaces. It is used to configure, view and control network interfaces.

$ ifconfig
$ sudo ifconfig eth0 up
$ sudo ifconfig eth0 down
$ sudo ifconfig eth0 172.16.25.125

36. ip command

ip command is used to display or manage routing, devices, policy routing and tunnels. It also works as a replacement for well known ifconfig command.

This command will assign an IP address to a specific interface (eth1 in this case).

$ sudo ip addr add 192.168.56.10 dev eth1

37. last command

last command display a listing of last logged in users.

$ last

38. login command

login command is used to create a new session with the system. You’ll be asked to provide a username and a password to login as below.

$ sudo login

39. passwd command

passwd command is used to create/update passwords for user accounts, it can also change the account or associated password validity period. Note that normal system users may only change the password of their own account, while root may modify the password for any account.

$ passwd tecmint

40. reboot command

reboot command may be used to halt, power-off or reboot a system as follows.

$ reboot

41. shutdown command

shutdown command schedules a time for the system to be powered down. It may be used to halt, power-off or reboot the machine like this.

$ shutdown --poweroff

42. su command

su command is used to switch to another user ID or become root during a login session. Note that when su is invoked without a username, it defaults to becoming root.

$ su 
$ su tecmint

43. users command

users command shows the user names of users currently logged in to the current host like this.

$ users

44. ping command

ping command is used to determine connectivity between hosts on a network (or the Internet):

$ ping google.com

45. nslookup command

nslookup (name server lookup) is a tool used to perform DNS lookups in Linux. It is used to display DNS details, such as the IP address of a particular computer, the MX records for a domain or the NS servers of a domain.


46. systemctl command

How To Use Systemctl to Manage Systemd Services and Units?

systemd is an init system and system manager that has widely become the new standard for Linux distributions. we will be discussing the systemctl command, which is the central management tool for controlling the init system.

Starting and Stopping Services

To start a systemd service, executing instructions in the service’s unit file, use the start command. If you are running as a non-root user, you will have to use sudo since this will affect the state of the operating system:

$ sudo systemctl start application.service

NB: application means httpd, mysqld, sshd etc

As we mentioned above, systemd knows to look for *.service files for service management commands, so the command could just as easily be typed like this:

$ sudo systemctl start application

Although you may use the above format for general administration, for clarity, we will use the .service suffix for the remainder of the commands, to be explicit about the target we are operating on.

To stop a currently running service, you can use the stop command instead:

$ sudo systemctl stop application.service

Enabling and Disabling Services

The above commands are useful for starting or stopping services during the current session. To tell systemd to start services automatically at boot, you must enable them.

To start a service at boot, use the enable command:

$ sudo systemctl enable application.service

To disable the service from starting automatically, you can type:

$ sudo systemctl disable application.service

This will remove the symbolic link that indicated that the service should be started automatically.

Keep in mind that enabling a service does not start it in the current session. If you wish to start the service and also enable it at boot, you will have to issue both the start and enable commands.

Checking the Status of Services

To check the status of a service on your system, you can use the status command:

$ systemctl status application.service

Listing Current Units

Units are the objects that systemd knows how to manage.  To see a list of all of the active units that systemd knows about, we can use the list-units command:

$ systemctl list-units

We can tell systemctl to output different information by adding additional flags. For instance, to see all of the units that systemd has loaded (or attempted to load), regardless of whether they are currently active, you can use the --all flag, like this:

$ systemctl list-units --all


To list all loaded services on your system (whether active; running, exited, or failed, use the list-units subcommand and --type switch with a value of service.

$ systemctl list-units --type=service



47. man command

man command in Linux is used to display the user manual of any command that we can run on
the terminal. It provides a detailed view of the command which includes NAME, SYNOPSIS,
DESCRIPTION, OPTIONS, EXIT STATUS, RETURN VALUES, ERRORS, FILES, VERSIONS, EXAMPLES,
AUTHORS and SEE ALSO.
Every manual is divided into the following sections:
 Executable programs or shell commands
 System calls (functions provided by the kernel)
 Library calls (functions within program libraries
 Games
 Special files (usually found in /dev)
 File formats and conventions eg /etc/passwd
 Miscellaneous (including macro packages and conventions),
 System administration commands (usually only for root)
 Kernel routines [Non standard]
Syntax :
$man [OPTION]... [COMMAND NAME]...
Options and Examples
No Option: It displays the whole manual of the command.
Syntax :
$ man [COMMAND NAME]


48. uptime command

The uptime command in Linux is a simple yet powerful tool that provides a snapshot of your system’s activity and performance. When you type uptime into your terminal and press Enter, the command returns four key pieces of information: the current time, how long the system has been running, how many users are currently logged in, and the system load averages.

$ uptime

# Output:
# 15:30:00 up 10 days,  8:00,  2 users,  load average: 0.05, 0.04, 0.05

In this example, the uptime command is showing:

  • The current system time (15:30:00)
  • The system’s uptime (10 days and 8 hours)
  • The number of users currently logged in (2 users)
  • The system load averages (0.05, 0.04, 0.05)

49. netstat command

`netstat` stands for network statistics. It allows users to display network-related information and diagnose various networking issues. The command has several options that can be combined to retrieve specific details.

Basic Syntax of `netstat`Command in Linux

Below is the general syntax of the netstat command:

netstat [options]

50. route command

The route command is used to view and make changes to the kernel routing table. running route at the command line without any options displays the routing table entries:


$ route

Kernel IP routing table

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface

default         192.168.1.2     0.0.0.0         UG    1024   0        0 eth0

192.168.1.0     *               255.255.255.0   U     0      0        0 eth0

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