An Introduction to the Linux Terminal and CLI
This guide is streamlined for complete beginners in the Linux community to get on their feet and get started with the terminal, use the Linux command line (LCI), and execute commands. To become a hacker at all, especially if you intend to maximize on the tools that the Linux Kali offers, you must be good with the Linux command line. The standard way to interact with the Linux service is via the command line.
Learning the LCI may seem like a daunting task, but it is pretty easy if you master the basics and begin developing the skill on your own from there. This is why this guide, rather than present you a list of ready commands to enter into the terminal and get instant results, focuses more on describing the different elements of the terminal, how and why they work, and what you will use them for. Let’s start.
3.1 The Terminal Emulator
A terminal emulator is essentially a program that enables you to use the terminal in a graphical user environment. Most people who are introduced to Linux are already familiar with other operating systems, mainly Microsoft Windows and Apple’s Mac OSX which use a full graphical user interface to interact with the user in the day-to-day computer operations.
They, too, however, have terminal emulators you may have heard of: Terminal (default) and iTerm 2 in Mac OSX; and PuTTY in Windows. In Linux, it is necessary that you use the terminal emulator such as the default Terminal, XTerm, or KDE Konsole.
Each of these terminal emulators has its set of unique and shared features, and they are all great to work with and easy to use as you will learn.
3.2 The Shell
The shell in the Linux system is a command-line interface that interprets the commands and scripts that the user enters, then tells the operating system what to do with them. There are many shells that you will get acquainted with such as the Bourne shell (sh) and the C shell (csh). Each shell in Linux has its sets of features and intricacies when it comes to how they interpret commands, but they all share certain features: they all have input and output redirection, condition and variable testing, etc.
The default shell for most Linux distros including CentOS, Ubuntu, and RedHat is the Bourne shell, also known as bash, which is also what is used in this ebook. I should point out that this is also the default shell for
3.3 The Command Prompt
When you start the Terminal application on Linux, you will be taken to the command prompt or shell prompt, which is where you can enter the commands to issue to the computer. The information presented by the command prompt can differ depending on the version and distribution of Linux or how the user chooses to customize it. Typically, Ubuntu-based distributions have a command prompt in this format:
username@hostname~$
The tilde symbol (~) represents the current directory in bash, the default Ubuntu shell. This special character expands to the entire path of the current user’s directory which in this case would be /home/username.
The prompt symbol ($) denotes the end of the command prompt after which the user’s keyboard input cursor will appear. However, when you log into the command prompt as a superuser, the $ symbol will be replaced by #. Try entering this in your command prompt after the $ symbol then presses enter:
username@hostname~$ su
You will be prompted to enter your root password because you entered a command to switch to a superuser with root access. Once your enter your password, the command prompt will change to something like this:
hostname username #
The symbol (#) is the standard symbol for root. The root is a superuser account in Linux, which means it is a special account that can perform system-wide administrative functions and must be used with caution. This is a free user account with privileges to perform any task. It is recommended that you do not switch to a superuser account unless it is necessary. Instead, you can use the sudo command that we will learn in a short while.
3.4 Executing Commands
Commands are executed by the Shell when you specify the name of an executable file in the form of a binary program or a script file. There are thousands of standard Linux commands and utilities that come with the operating system that you can use to navigate the file system, install and configure software packages, and run the system and third-party applications.
A running instance of an executed command is called a process. When you execute a command in the foreground, which is the default state in which commands are executed, you must wait for the process to complete before you can revert to the command prompt to issue a new command.
It is important to understand that almost all commands in Linux, including file and directory names, arguments, and options are case sensitive. If you enter a command you know right, and it does now cause the effect you expected, the first thing you should do is double check the spelling and the letters case.
3.4.1 Execute commands without arguments or options
You can run a command without any arguments or options by simply typing the name of the command then hitting return. Running a command like this causes it to exhibit its default behavior, which is dependent on the command. e.g.
Cd will bring your back to the current user’s home directory.
ls will print a list of the directories and files in the current working directory.
Ip will print how to use the ip command.
3.4.2 Execute commands with arguments
Many commands in Linux accept arguments, also called parameters, which have a direct impact on the behavior of the command. e.g.
cd /usr/bin Argument specifies which directory to change to, in this case, bin directory inside usr directory. ls /usr/bin Prints a list of directories and files in the directory /usr/bin.
3.4.3 Execute commands with options
Most commands accept options, also referred to as switches or flags. These options modify the behavior of the command. Options follow a command and are indicated by one - character preceding one or more options represented by single uppercase or lowercase letters. Note, however, that some options start with -- followed by one character or a multi-character word that is typically descriptive of what the option does e.g.
ls -a Prints a listing of directories and files including any hidden ones. ls -l Prints a long listing of files and directories including extra details. ls -l -a Prints a long listing of files and directories including hidden ones.
As you can see in the last example above, options can be grouped together. An alternative way to run the last command above is to combine options like this:
ls -la
3.4.4 Execute commands with options and arguments
As you get familiar with Linux, one thing you will quickly notice is that you can almost always combine options and arguments when running commands on the terminal. For instance, it is easy for you to check the contents of a directory regardless of the present working directory by just running this command ls -la /home
In this ls command, -la are the options while the /home is the argument that points to the file or directory that the command can refer to.
3.5 More comands
This chapter introduces the basics of the Linux terminal and is in no way exhaustive. However, the foundation laid in this hour should help you expand your knowledge of Linux commands, how to navigate view, and edit files and their permissions － and to become a formidable hacker.
Here is a summary of some of the most popular commands you can practice to reinforce the idea of how commands work in Linux: cd Change to the home directory. Also used to navigate the Linux file system. pwd (print working directory) Shows the path of the current working folder.
ls Print a list of directories and folders in the pwd. Items are color coded such that: Blue are directories, White are text files, Red are archives, Cyan are Links, Green are Executable files, and Pink are image files.
cat Displays the contents of a file. It must have a filename as an argument.
touch Creates a new file. It requires a filename as an argument.
mv Renames a file e.g. mv oldname.txt newname.txt.
cp Copies a file from one directory to another. Requires source and destination arguments.
rm Removes (Deletes) a file. Command expects filename as the argument.
find Use this command to search for files within a directory. Requires directory path as well as filename as arguments.
lsblk (List Block Devices) Prints block devices by their assigned names in a tree-like style on the standard output.
uname (Unix Name) prints detailed information about your machine including machine name, OS, and Kernel.
history (Event Record) prints a long history of commands executed in the terminal in the past.
sudo (superuser do) allows a permitted user to execute commands with superuser privileges.
mkdir (Make directory) create a new folder inside working directory.
Comments
Post a Comment