In linux what is command line interface

Linux Command Line Tutorial: the commands you need to know

The Linux command line, also called a terminal, shell, or console, is a text interface for your computer. With this computer program, you can interpret commands and write your own scripts.

It may look intimidating at first, but once you get used to it, it is easy to use. This article will walk you through some of the basics of the Linux command line, along with some real-world examples. We assume you have little or no prior knowledge.

Each Linux distribution comes with a command line. This article uses Ubuntu 20.04, the commands should work with most distributions.

Here’s what we will be covering today:

Learn advanced BASH commands and scripts

This course is for anyone interested in understanding the BASH shell. You’ll learn all the intricacies of advanced BASH scripting.

What is the Linux Command Line?

The Linux terminal gives you an interface where you can input commands and see the result printed as text. You can use the terminal to accomplish tasks like moving files or navigating a directory, without the use of a GUI. Terminals just transfer information. You need a shell for the operating system to understand this information.

With the Linux command line, you can execute commands manually by typing on the terminal. You can also automate commands using Shell Scripts.

Brief History of Linux Command Line

Let’s briefly go through the Linux command line history. Back in the early days of computing, there was an operating system called Unix, which was designed to run on mainframe computers as a multi-user system. Users connected to it remotely through terminals.

These terminals were very basic, as they could only send keystrokes and display data on the screen.

Since text is light on resources, users could interact quickly and efficiently. Every task the users performed was through this textual interface. To coordinate the execution of different programs, users would connect to single master programs.

The user commands were wrapped in “shell” programs. To make complex tasks easier, users could write shell scripts that automated a long series of shell commands.

Linux is a descendant of Unix. Its core part is designed to behave similarly to a Unix system. The Bourne Again Shell (BASH) was developed by Stephen Bourne and released in 1979 in the Version 7 Unix release. Over time, it has become the default shell for Linux systems.

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Introduction

Even though Ubuntu is a newbie friendly and polished graphical distribution, there are still situations where a significant amount of time and mouse-clicking can be spared by typing a bit. I don’t think this is a bad thing at all; no matter what you do, Linux has one of its real strengths in the Command Line!

Prerequisites

This assumes that you are running any version of Ubuntu Linux and have a desire to learn its inner workings. We will proceed by briefly describing the command line interface and giving some history.

The impatient can move right on to the Command Syntax section.

What is it?

A Command Line is, in all simplicity, a user interface based on lines of commands. You can say that it is a textual direct serial processor. Most commonly, the user interacts directly with the computer by typing one line (although it can be more than one), which triggers actions from the computer based on the syntax of the current processor.

History

In the early days of computers, there was only the Command Line. The concept of a Graphical User Interface (GUI) after which most GUI are modeled was developed by engineers at Xerox’s Palo Alto Research Center (PARC). A bit later, Apple paid a whole bunch of money to be allowed to «study» their GUI idea. And, after a while, Apple had their own GUI.

Not until 1986 did UNIX get its first GUI, developed by the MIT Project. They named it X. Linux, however, had to wait ten more years before XFree86 was released. XFree86 was (and remains) a free adaptation of the original X server.

As mentioned earlier, the CLI (Command Line Interface) was the only way to communicate with computers before the GUI was invented. In 1969, Bell Telephone Laboratories released V1 of the UNIX Timeshare System. UNIX had a shell called sh, which was the only means of communicating with the computer, and it would stay that way for quite some time.

Later on, there came derivatives of UNIX: HP-UX, 1BSD, Solaris, OpenVMS, IRIX, SCO XENIX, etc. As time progressed, GNU/Linux emerged. However, the history of Linux itself is way off the scope of this HOWTO. Suffice to say that alternative CLI to sh emerged: zsh, ksh, bourne shell, etc.

POSIX

• POSIX is the collective name of a family of related standards specified by the IEEE to define the application program interface (API) for software designed to run on variants of the Unix OS. They are formally designated as IEEE 1003 and the international standard name is ISO/IEC 9945. The standards emerged from a project, begun circa 1985. The term POSIX was suggested by Richard Stallman in response to an IEEE request for a memorable name; before that the standards effort was called IEEE-IX. POSIX is a near acronym for Portable Operating System Interface, with the X signifying the Unix heritage of the API.

POSIX is the underlying standard and functionality of how your CLI responds.

Advantages of using the command line

• It can save you time.
• It can help when you are unable to use the GUI, such as a system crash or a configuration issue.
• It can enable you to use Linux in ways that using a GUI exclusively can not (such as scripting repetitive tasks).

For example, you have been called by the systems administrator that you have used too much space. You want to quickly work out where the most space is used, so using a graphical interface, start your timer — go. Now, go to a command line and type: du | sort -n (we will describe more later). See? It is faster to do some things on the command line (and other times, easier for graphical).

How to invoke it

Methods of launching a CLI can vary by desktop environment:

• Dash -> Search for Terminal
  Dash -> More Apps -> ‘See More Results’ -> Terminal
  Dash -> More Apps -> Accessories -> Terminal

An alternative way to invoke the command line, only using keyboard shortcuts (since on the command line, you would mostly be interacting only through the keyboard) is:

On Unity (Ubuntu): Ctl + Alt + T
On GNOME (Ubuntu): Alt + F2 -> (Type within the text box) gnome-terminal (Press return)
On KDE (Kubuntu): Alt + F2 -> (Type within the text box) konsole (Press return)

dud@shadowplay:~ $ or [dud@shadowplay ~]$

dud@shadowplay:~ $ ls file1.txt file2.pdf file3.mp3 file1.pdf another_file.txt Yet-Another_file.txt file-with_other-NAME.TXT

dud@shadowplay:~ $ cat file1.txt Roses are red. Violets are blue, and you have the bird-flu!

dud@shadowplay:~ $ ls -r file-with_other-NAME.TXT Yet-Another_file.txt another_file.txt file1.pdf file3.mp3 file2.pdf file1.txt

Using the previous example of showing the current directory, we have added the -r option. As you can see, the listing of the current working directory has been displayed in the reverse order.

dud@shadowplay:~ $grep red file1.txt ; grep blue file1.txt Roses are red, Violets are blue,

dud@shadowplay:~ $ grep red file1.txt && grep blue file1.txt Roses are red, Violets are blue, dud@shadowplay:~ $ grep purple file1.txt && grep blue file1.txt dud@shadowplay:~ $

dud@shadowplay:~ $ grep red file1.txt || grep blue file1.txt Roses are red, dud@shadowplay:~ $ grep purple file1.txt || grep blue file1.txt Violets are blue, dud@shadowplay:~ $

dud@shadowplay:~ $ ls file?.pdf file1.pdf file2.pdf

dud@shadowplay:~ $ ls file2.* file2.pdf file2.mp3

dud@shadowplay:~ $ ls file10.pdf file2.pdf file23.pdf

dud@shadowplay:~ $ ls > file4.txt dud@shadowplay:~ $ cat file4.txt file1.txt file2.pdf file3.mp3 file1.pdf another_file.txt Yet-Another_file.txt file-with_other-NAME.TXT file4.txt

dud@shadowplay:~ $ ls >> file4.txt dud@shadowplay:~ $ cat file4.txt file1.txt file2.pdf file3.mp3 file1.pdf another_file.txt Yet-Another_file.txt file-with_other-NAME.TXT file4.txt file1.txt file2.pdf file3.mp3 file1.pdf another_file.txt Yet-Another_file.txt file-with_other-NAME.TXT file4.txt

dud@shadowplay:~ $ sort another_file.txt another_file.txt file1.txt file1.txt file2.pdf file2.pdf file3.mp3 file3.mp3 file4.txt file4.txt file-with_other-NAME.TXT file-with_other-NAME.TXT Yet-Another_file.txt Yet-Another_file.txt

dud@shadowplay:~ $ ls | sort another_file.txt file1.pdf file1.txt file2.pdf file3.mp3 file-with_other-NAME.TXT Yet-Another_file.txt