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The Linux kernel can run a wide variety of software across many hardware platforms

You can simulate almost any configuration without needing powerful hardware or server licensing.

Linux software generally falls into one of three categories:

Server software – software that has no direct interaction with the monitor and keyboard of the machine it runs on. Its purpose is to serve information to other computers, called clients. Sometimes server software may not talk to other computers but will just sit there and "crunch" data.

Desktop software – a web browser, text editor, music player, or other software that you interact with. In many cases, such as a web browser, the software is talking to a server on the other end and interpreting the data for you. Here, the desktop software is the client.

Tools – a loose category of software that exists to make it easier to manage your system. You might have a tool that helps you configure your display, or something that provides a Linux shell, or even more sophisticated tools that convert source code to something that the computer can execute

Any task you want to do in Linux can likely be accommodated by any number of applications.

Linux excels at running server applications because of its reliability and efficiency.

A web server hosts content for web pages, which are viewed by a web browser using the Hypertext Transfer Protocol (HTTP) or its encrypted flavor, HTTPS. The web page itself can be static or dynamic.

Apache is the dominant web server in use today. It was originally a standalone project but the group has since formed the Apache Software Foundation and maintains over a hundred open source software projects.

Another web server is nginx, which is based out of Russia. It focuses on performance by making use of more modern UNIX kernels and only does a subset of what Apache can do.

3 different roles required to get email between people:

Mail Transfer Agent (MTA) – figures out which server needs to receive the email and uses the Simple Mail Transfer Protocol (SMTP) to move the email to that server. It is not unusual for an email to take several “hops” to get to its final destination, since an organization might have several MTAs.

The most well known MTA is sendmail. Postfix is another popular one and aims to be simpler and more secure than sendmail. If you’re using standard file formats for storing emails, your MTA can also deliver mail. Alternatively, you can use something like procmail, which lets you define custom filters to process mail and filter it.

Mail Delivery Agent (MDA, also called the Local Delivery Agent) – takes care of storing the email in the user’s mailbox. Usually invoked from the final MTA in the chain.

POP/IMAP server – The Post Office Protocol and Internet Message Access Protocol are two communication protocols that let an email client running on your computer talk to a remote server to pick up the email.Dovecot is a popular POP/IMAP server owing to its ease of use and low maintenance. Cyrus IMAP is another option. For file sharing, Samba is the clear winner. Samba allows a Linux machine to look like a Windows machine so that it can share files and participate in a Windows domain.

The native file sharing protocol for UNIX is called the Network File System (NFS). NFS is usually part of the kernel.

As your computer network gets larger, you will need to implement some kind of directory.

The DNS is largely focused on computer names and IP addresses and is not easily searchable.

One final piece of network infrastructure is called the Dynamic Host Configuration Protocol (DHCP). When a computer boots up, it needs an IP address for the local network so it can be uniquely identified. DHCP’s job is to listen for requests and to assign a free address from the DHCP pool.

A database stores information and also allows for easy retrieval and querying. The most popular databases here are MySQL and PostgreSQL.

The Linux X11 server is X.org, which provides a way for software to operate in a graphical mode and accept input from a keyboard and a mouse. Windows and icons are handled by another piece of software called the window manager or desktop environment. A window manager is a simpler version of desktop environment as it only provides the code to draw menus and manage the application windows on the screen. A desktop environment layers in features like login windows, sessions, a file manager, and other utilities. Window managers include Compiz, FVWM, and Enlightenment, though there are many more. Desktop environments are tprimarily KDE and GNOME, both of which have their own window managers

In 1999 Sun Microsystems acquired a relatively obscure German company that was making an office suite for Linux called StarOffice. Sun was acquired by Oracle, who later turned the project over to the Apache Foundation.2.2.2

At the basic level, you interact with a Linux system through a shell no matter if you are connecting to the system remotely or from an attached keyboard. The shell’s job is to accept commands, such as file manipulations and starting applications, and to pass those to the Linux kernel for execution. Comands a 2.2.

A text editor is used at the console to edit configuration files. The two main camps are vi (or the more modern vim) and emacs. Both are remarkably powerful tools to edit text files, they differ in the format of the commands and how you write plugins for them. Plugins could be anything from syntax highlighting of software projects to integrated calendars.

If you have a Linux system you will need to add, remove, and update software. distributions created packages which are compressed copies of the application. A package manager takes care of keeping track of which files belong to which package and even downloading updates from a remote server called a repository.

Languages fall into one of two camps: interpreted or compiled. An interpreted language translates the written code into computer code as the program runs, and a compiled language is translated all at once.

C’s main benefit is that the language itself maps closely to the generated machine code so that a skilled programmer can write code that is small and efficient. C is still helpful for writing code that must run fast, such as an operating system. At a much lower level is the C library. This provides a basic set of functions for reading and writing to files and displays, which is used by applications and other languages alike.

Interpreted languages also tend to offer more features than compiled languages, meaning that often less code is needed.

Perl is an interpreted language. Perl was originally developed to perform text manipulation. Over the years, it gained favor with systems administrators and still continues to be improved and used in everything from automation to building web applications. PHP is a language that was originally built to create dynamic web pages. Ruby, Phyton,OpenSSL 2.2.4

When we talk about buying software there are three distinct components:

Ownership – Who owns the intellectual property behind the software?

Money transfer – How does money change hands, if at all?

Licensing – What do you get? What can you do with the software? Can you use it on only one computer? Can you give it to someone else?

the ownership of the software remains with the person or company that created it. Users are only being granted a license to use the software. This is a matter of copyright law. The money transfer depends on the business model of the creator. It’s the licensing that really differentiates open source software from closed source software.

GNU Public License version 2 (GPLv2). This license, among other things, says that the source code must be made available to anyone who asks and that you are allowed to make any changes you want. One caveat to this is that if you make changes and distribute them, you must put your changes under the same license so that others can benefit.2.3

Free and Open Source Software (FOSS) refers to software where this right has been given up and you are allowed to view the source code and redistribute it. Linus Torvalds has done that with Linux – even though he created Linux he can’t tell you that you can’t use it on your computer because he has given up that right through the GPLv2 license.

End User License Agreement (EULA), is a custom legal document that you must click through, indicating your acceptance, in order to install the software. Microsoft keeps the source code and distributes only binary copies through authorized channels. For most consumer products you are allowed to install the software on one computer and are not allowed to make copies of the disk other than for a backup. You are not allowed to reverse engineer the software.

open source: The Free Software Foundation (FSF) and the Open Source Initiative (OSI).

The Free Software Foundation was founded in 1985 by Richard Stallman (RMS). The goal of the FSF is to promote Free Software. Free Software does not refer to the price, but to the freedom to share, study, and modify the underlying source code. It is the view of the FSF that proprietary software (software distributed under a closed source license) is bad. FSF also advocates that software licenses should enforce the openness of modifications. It is their view that if you modify Free Software that you should be required to share your changes. This specific philosophy is called copyleft. Digital Rights Management (DRM) that could restrict what you could do with the service.

The FSF have developed their own set of licenses, such as the GPLv2 and GPLv3, and the Lesser GPL licenses versions 2 and 3 (LGPLv2 & LGPLv3). The lesser licenses are much like the regular licenses except they have provisions for linking against non-Free Software. For example, under GPLv2 you can’t redistribute software that uses a closed source library (such as a hardware driver) but the lesser variant allows this.

OSI believes that not only should the source be freely available, but also that no restrictions should be placed on the use of the software no matter what the intended use. Unlike the FSF, the OSI does not have its own set of licenses. Instead, the OSI has a set of principles and adds other licenses to that list if they meet those principles, called Open Source licenses. Software that conforms to an Open Source license is therefore Open Source Software. Some of the Open Source licenses are the BSD family of licenses, which are much simpler than GPL. MIT 2.3.1

When software has been placed in the Public domain, the author has relinquished all rights, including the copyright on the work. In some countries, this is the default when the work is done by a government agency. In some countries, copyrighted work becomes public domain after the author has died and a lengthy waiting period has elapsed.

The Creative Commons (CC) organization has created the Creative Commons Licenses which try to address the intentions behind FOSS licenses for non software entities. CC licenses can also be used to restrict commercial use if that is the desire of the copyright holder. The CC licenses are. 2.3.3

The simplest way to make money is to sell support or warranty around the software. You may make money by installing the software for people, helping people when they have problems, or fixing bugs for money. You are effectively a consultant. You can also charge for a service or subscription that enhances the software. You can package hardware or add extra closed source software to sell alongside the free software. You can also develop open source software as part of your job.