● Detecting and preparing hardware– When the Linux system boots up (when you turn on your computer), it looks at the components on your computer (CPU, hard drive, network cards, and so on) and loads the software (drivers and modules) needed to access those particular hardware devices.
● Managing processes– The operating system must keep track of multiple processes running at the same time and decide which have access to the CPU and when. The system also must offer ways of starting, stopping, and changing the status of processes.
● Managing memory– RAM and swap space (extended memory) must be allocated to applications as they need memory. The operating system decides how requests for memory are handled.
● Providing user interfaces– An operating system must provide ways of accessing the system. The first Linux systems were accessed from a command-line interpreter called a shell. Today, graphical desktop interfaces are commonly available as well.
● Controlling filesystems– Filesystem structures are built into the operating system (or loaded as modules). The operating system controls ownership of and access to the files and directories (folders) that the filesystems contain.
● Providing user access and authentication– Creating user accounts and allowing boundaries to be set between users is a basic feature of Linux. Separate user and group accounts enable users to control their own files and processes.
● Offering administrative utilities– In Linux, hundreds (perhaps thousands) of commands and graphical windows are available to do such things as add users, manage disks, monitor the network, install software, and generally secure and manage your computer.
● Starting up services– To use printers, handle log messages, and provide a variety of system and network services, processes called daemon processes run in the background, waiting for requests to come in. Many types of services run in Linux. Linux provides different ways of starting and stopping these services. In other words, while Linux includes web browsers to view web pages, it can also be the computer that serves up web pages to others. Popular server features include web, mail, database, printer, file, DNS, and DHCP servers.
● Programming tools– A wide variety of programming utilities for creating applications and libraries for implementing specialty interfaces are available with Linux.
As someone managing Linux systems, you need to learn how to work with those features just described. While many features can be managed using graphical interfaces, an understanding of the shell command line is critical for someone administering Linux systems.
Modern Linux systems now go way beyond what the first UNIX systems (on which Linux was based) could do. Advanced features in Linux, often used in large enterprises, include the following:
● Clustering– Linux can be configured to work in clusters so that multiple systems can appear as one system to the outside world. Services can be configured to pass back and forth between cluster nodes, while appearing to those using the services that they are running without interruption.
● Virtualization– To manage computing resources more efficiently, Linux can run as a virtualization host. On that host, you could run other Linux systems, Microsoft Windows, BSD, or other operating systems as virtual guests. To the outside world, each of those virtual guests appears as a separate computer. KVM and Xen are two technologies in Linux for creating virtual hosts.
● Cloud computing– To manage large-scale virtualization environments, you can use full-blown cloud computing platforms based on Linux. Projects such as OpenStack and Red Hat Enterprise Virtualization can simultaneously manage many virtualization hosts, virtual networks, user and system authentication, virtual guests, and networked storage.
● Real-time computing– Linux can be configured for real-time computing, where high-priority processes can expect fast, predictable attention.
● Specialized storage– Instead of just storing data on the computer's hard disk, many specialized local and networked storage interfaces are available in Linux. Shared storage devices available in Linux include iSCSI, Fibre Channel, and Infiniband. Entire open source storage platforms include projects such as Ceph (http://ceph.com) and GlusterFS (http://gluster.org).
Some of these advanced topics are not covered in this book. However, the features covered here for using the shell, working with disks, starting and stopping services, and configuring a variety of servers should serve as a foundation for working with those advanced features.
Understanding How Linux Differs from Other Operating Systems
If you are new to Linux, chances are good that you have used a Microsoft Windows or Apple Mac OS operating system. Although Mac OS X has its roots in a free software operating system, referred to as the Berkeley Software Distribution (more on that later), operating systems from both Microsoft and Apple are considered proprietary operating systems. What that means is:
● You cannot see the code used to create the operating system.
● You, therefore, cannot change the operating system at its most basic levels if it doesn't suit your needs – and you can't use the operating system to build your own operating system from source code.
● You cannot check the code to find bugs, explore security vulnerabilities, or simply learn what that code is doing.
● You may not be able to easily plug your own software into the operating system if the creators of that system don't want to expose the programming interfaces you need to the outside world.
You might look at those statements about proprietary software and say, “What do I care? I'm not a software developer. I don't want to see or change how my operating system is built.”
That may be true. But the fact that others can take free and open source software and use it as they please has driven the explosive growth of the Internet (think Google), mobile phones (think Android), special computing devices (think Tivo), and hundreds of technology companies. Free software has driven down computing costs and allowed for an explosion of innovation.
Maybe you don't want to use Linux – as Google, Facebook, and other companies have done – to build the foundation for a multi-billion-dollar company. But those and other companies who now rely on Linux to drive their computer infrastructures need more and more people with the skills to run those systems.
You may wonder how a computer system that is so powerful and flexible has come to be free as well. To understand how that could be, you need to see where Linux came from. So the next section of this chapter describes the strange and winding path of the free software movement that led to Linux.
Exploring Linux History
Some histories of Linux begin with this message posted by Linus Torvalds to the comp.os.minix newsgroup on August 25, 1991 (http://groups.google.com/group/comp.os.minix/msg/b813d52cbc5a044b?pli=1):
Linus Benedict Torvalds
Hello everybody out there using minix -
I'm doing a (free) operating system (just a hobby, won't be big and professional like gnu) for 386(486) AT clones. This has been brewing since april, and is starting to get ready. I'd like any feedback on things people like/dislike in minix, as my OS resembles it somewhat (same physical layout of the file-system (due to practical reasons, among other things)…Any suggestions are welcome, but I won't promise I'll implement them: – )
Linus ([email protected])
PS. Yes – it's free of any minix