What is an Operating System?
An operating system (OS) is software that manages computer resources and provides programmers with an interface used to access those resources. An operating system processes system data and user input, and responds by allocating and managing tasks and internal system resources as a service to users and programs of the system. An operating system performs basic tasks such as controlling and allocating memory, prioritizing system requests, controlling input and output devices, facilitating computer networking and managing files. Operating systems can be found on almost anything made with integrated circuits, such as personal computers, internet servers, cell phones, music players, routers, switches, wireless access points.
The Operating System as a Resource Manager
The concept of the operating system as primarily providing its users with a convenient interface is a top-down view. An alternative, bottom-up, view holds that the operating system is there to manage all the pieces of a complex system.
When a computer (or network) has multiple users, the need for managing and protecting the memory, I/O devices, and other resources is even greater, since the users might otherwise interfere with one another. In addition, users often need to share not only hardware, but information (files, databases, etc.) as well. In short, this view of the operating system holds that its primary task is to keep track of who is using which resource, to grant resource requests, to account for usage, and to mediate conflicting requests from different programs and users.
Resource management includes multiplexing (sharing) resources in two ways:
- In time and
- In space.
When a resource is time multiplexed, different programs or users take turns using it. First one of them gets to use the resource, then another, and so on.
The other kind of multiplexing is space multiplexing. Instead of the customers taking turns, each one gets part of the resource. Resources that are space multiplexed are- the (hard) disk, main memory (it is normally divided up among several running programs, so each one can be resident at the same time). In many systems a single disk can hold files from many users at the same time. Allocating disk space and keeping track of who is using which disk blocks is a typical operating system resource management task.
Operating System Concepts
The interface between the operating system and the user programs is defined by the set of "extended instructions" that the operating system provides. These extended instructions have been traditionally known as system calls, although they can be implemented in several ways. The calls available in the interface vary from operating system to operating system.
A key concept in all operating systems is the process. A process is basically a program in execution. Associated with each process is its address space, a list of memory locations from some minimum (usually 0) to some maximum, which the process can read and write. The address space contains the executable program, the program's data, and its stack. Also associated with each process is some set of registers, including the program counter, stack pointer, and other hardware registers, and all the other information needed to run the program.
When a process is suspended temporarily like this, it must later be restarted in exactly the same state it had when it was stopped.
In many operating systems, all the information about each process, other than the contents of its own address space, is stored in an operating system table called the process table, which is an array (or linked list) of structures, one for each process currently in existence.
Thus, a (suspended) process consists of its address space, usually called the core image (in honor of the magnetic core memories used in days of yore), and its process table entry, which contains its registers, among other things.
The key process management system calls are those dealing with the creation and termination of processes. Consider a typical example. A process called the command interpreter or shell reads commands from a terminal. The user has just typed a command requesting that a program be compiled. The shell must now create a new process that will run the compiler. When that process has finished the compilation, it executes a system call to terminate itself.
A process that is communicating with another process on a different computer does so by sending messages to the remote process over a network. Each person authorized to use an operating system is assigned a UID (User IDentification) by the system administrator. Every process started has the UID of the person who started it. A child process has the same UID as its parent. Users can be members of groups, each of which has a GID (Group IDentification)
One UID, called the superuser (in UNIX), has special power and may violate many of the protection rules. In large installations, only the system administrator knows the password needed to become superuser, but many of the ordinary users (especially students) devote considerable effort to trying to find flaws in the system that allow them to become superuser without the password.
The other broad category of system calls relates to the file system. A major function of the operating system is to hide the peculiarities of the disks and other I/O devices and present the programmer with a nice, clean abstract model of device-independent files. System calls are obviously needed to create files, remove files, read files, and write files.
To provide a place to keep files, Operating systems has the concept of a directory as a way of grouping files together. A student, for example, might have one directory for each course he is taking (for the programs needed for that course), another directory for his electronic mail, and still another directory for his World Wide Web home page. System calls are then needed to create and remove directories. Calls are also provided to put an existing file into a directory, and to remove a file from a directory. Directory entries may be either files or other directories.
The process and file hierarchies both are organized as trees, but the similarity stops there. Process hierarchies usually are not very deep (more than three levels is unusual), whereas file hierarchies are commonly four, five, or even more levels deep. Process hierarchies are typically short-lived, generally a few minutes at most, whereas the directory hierarchy may exist for years. Ownership and protection also differ for processes and files. Typically, only a parent process may control or even access a child process, but mechanisms nearly always exist to allow files and directories to be read by a wider group than just the owner.
Every file within the directory hierarchy can be specified by giving its path name from the top of the directory hierarchy, the root directory. Such absolute path names consist of the list of directories that must be traversed from the root directory to get to the file, with slashes separating the components. The path for file CS101 is /Faculty/Prof.Brown/Courses/CS101. The leading slash indicates that the path is absolute that is, starting at the root directory. As an aside, in Windows, the backslash (\) character is used as the separator instead of the slash (/) character, so the file path given above would be written as \Faculty\Prof.Brown\Courses\CS101.
File Types - Name, Extension
In computing, a shell is a piece of software that provides an interface for users. Typically, the term refers to an operating system shell which provides access to the services of a kernel. However, the term is also applied very loosely to applications and may include any software that is "built around" a particular component, such as web browsers and email clients that are "shells" for HTML rendering engines. The name 'shell' originates from shells being an outer layer of interface between the user and the innards of the operating system (the kernel).
Operating system shells generally fall into one of two categories:
- Command line and
Command line shells provide a command line interface (CLI) to the operating system, while graphical shells provide a graphical user interface (GUI). In either category the primary purpose of the shell is to invoke or "launch" another program; however, shells frequently have additional capabilities such as viewing the contents of directories.
CLI proponents claim that certain operations can be performed much faster under CLI shells than under GUI shells (such as moving files, for example). However, GUI proponents advocate the comparative usability and simplicity of GUI shells. The best choice is often determined by the way in which a computer will be used. On a server mainly used for data transfers and processing with expert administration, a CLI is likely to be the best choice. On the other hand, a GUI would be more appropriate for a computer to be used for image or video editing and the development of the above data.
A UNIX shell, also called "the command line", provides the traditional user interface for the UNIX operating system and for Unix-like systems. Users direct the operation of the computer by entering command input as text for a shell to execute. Within the Microsoft Windows suite of operating systems the analogous program is command.com, or cmd.exe for Windows NT-based operating systems.
The most generic sense of the term shell means any program that users use to type commands. Since in the UNIX operating system users can select which shell they want to use (which program should execute when they login), many shells have been developed. It is called a "shell" because it hides the details of the underlying operating system behind the shell's interface (in contrast with the "kernel", which refers to the lowest-level or 'inner-most' component of an operating system). Similarly, graphical user interfaces for UNIX, such as GNOME, KDE, and Xfce can be called visual shells or graphical shells. By itself, the term shell is usually associated with the command line. In UNIX, any program can be the user's shell. Users who want to use a different syntax for typing commands can specify a different program as their shell.
The term shell also refers to a particular program, such as the Bourne shell, sh. The Bourne shell was the shell used in early versions of UNIX and became a de facto standard; every Unix-like system has at least one shell compatible with the Bourne shell. The Bourne shell program is located in the UNIX file hierarchy at /bin/sh. On some systems, such as BSD, /bin/sh is a Bourne shell or equivalent, but on other systems such as Linux, /bin/sh is likely to be a link to a compatible, but more feature-rich shell. POSIX specifies its standard shell as a strict subset of the Korn shell.
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