File Name: kernel and shell in operating system .zip
A shell is a program that provides an interface between a user and an operating system OS kernel. An OS starts a shell for each user when the user logs in or opens a terminal or console window. By interfacing with a kernel, a shell provides a way for a user to execute utilities and programs.
- Kernel (operating system)
- What is the Unix Operating System : Architecture & Its Properties
- Understanding the Linux Kernel, 3rd Edition by Daniel P. Bovet, Marco Cesati
- What is Shell and Kernel
Kernel (operating system)
Each computer system includes a basic set of programs called the operating system. The most important program in the set is called the kernel. It is loaded into RAM when the system boots and contains many critical procedures that are needed for the system to operate. The other programs are less crucial utilities; they can provide a wide variety of interactive experiences for the user—as well as doing all the jobs the user bought the computer for—but the essential shape and capabilities of the system are determined by the kernel.
The kernel provides key facilities to everything else on the system and determines many of the characteristics of higher software. Interact with the hardware components, servicing all low-level programmable elements included in the hardware platform. Provide an execution environment to the applications that run on the computer system the so-called user programs.
Some operating systems allow all user programs to directly play with the hardware components a typical example is MS-DOS. In contrast, a Unix-like operating system hides all low-level details concerning the physical organization of the computer from applications run by the user. When a program wants to use a hardware resource, it must issue a request to the operating system.
The kernel evaluates the request and, if it chooses to grant the resource, interacts with the proper hardware components on behalf of the user program. To enforce this mechanism, modern operating systems rely on the availability of specific hardware features that forbid user programs to directly interact with low-level hardware components or to access arbitrary memory locations.
In particular, the hardware introduces at least two different execution modes for the CPU: a nonprivileged mode for user programs and a privileged mode for the kernel. Unix calls these User Mode and Kernel Mode , respectively. In the rest of this chapter, we introduce the basic concepts that have motivated the design of Unix over the past two decades, as well as Linux and other operating systems.
While the concepts are probably familiar to you as a Linux user, these sections try to delve into them a bit more deeply than usual to explain the requirements they place on an operating system kernel.
These broad considerations refer to virtually all Unix-like systems. The other chapters of this book will hopefully help you understand the Linux kernel internals. A multiuser system is a computer that is able to concurrently and independently execute several applications belonging to two or more users.
Concurrently means that applications can be active at the same time and contend for the various resources such as CPU, memory, hard disks, and so on.
Independently means that each application can perform its task with no concern for what the applications of the other users are doing. Switching from one application to another, of course, slows down each of them and affects the response time seen by the users. Many of the complexities of modern operating system kernels, which we will examine in this book, are present to minimize the delays enforced on each program and to provide the user with responses that are as fast as possible.
A protection mechanism against buggy user programs that could block other applications running in the system. A protection mechanism against malicious user programs that could interfere with or spy on the activity of other users.
An accounting mechanism that limits the amount of resource units assigned to each user. To ensure safe protection mechanisms, operating systems must use the hardware protection associated with the CPU privileged mode. Otherwise, a user program would be able to directly access the system circuitry and overcome the imposed bounds.
Unix is a multiuser system that enforces the hardware protection of system resources. In a multiuser system, each user has a private space on the machine; typically, he owns some quota of the disk space to store files, receives private mail messages, and so on. The operating system must ensure that the private portion of a user space is visible only to its owner.
In particular, it must ensure that no user can exploit a system application for the purpose of violating the private space of another user. Usually only a restricted number of persons are allowed to make use of a computer system. When one of these users starts a working session, the system asks for a login name and a password. If the user does not input a valid pair, the system denies access. To selectively share material with other users, each user is a member of one or more user groups , which are identified by a unique number called a user group ID.
Each file is associated with exactly one group. For example, access can be set so the user owning the file has read and write privileges, the group has read-only privileges, and other users on the system are denied access to the file. Any Unix-like operating system has a special user called root or superuser. The system administrator must log in as root to handle user accounts, perform maintenance tasks such as system backups and program upgrades, and so on.
The root user can do almost everything, because the operating system does not apply the usual protection mechanisms to her. In particular, the root user can access every file on the system and can manipulate every running user program. All operating systems use one fundamental abstraction: the process. In traditional operating systems, a process executes a single sequence of instructions in an address space ; the address space is the set of memory addresses that the process is allowed to reference.
Modern operating systems allow processes with multiple execution flows — that is, multiple sequences of instructions executed in the same address space. Multiuser systems must enforce an execution environment in which several processes can be active concurrently and contend for system resources, mainly the CPU. Systems that allow concurrent active processes are said to be multiprogramming or multiprocessing. On uniprocessor systems, just one process can hold the CPU, and hence just one execution flow can progress at a time.
In general, the number of CPUs is always restricted, and therefore only a few processes can progress at once. An operating system component called the scheduler chooses the process that can progress. Some operating systems allow only nonpreemptable processes, which means that the scheduler is invoked only when a process voluntarily relinquishes the CPU. But processes of a multiuser system must be preemptable ; the operating system tracks how long each process holds the CPU and periodically activates the scheduler.
Unix is a multiprocessing operating system with preemptable processes. Even when no user is logged in and no application is running, several system processes monitor the peripheral devices. In particular, several processes listen at the system terminals waiting for user logins. When a user inputs a login name, the listening process runs a program that validates the user password.
If the user identity is acknowledged, the process creates another process that runs a shell into which commands are entered. When a graphical display is activated, one process runs the window manager, and each window on the display is usually run by a separate process.
When a user creates a graphics shell, one process runs the graphics windows and a second process runs the shell into which the user can enter the commands. For each user command, the shell process creates another process that executes the corresponding program. Whenever a process makes a system call i. In this way, the operating system acts within the execution context of the process in order to satisfy its request.
Whenever the request is fully satisfied, the kernel procedure forces the hardware to return to User Mode and the process continues its execution from the instruction following the system call. As stated before, most Unix kernels are monolithic: each kernel layer is integrated into the whole kernel program and runs in Kernel Mode on behalf of the current process.
In contrast, microkernel operating systems demand a very small set of functions from the kernel, generally including a few synchronization primitives, a simple scheduler, and an interprocess communication mechanism. Several system processes that run on top of the microkernel implement other operating system-layer functions, like memory allocators, device drivers, and system call handlers. Although academic research on operating systems is oriented toward microkernels , such operating systems are generally slower than monolithic ones, because the explicit message passing between the different layers of the operating system has a cost.
However, microkernel operating systems might have some theoretical advantages over monolithic ones. Microkernels force the system programmers to adopt a modularized approach, because each operating system layer is a relatively independent program that must interact with the other layers through well-defined and clean software interfaces.
Moreover, an existing microkernel operating system can be easily ported to other architectures fairly easily, because all hardware-dependent components are generally encapsulated in the microkernel code. To achieve many of the theoretical advantages of microkernels without introducing performance penalties, the Linux kernel offers modules.
A module is an object file whose code can be linked to and unlinked from the kernel at runtime. The module, unlike the external layers of microkernel operating systems, does not run as a specific process. Instead, it is executed in Kernel Mode on behalf of the current process, like any other statically linked kernel function.
Because any module can be linked and unlinked at runtime, system programmers must introduce well-defined software interfaces to access the data structures handled by modules. This makes it easy to develop new modules. A module can be linked to the running kernel when its functionality is required and unlinked when it is no longer useful; this is quite useful for small embedded systems.
Once linked in, the object code of a module is equivalent to the object code of the statically linked kernel. Therefore, no explicit message passing is required when the functions of the module are invoked. However, this penalty can be compared to the penalty caused by the creation and deletion of system processes in microkernel operating systems. Skip to main content. Bovet, Marco Cesati. Start your free trial. Basic Operating System Concepts. Multiuser Systems. Users and Groups. Kernel Architecture.
Frugal main memory usage A module can be linked to the running kernel when its functionality is required and unlinked when it is no longer useful; this is quite useful for small embedded systems. No performance penalty Once linked in, the object code of a module is equivalent to the object code of the statically linked kernel.
What is the Unix Operating System : Architecture & Its Properties
This tutorial teaches some basics about UNIX. By operating system, we mean the suite of programs which make the computer work. Because jobs must be submitted to the supercomputer using UNIX commands, it is pertinent that you, as a user of the supercomputer, obtain at least a basic knowledge of the UNIX environment. In general, the UNIX operating system is made up of three parts; the kernel , the shell , and the programs. If we think of the UNIX operating system in terms of layers, the kernel is the lowest layer. It interfaces directly with the computer hardware and is responsible for allocating and managing the resouces available to programs. It allocates processor time and memory to each program and determines when each program will run.
Understanding the Linux Kernel, 3rd Edition by Daniel P. Bovet, Marco Cesati
A computer system contains hardware and software components. We can install and use many types of software on our computers. The system software is specially designed to provide a platform for other software on the computer.
The kernel is a computer program at the core of a computer's operating system that has complete control over everything in the system. On most systems, the kernel is one of the first programs loaded on startup after the bootloader. The critical code of the kernel is usually loaded into a separate area of memory, which is protected from access by application programs or other, less critical parts of the operating system. The kernel performs its tasks, such as running processes, managing hardware devices such as the hard disk , and handling interrupts, in this protected kernel space.
What is Shell and Kernel
Each computer system includes a basic set of programs called the operating system. The most important program in the set is called the kernel. It is loaded into RAM when the system boots and contains many critical procedures that are needed for the system to operate.
UNIX is an operating system which was first developed in the s, and has been under constant development ever since. By operating system, we mean the suite of programs which make the computer work. It is a stable, multi-user, multi-tasking system for servers, desktops and laptops. However, knowledge of UNIX is required for operations which aren't covered by a graphical program, or for when there is no windows interface available, for example, in a telnet session. There are many different versions of UNIX, although they share common similarities. The kernel of UNIX is the hub of the operating system: it allocates time and memory to programs and handles the filestore and communications in response to system calls. As an illustration of the way that the shell and the kernel work together, suppose a user types rm myfile which has the effect of removing the file myfile.
Types of UNIX
What are the tasks performed by kernel? Answer: There are following main tasks performed by kernel What are the different types of shells available? Answer: Some most popular commercial shells are Shell variable substitution Stores data in user-defined variables and predefined shell variables. Kernel is also called as the heart of the Operating System. A shell is software that provides an interface for users of an operating system to access the services of a kernel.
Мы слухачи, стукачи, нарушители прав человека. - Стратмор шумно вздохнул. - Увы, в мире полно наивных людей, которые не могут представить себе ужасы, которые нас ждут, если мы будем сидеть сложа руки. Я искренне верю, что только мы можем спасти этих людей от их собственного невежества. Сьюзан не совсем понимала, к чему он клонит. Коммандер устало опустил глаза, затем поднял их вновь.
Беккер отлично говорил по-французски, тем не менее обратился к этому человеку на языке, который тот, как он надеялся, должен был знать хуже. Убедить абсолютно незнакомого человека отдать вам золотое кольцо скорее всего будет весьма непросто, поэтому Беккер хотел заручиться хотя бы одним преимуществом. Пока старик собирался с мыслями, Беккер не произнес ни слова. Тот огляделся вокруг, указательным пальцем разгладил усы и наконец заговорил: - Что вам нужно? - Он произносил английские слова немного в нос. - Сэр, - начал Беккер чуть громче, словно обращаясь к глуховатому человеку, - я хотел бы задать вам несколько вопросов.
Беккер еще раз просмотрел сообщение. - Нет. Они сказали - агентство. АНБ. - Никогда о таком не слышал.
Сьюзан потребовалось всего мгновение. Стратмор сразу заметил изумление, мелькнувшее в ее глазах, и взволнованно изложил свой план: - Если бы я получил ключ, то смог бы взломать наш экземпляр Цифровой крепости и внести нужные изменения… - Черный ход, - сказала Сьюзан, мгновенно забыв о том, что Стратмор ей лгал. Она все поняла.
- Она показала ему другую колонку. - Видишь. - Вижу, - сказал Бринкерхофф, стараясь сосредоточиться на документе. - Это данные о сегодняшней производительности. Взгляни на число дешифровок.