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I want to understand how this applies to an operating system and also to those things that are not infact operating systems. I can't understand the difference between the three and their essence. API is the functions we can call but what is Shell? If we have an API than what exactly is the Kernel of the operating system? I understand the an operating system has a Core that is not going to change and this core does the fundamental Job of a typical OS while we may have different user interfaces like GUI or command line with the same Kernel. So the problem is I am confused how these things are different. Aaaaaaarhg!

Can the functions like printf and fopen in C be called API calls?

quantum231
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4 Answers4

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  • A command-line interface (CLI) shell is a command interpreter, i.e. the program that either processes the command you enter in your command line (aka terminal) or processes shell scripts (text files containing commands) (batch mode). In early Unix times, it used to be the unique way for users to interact with their machines. Nowadays, graphical user interfaces (GUIs) are becoming the preferred type of shell for most users.

  • A kernel is a low level program interfacing with the hardware (CPU, RAM, disks, network, ...) on top of which applications are running. It is the lowest level program running on computers although with virtualization you can have multiple kernels running on top of virtual machines which themselves run on top of another operating system.

  • An API is a generic term defining the interface developers have to use when writing code using libraries and a programming language. Kernels have no APIs as they are not libraries. They do have an ABI, which, beyond other things, define how do applications interact with them through system calls. Unix application developers use the standard C library (eg: libc, glibc) to build ABI compliant binaries. printf(3) and fopen(3) are not wrappers to system calls but (g)libc standard facilities. The low level system calls they eventually use are write(2) and open(2) and possibly others like brk, mmap. The number in parentheses is a convention to tell in what manual the command is to be found.

The first volume of the Unix manual pages contains the shell commands.

The second one contains the system call wrappers like write and open. They form the interface to the kernel.

The third one contains the standard library (including the Unix standard API) functions (excluding system calls) like fopen and printf. These are not wrappers to specific system calls but just code using system calls when required.

jyapx
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jlliagre
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  • Than I wish to know one more thing. Is there any such thing as Windows Shell? What about situations where we have a GUI instead of a command line prompt? This is the only last thing I do not yet understand. – quantum231 Aug 31 '12 at 23:42
  • shell is a term initially and still mostly used only in an Unix context. Anyway, the fact there is a GUI doesn't make a difference. Whether you use windows, Gnome or KDE, you can run a shell or equivalent in a command line window (terminal emulator). – jlliagre Sep 01 '12 at 20:54
  • IMHO, "system call" is OS's API for implementing certain programming language's cross-platform standard library. Actually, fopen & printf are indeed kind of "wrapper" to system calls. some references: http://stackoverflow.com/questions/13179102 – Bossliaw Aug 01 '13 at 05:54
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    @Bossliaw `fopen` and `printf` have a lot of specific code and share tables, buffers and context with each other. For that reason, I don't call them wrappers to system calls, especially as for example you can call `printf` without any system call being involved. They just sometimes happen to eventually use system calls but this is true for every library function. The link you posted as reference which I totally agree with doesn't say anything that contradict that point. – jlliagre Aug 01 '13 at 09:06
  • I want to clearly state that a GUI is a type of shell. It is named a shell because it is the outermost layer around the operating system. – jyapx May 28 '20 at 21:49
  • @jyapx You might indeed call a GUI a shell but the usual definition is restricting it to CLIs. In any case, a shell, whether text only or graphical is not "the" outermost layer of an OS. That's an obsolete representation that was only valid in the early Unix days when there was a single shell implementation and that shell was the only way to launch a program. There is nowadays no single outermost layer as any userland program is part of this layer as far as the kernel is concerned. – jlliagre May 28 '20 at 23:47
  • @jiliagre I can see how you found my comment misleading, but if a kernel has both a CLI and a GUI then they are both shells of the same kernel. It is a metaphor for a shell and a kernel in nature where the shell wraps the inside (or kernel) of the nut or seed. – jyapx May 29 '20 at 15:36
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    @jyapx Yes, I do not question the metaphor. My point is that a shell, or a collection of shells are not the only outermost layer of an operation system. There are so much bogus diagrams wrongly presenting the shell(s) as a real layer while they are just programs like the others. See for example https://unix.stackexchange.com/questions/186474/what-are-the-very-fundamental-differences-in-architecture-between-unix-and-linux – jlliagre May 29 '20 at 16:14
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The Shell is the way to communicate with the OS and kernel by command line. The Shell does this by also calling the API. The kernel is indeed the core of the OS and does memory management, task scheduling, handles with filesystems, I/O handling,... All the things that the kernel does, can in some way be invoced by the API the OS provides.

printf and fopen are wraps around the system calls (API) provided by the OS and kernel

hamon
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Shell: It is like a command line interface to your operating system. Commands like ls, ps, kill and many more can be used to request to complete the specific operation to the OS. It is like "cmd" on windows.

Kernel: It is the main code of any operating system. Any request you give on shell or through GUI (like memory allocation, opening a file etc) are finally fulfilled by Kernel.

And yes, the calls you mentioned are regarded as API calls. The request to these calls are also handled by Kernel. Please go the below link to find API calls in unix. http://www.mkssoftware.com/docs/api_index.asp

This is the overall picture in a unix os:

Applications => (shell+library routines) => system calls => kernel

look the final request handler is the Kernel. Thx!

rahul
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  • Ahah, so Shell is merely a command line. So the cmd in windows is the Shell for windows or DOS? – quantum231 Aug 26 '12 at 18:09
  • Neither, because windows (and other OS) do not need to have a shell. Maybe shell should be renamed as default interface to the OS services? – Christopher Aug 26 '12 at 18:17
  • So our windows GUI environment with all the icons is not Shell or is it? – quantum231 Aug 26 '12 at 19:52
  • Is the Dos prompt Dos shell? Also, in order to have Linux Shell we shall have to run the Shell on the Windows OS right? – quantum231 Aug 26 '12 at 19:54
  • SO we communicate with the Kernel using API and these API calls can be made through a Shell when we are making direct requests to the OS e.g Open file, change directory e.t.c. Thus the DIR shall be an API call in DOS to its Kernel? – quantum231 Aug 26 '12 at 19:55
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Consider an example, You are watching the movie is on shell and actually process done over hardware is the kernel. shell is approximately work as that of os for user and software interface and kernel work as that of os for software and hardware.

Uwe Plonus
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Sanket
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