What is the difference between Shell, Kernel and API

Question

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?

Answer 1

• 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.