A kernel is the foundational layer of an operating system OS. A Kernel is a computer program that is the heart and core of an Operating System. Since the Operating System has control over the system so, the Kernel also has control over everything in the system.
It is the most important part of an Operating System. The main difference between kernel and shell is that the kernel is the core of the operating system that controls all the tasks of the system while the shell is the interface that allows the users to communicate with the kernel.
When his sons Marcus junior and Samuel were looking for a name for the kerosene that they were exporting to Asia, they chose Shell. Shell functions are a way to group commands for later execution using a single name for the group. When the name of a shell function is used as a simple command name, the list of commands associated with that function name is executed.
A core feature of any operating system, the kernel manages communication between hardware and software. It also handles device signals, task scheduling, and other essential duties. There are special files too, and in fact even devices and directories are represented as files. How does the output of ls end up in foo? To understand that we have to know a bit more about files work in Unix.
The system call open is used to see the contents of a file. When we open a file to read or write to it, we get a file descriptor back. This file descriptor is just an integer that represents a specific file that a process can read or write to.
There are three special integers, 0, 1, and 2. Another word for these is standard streams. There is some confusion about the difference between files and streams, and people can mean different things when they talk about them. For our present purposes, we can treat them as equivalent - as long we let go of our preconceptions of what a file is. File descriptors are handed out by the kernel starting from the lowest available file descriptor, and when the shell starts it opens the three standard streams.
This is a simplified version of the actual code and it has no error handling. The second argument to open is mode, which is where we say if we want to read or write to the file. When we then open the file, foo in this case, to write to it, it will pick the lowest available file descriptor, which is 1. If we want to have a list of all my processes sorted by their process id we can pipe the result of running ps to the program sort. A pipe command has two sides: a left and a right side.
A pipe is a small buffer 6 that lives in kernel space and allows processes to talk to each other, which is called inter-process communication. This communication happens continuously as new data is written to the pipe. We create an array of two integers, which is where we will keep track of our file descriptors. We then use the system call pipe, which creates a pipe between two file descriptors and and puts these in p, where p[0] is for reading and p[1] is for writing.
After that, we create two child processes - one for the left process and one for the right one. These are the two if-blocks that check if fork returns 0, which it does in the child processes. Then we use another system call dup that duplicates a file descriptor.
What this means is that we can refer to the same file or stream but using a different file descriptor. Recall that a child process has almost exactly the same memory as a parent process. This includes file descriptors, so after we have connected the left process to STDOUT we want to close the file descriptors in p. Click here to post comments. Join in and write your own page! It's easy to do. What is the difference between kernel and shell?
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