The objective of this project is for you to create a simple shell. Yes, your own little bash or zsh. You will learn a lot about processes and file descriptors.
Tutorial - Write a Shell in C - Stephen Brennan
Want to build a simple shell? Here's how you do it.
The shell is a command line interface (CLI) program that takes commands from the keyboard and gives them to the operating system to perform.
fork
creates a new process by duplicating the calling process. The new process is referred to as the child process. The calling process is referred to as the parent process. After a new child process is created, both processes will execute the next instruction following the fork() system call. A child process uses the same pc(program counter), same CPU registers, same open files which use in the parent process.
It takes no parameters and returns an integer value. Below are different values returned by fork().
Negative Value: the creation of a child process was unsuccessful. Zero: Returned to the newly created child process.Positive value: Returned to parent or caller. The value contains the process ID of the newly created child process.
wait
A call to wait() blocks the calling process until one of its child processes exits or a signal is received. After the child process terminates, the parent continues its execution after wait system call instruction.
We know if more than one child processes are terminated, then wait() reaps any arbitrarily child process but if we want to reap any specific child process, we use waitpid function.
wait3 wait4
The wait3() and wait4() system calls are similar to waitpid(2), but additionally, return resource usage information about the child in the structure pointed to by rusage.
The difference is wait3() waits of any child, while wait4() can be used to select a specific child, or children, on which to wait.
signal
A signal is a software generated interrupt that is sent to a process by the OS because of when the user press ctrl-c or another process tells something to this process. There are fixed set of signals that can be sent to a process. signal are identified by integers.
#include<stdio.h>
#include<signal.h>
**int** main()
{
**signal**(SIGINT, handle_sigint);
while(1)
{
**printf**(“hello world\n”);
sleep(1);
}
return 0;
}Output: Print hello world infinite times. If user presses ctrl-c to terminate the process because of SIGINT signal sent and its default handler to terminate the process.
dup dup2
https://www.youtube.com/watch?v=EqndHT606Tw
pipe
Conceptually, a pipe is a connection between two processes, such that the standard output from one process becomes the standard input of the other process.
execve
execve() executes the program pointed to by filename. filename must be either a binary executable or a script starting with a line of the form "#! interpreter [arg]". On success, execve() does not return, on error -1 is returned, and errno is set appropriately.
getenv
searches for the environment string pointed to by name and returns the associated value to the string.
isatty
is a function that returns 1 if the file descriptor refers to a terminal.
Further investigation would lead you to the discovery that file descriptors 0, 1 and 2 (aka STDIN_FILENO, STDOUT_FILENO and STDERR_FILENO) are by convention set up to point to your terminal when your program is running from a terminal.