Question

Write a simple shell that is similar to this: while(1 //repeat forever //display prompt on screen type_prompt )i read_command ( command, parameters); // read input from terminal if ( fork) 0) // parent //wait for child wait NULL elset execve (commamd, parameters, 0 // execute command but contains enough code that it actually works so you can test it. For simplicity, you may assume that all commands are in the directory /bin

please write down the code

Thank you.

Answer 1

A shell script is a computer program designed to be run by the Unix/Linux shell which could be one of the following:

• The Bourne Shell
• The C Shell
• The Korn Shell
• The GNU Bourne-Again Shell   

The following script uses the read command which takes the input from the keyboard and assigns it as the value of the variable PERSON and finally prints it on STDOUT.

#!/bin/sh
echo "What is your name?"
read PERSON
echo "Hello, $PERSON"

Sample run of Output:

$./test.sh
What is your name?
Zara Ali
Hello, Zara Ali
$

Fork() System Call:

Process creation is achieved through the fork() system call. The newly created process is called the child process and the process that initiated it (or the process when execution is started) is called the parent process. After the fork() system call, now we have two processes - parent and child processes

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: 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 process ID of newly created child process.

Sample Program:

#include <stdio.h>

#include <sys/types.h>

#include <unistd.h>

void forkexample()

{

    // child process because return value zero

    if (fork() == 0)

        printf("Hello from Child!\n");

  

    // parent process because return value non-zero.

    else

        printf("Hello from Parent!\n");

}

int main()

{

    forkexample();

    return 0;

}

Output:

1.
Hello from Child!
Hello from Parent!
     (or)
2.
Hello from Parent!
Hello from Child!

Explanation:

In the above code, a child process is created, fork() returns 0 in the child process and positive integer to the parent process.
Here, two outputs are possible because the parent process and child process are running concurrently. So we don’t know if OS first give control to which process a parent process or a child process.

Important: Parent process and child process are running the same program, but it does not mean they are identical. OS allocate different data and state for these two processes and also control the flow of these processes can be different. See next example