Thread: Implementing pipe in my own unix shell

Hi,

I am trying to implement pipes in my own unix shell. Below is the code.

Code:

#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include "scanner.h"


int main(int argc, char **argv)
{
    
    int pipefd[2];
    int pid;
    printf("\n[myShell--%s]$", getenv("PWD")); 

    while(1)
    {
        char *cat_args[] = {"cat", "f1", NULL};
        char *wc_args[] = {"wc", "-l", NULL};
        int pid1, status;
     // make a pipe (fds go in pipefd[0] and pipefd[1])

        pipe(pipefd);
        pid1 = fork();
        if(pid1 == 0)
        {

            pid = fork();

            if (pid == 0)
            {
                // replace standard input with input part of pipe
                dup2(pipefd[0], 0);
                // close unused hald of pipe
                close(pipefd[1]);
                // execute wc
                execvp("wc", wc_args);
            }
            else
            {
                // replace standard output with output part of pipe
                dup2(pipefd[1], 1);
                // close unused unput half of pipe
                close(pipefd[0]);
                  // execute cat
                execvp("cat", cat_args);
            }
        }
        else
        {
            wait(NULL);
            printf("[myShell--%s]$", getenv("PWD"));
        }
    }
}

I want to execute the command and go back to my own prompt. But instead it goes on printing "[myShell]. If I remove one parent fork, then it execute fine, but exits my shell.


Thanks in advance !!

With only one fork(), the parent process image is replaced (cf man execv) with that of "wc", so of course your loop doesn't exist anymore;
"wc" terminates and its child ("cat" in your case) is terminated.

The inner part of your program (piping cat and wc) is fine, but you need to rework the logic around that.
You outer fork() is just looping on the creation of child processes printing "[myShell..." and serve no purpose.
Try to implement a function that wrap execv() in order to execute your composite shell command without disturbing your main process looping on a (so far non-existing!) prompt.

PS: also, check system calls returns for errors.