Using dup2 for piping

Question

How do I use dup2 to perform the following command?

ls -al | grep alpha | more

Answer 1

You would use pipe(2,3p) as well. Create the pipe, fork, duplicate the appropriate end of the pipe onto FD 0 or FD 1 of the child, then exec.

Answer 2

A Little example with the first two commands. You need to create a pipe with the pipe() function that will go between ls and grep and other pipe between grep and more. What dup2 does is copy a file descriptor into another. Pipe works by connecting the input in fd[0] to the output of fd[1]. You should read the man pages of pipe and dup2. I may try and simplify the example later if you have some other doubts.

#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>

#define READ_END 0
#define WRITE_END 1

int 
main (int argc, char **argv) 
{
    pid_t pid;
    int fd[2];

    pipe(fd);
    int i;
    pid=fork();

    if (pid==0) {

        printf("i'm the child used for ls \n");
        dup2(fd[WRITE_END], STDOUT_FILENO);
        close(fd[WRITE_END]);
        execlp("ls", "ls", "-al", NULL);

   } else { 
    pid=fork();

    if (pid==0) {
        printf("i'm in the second child, which will be used to run grep\n");
        dup2(fd[READ_END], STDIN_FILENO);
        close(fd[READ_END]);


       execlp("grep", "grep","alpha",NULL);
    }

    }


return 0;
}