Question in process redirection

I am trying to write a program to run ls ../ | grep sg | sort
with execvp and pipe in linux
The program does what I want it to do but the sequence is wrong.
The shell always prompt before the result.
How can I print the result before the shell prompt?
Here is my code:

Code:

#include <stdio.h>
#include <unistd.h>
int main(){
        char cmd[255];
        int fd[2];
        int fd2[2];
        int i;
        pid_t id;
        pipe(fd);
        pipe(fd2);
        for(i = 1; i < 4; i++){
                id = fork();
                if(id==0){
                        switch (i){
                                case(1): close(fd[0]);
                                    close(fd2[0]);
                                    close(fd2[1]);
                                    dup2(fd[1],STDOUT_FILENO);
                                    char * args1[] = { "ls", "../", NULL};
                                    execvp(args1[0],args1);
                                    break;
                                case(2): close(fd2[0]);
                                    dup2(fd2[1],STDOUT_FILENO);
                                    close(fd[1]);
                                    dup2(fd[0],STDIN_FILENO);
                                    char *args2[] = {"grep","sg",NULL};
                                    execvp(args2[0],args2);
                                    break;
                                case(3): close(fd[0]);
                                    close(fd[1]);
                                    close(fd2[1]);
                                    dup2(fd2[0],STDIN_FILENO);
                                    char *args3[] = {"sort",NULL};
                                    execvp(args3[0],args3);
                                }
                }else{
                        waitpid(id);
                }
        }
        return 0;
}

You are doing a VERY wierd thing here, teleologically speaking.

Anyway, there is only one thing wrong: in case(2) you need to move the stdout duplication after the execvp call. Otherwise, the pipe is waiting with nothing.

Code:

                                case(2): close(fd2[0]);
                                    close(fd[1]);
                                    dup2(fd[0],STDIN_FILENO);
                                    char *args2[] = {"grep","sg",NULL};
                                    execvp(args2[0],args2);
                                    dup2(fd2[1],STDOUT_FILENO);
                                    break;

Your program remains very unorthodox but it should work perfectly now.
ps. you do know that there is no point calling sort at the end of the command?

Originally Posted by MK27

Anyway, there is only one thing wrong: in case(2) you need to move the stdout duplication after the execvp call. Otherwise, the pipe is waiting with nothing.

Actually that's not true: the reason it works is because it now skips the last step (sort) and the output would be identical anyway.

This is really not a good use for c. I was warned.