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:

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#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;
}

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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:

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                                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;

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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?

Quote:

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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.

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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.