int main(int argc, char *argv[])
{
  if (argc!=3) {printf("\nusage: <dir> <filter>\n"); return 1;}

  int fd1[2];
  pipe(fd1);
  int fd2[2];
  pipe(fd2);
  //int stdin = dup(STDIN_FILENO);
  //int stdout = dup(STDOUT_FILENO);

  pid_t pid;

  pid = fork();
  //send ls result via fd1
  if(pid==0) //son
  {
    close(fd1[READ]);
    dup2(fd1[WRITE],STDOUT_FILENO);
    execlp("ls","ls",argv[1],"-laR",NULL);
    exit(1);
  }//end_son, start father

  wait(NULL);

  pid = fork();

  //receive ls from fd1 and send grep vi fd2
  if(pid==0) //son
  {
     close(fd1[WRITE]);
    dup2(fd1[READ],STDIN_FILENO);
    close(fd2[READ]);
    dup2(fd2[WRITE],STDOUT_FILENO);
     execlp("grep","grep",argv[2],NULL);
     exit(1);
  }//end_son, start father

  //wait(NULL);

  pid = fork();
  //receive via fd2
  if(pid==0) //son
  {
    close(fd2[WRITE]);
    dup2(fd2[READ],STDIN_FILENO);
    execlp("sort","sort",NULL);
    exit(1);
  }//end_son, start father

  //wait(NULL);
  //wait(NULL);
  //wait(NULL);

  //  close(fd1[WRITE]);
  //close(fd1[READ]);
  //close(fd2[WRITE]);
  //close(fd2[READ]);

  return 0;
}